[syzbot] [fs?] KASAN: slab-use-after-free Read in debugfs_u32_get (2)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 062871f1371b Merge tag 'pinctrl-v7.1-3' of git://git.kerne..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=148da8ae580000
kernel config: https://syzkaller.appspot.com/x/.config?x=4e4d2284f2ffa41
dashboard link: https://syzkaller.appspot.com/bug?extid=6c25f4750230faf70be9
compiler: gcc (Debian 14.2.0-19) 14.2.0, GNU ld (GNU Binutils for Debian) 2.44
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=123143b6580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/d66ac4b1a707/disk-062871f1.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/5fb941439264/vmlinux-062871f1.xz
kernel image: https://storage.googleapis.com/syzbot-assets/ed65e46ebbc3/bzImage-062871f1.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+6c25f4...@syzkaller.appspotmail.com

==================================================================
BUG: KASAN: slab-use-after-free in debugfs_u32_get+0x65/0x80 fs/debugfs/file.c:657
Read of size 4 at addr ffff88807c8e5b00 by task syz.2.10230/16881

CPU: 0 UID: 0 PID: 16881 Comm: syz.2.10230 Not tainted syzkaller #0 PREEMPT(full)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/09/2026
Call Trace:
<TASK>
__dump_stack lib/dump_stack.c:94 [inline]
dump_stack_lvl+0x100/0x190 lib/dump_stack.c:120
print_address_description mm/kasan/report.c:378 [inline]
print_report+0x13d/0x4b0 mm/kasan/report.c:482
kasan_report+0xdf/0x1d0 mm/kasan/report.c:595
debugfs_u32_get+0x65/0x80 fs/debugfs/file.c:657
simple_attr_read+0x16b/0x350 fs/libfs.c:1368
debugfs_attr_read+0x74/0xb0 fs/debugfs/file.c:523
vfs_read+0x1e4/0xb30 fs/read_write.c:572
ksys_read+0x12a/0x250 fs/read_write.c:717
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x115/0x840 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fa7fd79ce59
Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 e8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007ffe85aad168 EFLAGS: 00000246 ORIG_RAX: 0000000000000000
RAX: ffffffffffffffda RBX: 00007fa7fda15fa0 RCX: 00007fa7fd79ce59
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000003
RBP: 00007fa7fd832d6f R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007fa7fda15fac R14: 00007fa7fda15fa0 R15: 00007fa7fda15fa0
</TASK>

Allocated by task 14078:
kasan_save_stack+0x30/0x50 mm/kasan/common.c:57
kasan_save_track+0x14/0x30 mm/kasan/common.c:78
poison_kmalloc_redzone mm/kasan/common.c:398 [inline]
__kasan_kmalloc+0xaa/0xb0 mm/kasan/common.c:415
kasan_kmalloc include/linux/kasan.h:263 [inline]
__do_kmalloc_node mm/slub.c:5296 [inline]
__kvmalloc_node_noprof+0x360/0xa00 mm/slub.c:6833
alloc_netdev_mqs+0xd7/0x1560 net/core/dev.c:12029
nsim_create+0x94/0x1420 drivers/net/netdevsim/netdev.c:1133
__nsim_dev_port_add+0x3ba/0x8f0 drivers/net/netdevsim/dev.c:1509
nsim_dev_port_add_all drivers/net/netdevsim/dev.c:1570 [inline]
nsim_drv_probe+0xeb7/0x14b0 drivers/net/netdevsim/dev.c:1731
call_driver_probe drivers/base/dd.c:631 [inline]
really_probe+0x241/0xa60 drivers/base/dd.c:709
__driver_probe_device+0x22e/0x480 drivers/base/dd.c:871
driver_probe_device+0x4c/0x1b0 drivers/base/dd.c:901
__device_attach_driver+0x1df/0x340 drivers/base/dd.c:1029
bus_for_each_drv+0x159/0x1e0 drivers/base/bus.c:500
__device_attach+0x1e4/0x4d0 drivers/base/dd.c:1101
device_initial_probe+0xaf/0xd0 drivers/base/dd.c:1156
bus_probe_device+0x64/0x160 drivers/base/bus.c:613
device_add+0x1210/0x1950 drivers/base/core.c:3706
nsim_bus_dev_new drivers/net/netdevsim/bus.c:471 [inline]
new_device_store+0x40b/0x700 drivers/net/netdevsim/bus.c:191
bus_attr_store+0x74/0xb0 drivers/base/bus.c:172
sysfs_kf_write+0xf2/0x150 fs/sysfs/file.c:142
kernfs_fop_write_iter+0x3e0/0x5f0 fs/kernfs/file.c:352
new_sync_write fs/read_write.c:595 [inline]
vfs_write+0x6ac/0x1070 fs/read_write.c:688
ksys_write+0x12a/0x250 fs/read_write.c:740
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x115/0x840 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

Freed by task 13:
kasan_save_stack+0x30/0x50 mm/kasan/common.c:57
kasan_save_track+0x14/0x30 mm/kasan/common.c:78
kasan_save_free_info+0x3b/0x70 mm/kasan/generic.c:584
poison_slab_object mm/kasan/common.c:253 [inline]
__kasan_slab_free+0x5f/0x80 mm/kasan/common.c:285
kasan_slab_free include/linux/kasan.h:235 [inline]
slab_free_hook mm/slub.c:2689 [inline]
slab_free mm/slub.c:6251 [inline]
kfree+0x223/0x6c0 mm/slub.c:6566
device_release+0xd2/0x270 drivers/base/core.c:2566
kobject_cleanup lib/kobject.c:689 [inline]
kobject_release lib/kobject.c:720 [inline]
kref_put include/linux/kref.h:65 [inline]
kobject_put+0x1f7/0x640 lib/kobject.c:737
put_device+0x1f/0x30 drivers/base/core.c:3814
free_netdev+0x5ef/0x8c0 net/core/dev.c:12223
__nsim_dev_port_del+0x194/0x2b0 drivers/net/netdevsim/dev.c:1547
nsim_dev_port_del_all drivers/net/netdevsim/dev.c:1561 [inline]
nsim_dev_reload_destroy+0x10a/0x4a0 drivers/net/netdevsim/dev.c:1785
nsim_dev_reload_down+0x66/0xd0 drivers/net/netdevsim/dev.c:1038
devlink_reload+0x1ae/0x7a0 net/devlink/dev.c:462
devlink_pernet_pre_exit+0x247/0x370 net/devlink/core.c:560
ops_pre_exit_list net/core/net_namespace.c:161 [inline]
ops_undo_list+0x187/0xab0 net/core/net_namespace.c:234
cleanup_net+0x499/0x920 net/core/net_namespace.c:702
process_one_work+0xa0e/0x1980 kernel/workqueue.c:3314
process_scheduled_works kernel/workqueue.c:3397 [inline]
worker_thread+0x5ef/0xe50 kernel/workqueue.c:3478
kthread+0x370/0x450 kernel/kthread.c:436
ret_from_fork+0x72b/0xd50 arch/x86/kernel/process.c:158
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

The buggy address belongs to the object at ffff88807c8e4000
which belongs to the cache kmalloc-cg-8k of size 8192
The buggy address is located 6912 bytes inside of
freed 8192-byte region [ffff88807c8e4000, ffff88807c8e6000)

The buggy address belongs to the physical page:
page: refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x7c8e0
head: order:3 mapcount:0 entire_mapcount:0 nr_pages_mapped:0 pincount:0
memcg:ffff88807c8e2011
flags: 0xfff00000000040(head|node=0|zone=1|lastcpupid=0x7ff)
page_type: f5(slab)
raw: 00fff00000000040 ffff88813fe47640 dead000000000100 dead000000000122
raw: 0000000000000000 0000400000020002 00000000f5000000 ffff88807c8e2011
head: 00fff00000000040 ffff88813fe47640 dead000000000100 dead000000000122
head: 0000000000000000 0000400000020002 00000000f5000000 ffff88807c8e2011
head: 00fff00000000003 fffffffffffffe01 00000000ffffffff 00000000ffffffff
head: ffffffffffffffff 0000000000000000 00000000ffffffff 0000000000000008
page dumped because: kasan: bad access detected
page_owner tracks the page as allocated
page last allocated via order 3, migratetype Unmovable, gfp_mask 0xd60c0(__GFP_IO|__GFP_FS|__GFP_NOWARN|__GFP_RETRY_MAYFAIL|__GFP_NORETRY|__GFP_COMP|__GFP_NOMEMALLOC), pid 5738, tgid 5738 (syz-executor), ts 350789534111, free_ts 350786879056
set_page_owner include/linux/page_owner.h:32 [inline]
post_alloc_hook+0xfd/0x120 mm/page_alloc.c:1853
prep_new_page mm/page_alloc.c:1861 [inline]
get_page_from_freelist+0x11a6/0x3410 mm/page_alloc.c:3941
__alloc_frozen_pages_noprof+0x27c/0x2bc0 mm/page_alloc.c:5221
alloc_slab_page mm/slub.c:3278 [inline]
allocate_slab mm/slub.c:3467 [inline]
new_slab+0xa6/0x6c0 mm/slub.c:3525
refill_objects+0x277/0x420 mm/slub.c:7272
refill_sheaf mm/slub.c:2816 [inline]
__pcs_replace_empty_main+0x375/0x650 mm/slub.c:4652
alloc_from_pcs mm/slub.c:4750 [inline]
slab_alloc_node mm/slub.c:4884 [inline]
__do_kmalloc_node mm/slub.c:5295 [inline]
__kvmalloc_node_noprof+0x7da/0xa00 mm/slub.c:6833
alloc_netdev_mqs+0xd7/0x1560 net/core/dev.c:12029
rtnl_create_link+0xc13/0xf80 net/core/rtnetlink.c:3672
rtnl_newlink_create net/core/rtnetlink.c:3854 [inline]
__rtnl_newlink net/core/rtnetlink.c:3995 [inline]
rtnl_newlink+0x13bd/0x2380 net/core/rtnetlink.c:4110
rtnetlink_rcv_msg+0x95e/0xe90 net/core/rtnetlink.c:6997
netlink_rcv_skb+0x159/0x420 net/netlink/af_netlink.c:2555
netlink_unicast_kernel net/netlink/af_netlink.c:1318 [inline]
netlink_unicast+0x585/0x850 net/netlink/af_netlink.c:1344
netlink_sendmsg+0x8b0/0xda0 net/netlink/af_netlink.c:1899
sock_sendmsg_nosec net/socket.c:787 [inline]
__sock_sendmsg net/socket.c:802 [inline]
__sys_sendto+0x468/0x4b0 net/socket.c:2266
__do_sys_sendto net/socket.c:2273 [inline]
__se_sys_sendto net/socket.c:2269 [inline]
__x64_sys_sendto+0xe0/0x1c0 net/socket.c:2269
page last free pid 5762 tgid 5762 stack trace:
reset_page_owner include/linux/page_owner.h:25 [inline]
__free_pages_prepare mm/page_alloc.c:1397 [inline]
__free_frozen_pages+0x794/0x10a0 mm/page_alloc.c:2938
qlink_free mm/kasan/quarantine.c:163 [inline]
qlist_free_all+0x47/0xf0 mm/kasan/quarantine.c:179
kasan_quarantine_reduce+0x1a0/0x1f0 mm/kasan/quarantine.c:286
__kasan_slab_alloc+0x69/0x90 mm/kasan/common.c:350
kasan_slab_alloc include/linux/kasan.h:253 [inline]
slab_post_alloc_hook mm/slub.c:4570 [inline]
slab_alloc_node mm/slub.c:4899 [inline]
__kmalloc_cache_noprof+0x243/0x6f0 mm/slub.c:5415
kmalloc_noprof include/linux/slab.h:950 [inline]
kzalloc_noprof include/linux/slab.h:1188 [inline]
ref_tracker_alloc+0x190/0x590 lib/ref_tracker.c:270
__netdev_tracker_alloc include/linux/netdevice.h:4471 [inline]
netdev_hold include/linux/netdevice.h:4500 [inline]
netdev_hold include/linux/netdevice.h:4495 [inline]
netdev_queue_add_kobject net/core/net-sysfs.c:1979 [inline]
netdev_queue_update_kobjects+0x2db/0x6f0 net/core/net-sysfs.c:2041
register_queue_kobjects net/core/net-sysfs.c:2104 [inline]
netdev_register_kobject+0x2b3/0x3d0 net/core/net-sysfs.c:2347
register_netdevice+0x151c/0x24b0 net/core/dev.c:11423
__ip_tunnel_create+0x52b/0x670 net/ipv4/ip_tunnel.c:268
ip_tunnel_init_net+0x230/0x780 net/ipv4/ip_tunnel.c:1147
ops_init+0x1e2/0x5f0 net/core/net_namespace.c:137
setup_net+0x118/0x3a0 net/core/net_namespace.c:446
copy_net_ns+0x46f/0x7c0 net/core/net_namespace.c:579
create_new_namespaces+0x3ea/0xac0 kernel/nsproxy.c:132
unshare_nsproxy_namespaces+0xf2/0x220 kernel/nsproxy.c:234

Memory state around the buggy address:
ffff88807c8e5a00: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
ffff88807c8e5a80: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>ffff88807c8e5b00: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
^
ffff88807c8e5b80: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
ffff88807c8e5c00: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
==================================================================


---
This report is generated by a bot. It may contain errors.
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If the report is already addressed, let syzbot know by replying with:
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If you want syzbot to run the reproducer, reply with:
#syz test: git://repo/address.git branch-or-commit-hash
If you attach or paste a git patch, syzbot will apply it before testing.

If you want to overwrite report's subsystems, reply with:
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If you want to undo deduplication, reply with:
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[syzbot]

For archival purposes, forwarding an incoming command email to
linux-...@vger.kernel.org, syzkall...@googlegroups.com.

***

Subject: netdevsim: fix use-after-free in __nsim_dev_port_del
Author: hrushi...@gmail.com

#syz test: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
master

From b3aa94bf5977395e788b457e3bb073c2b7f4580d Mon Sep 17 00:00:00 2001
From: Hrushiraj Gandhi <hrushi...@gmail.com>
Date: Thu, 18 Jun 2026 13:03:56 +0530
Subject: [PATCH] netdevsim: fix use-after-free in __nsim_dev_port_del

debugfs files created under a port's ddir (ethtool/get_err,
ethtool/set_err, ring params, bpf_offloaded_id, udp_ports/inject_error,
etc.) store raw pointers directly into the netdevsim struct, which lives
in the net_device private data kmalloc slab.

In __nsim_dev_port_del(), nsim_destroy() was called before
nsim_dev_port_debugfs_exit(), meaning free_netdev() freed the
netdevsim slab while debugfs files still held live pointers into it.
A concurrent reader with the file already open could pass
debugfs_file_get(), then dereference the freed pointer in
debugfs_u32_get(), triggering a slab-use-after-free.

Fix by calling nsim_dev_port_debugfs_exit() first, so
debugfs_remove_recursive() tears down the entire port ddir subtree
(invalidating all stale data pointers) before free_netdev() releases
the backing memory.

Reported-by: syzbot+6c25f4...@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=6c25f4750230faf70be9
Fixes: e05b2d141fef ("netdevsim: move netdev creation/destruction to dev probe")
Signed-off-by: Hrushiraj Gandhi <hrushi...@gmail.com>
---
drivers/net/netdevsim/dev.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/netdevsim/dev.c b/drivers/net/netdevsim/dev.c
index f00fc2f9ebde..77417dd0f752 100644
--- a/drivers/net/netdevsim/dev.c
+++ b/drivers/net/netdevsim/dev.c
@@ -1544,8 +1544,8 @@ static void __nsim_dev_port_del(struct
nsim_dev_port *nsim_dev_port)
list_del(&nsim_dev_port->list);
if (nsim_dev_port_is_vf(nsim_dev_port))
devl_rate_leaf_destroy(&nsim_dev_port->devlink_port);
- nsim_destroy(nsim_dev_port->ns);
nsim_dev_port_debugfs_exit(nsim_dev_port);
+ nsim_destroy(nsim_dev_port->ns);
if (nsim_dev_port_is_pf(nsim_dev_port))
devl_port_resources_unregister(devlink_port);
devl_port_unregister(devlink_port);
--
2.47.3

[syzbot]

Hello,

syzbot tried to test the proposed patch but the build/boot failed:

failed to apply patch:
error: corrupt patch at line 6



Tested on:

commit: e771677c Merge tag 'for-linus-iommufd' of git://git.ke..
git tree: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master

kernel config: https://syzkaller.appspot.com/x/.config?x=4e4d2284f2ffa41
dashboard link: https://syzkaller.appspot.com/bug?extid=6c25f4750230faf70be9
compiler:

patch: https://syzkaller.appspot.com/x/patch.diff?x=125c9986580000

[syzbot]

For archival purposes, forwarding an incoming command email to
linux-...@vger.kernel.org, syzkall...@googlegroups.com.

***

Subject: Re: [syzbot] netdevsim fix

Author: hrushi...@gmail.com

#syz test: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
master

[syzbot]

patch: https://syzkaller.appspot.com/x/patch.diff?x=102f6d56580000

[syzbot]

For archival purposes, forwarding an incoming command email to
linux-...@vger.kernel.org, syzkall...@googlegroups.com.

***

Subject: [PATCH] netdevsim: fix use-after-free in __nsim_dev_port_del

Author: hrushi...@gmail.com

#syz test: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master

debugfs files created under a port's ddir (ethtool/get_err,
ethtool/set_err, ring params, bpf_offloaded_id, udp_ports/inject_error,
etc.) store raw pointers directly into the netdevsim struct, which lives
in the net_device private data kmalloc slab.

In __nsim_dev_port_del(), nsim_destroy() was called before
nsim_dev_port_debugfs_exit(), meaning free_netdev() freed the
netdevsim slab while debugfs files still held live pointers into it.
A concurrent reader with the file already open could pass
debugfs_file_get(), then dereference the freed pointer in
debugfs_u32_get(), triggering a slab-use-after-free.

Fix by calling nsim_dev_port_debugfs_exit() first, so
debugfs_remove_recursive() tears down the entire port ddir subtree
(invalidating all stale data pointers) before free_netdev() releases
the backing memory.

Reported-by: syzbot+6c25f4...@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=6c25f4750230faf70be9
Fixes: e05b2d141fef ("netdevsim: move netdev creation/destruction to dev probe")
Signed-off-by: Hrushiraj Gandhi <hrushi...@gmail.com>
---
drivers/net/netdevsim/dev.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/netdevsim/dev.c b/drivers/net/netdevsim/dev.c
index f00fc2f9ebde..77417dd0f752 100644
--- a/drivers/net/netdevsim/dev.c
+++ b/drivers/net/netdevsim/dev.c
@@ -1544,8 +1544,8 @@ static void __nsim_dev_port_del(struct nsim_dev_port *nsim_dev_port)
list_del(&nsim_dev_port->list);
if (nsim_dev_port_is_vf(nsim_dev_port))
devl_rate_leaf_destroy(&nsim_dev_port->devlink_port);
- nsim_destroy(nsim_dev_port->ns);
nsim_dev_port_debugfs_exit(nsim_dev_port);
+ nsim_destroy(nsim_dev_port->ns);
if (nsim_dev_port_is_pf(nsim_dev_port))
devl_port_resources_unregister(devlink_port);
devl_port_unregister(devlink_port);

--
2.47.3

[syzbot]

Hello,

syzbot tried to test the proposed patch but the build/boot failed:

[ 90.657969][ T12] netdevsim netdevsim1 netdevsim2: set [1, 0] type 2 family 0 port 6081 - 0
[ 90.667366][ T12] netdevsim netdevsim1 netdevsim3: set [1, 0] type 2 family 0 port 6081 - 0
2026/06/18 09:21:13 executed programs: 0
[ 90.770444][ T50] Bluetooth: hci0: unexpected cc 0x0c03 length: 249 > 1
[ 90.781106][ T50] Bluetooth: hci0: unexpected cc 0x1003 length: 249 > 9
[ 90.792650][ T50] Bluetooth: hci0: unexpected cc 0x1001 length: 249 > 9
[ 90.802067][ T50] Bluetooth: hci0: unexpected cc 0x0c23 length: 249 > 4
[ 90.809763][ T50] Bluetooth: hci0: unexpected cc 0x0c38 length: 249 > 2
[ 91.161772][ T5757] bridge0: port 1(bridge_slave_0) entered blocking state
[ 91.169484][ T5757] bridge0: port 1(bridge_slave_0) entered disabled state
[ 91.177477][ T5757] bridge_slave_0: entered allmulticast mode
[ 91.185001][ T5757] bridge_slave_0: entered promiscuous mode
[ 91.193706][ T5757] bridge0: port 2(bridge_slave_1) entered blocking state
[ 91.201868][ T5757] bridge0: port 2(bridge_slave_1) entered disabled state
[ 91.209601][ T5757] bridge_slave_1: entered allmulticast mode
[ 91.217118][ T5757] bridge_slave_1: entered promiscuous mode
[ 91.238425][ T5757] bond0: (slave bond_slave_0): Enslaving as an active interface with an up link
[ 91.250819][ T5757] bond0: (slave bond_slave_1): Enslaving as an active interface with an up link
[ 91.276001][ T5757] team0: Port device team_slave_0 added
[ 91.283694][ T5757] team0: Port device team_slave_1 added
[ 91.302541][ T5757] batman_adv: batadv0: Adding interface: batadv_slave_0
[ 91.309669][ T5757] batman_adv: batadv0: The MTU of interface batadv_slave_0 is too small (1500) to handle the transport of batman-adv packets. Packets going over this interface will be fragmented on layer2 which could impact the performance. Setting the MTU to 1532 would solve the problem.
[ 91.336872][ T5757] batman_adv: batadv0: Not using interface batadv_slave_0 (retrying later): interface not active
[ 91.349188][ T5757] batman_adv: batadv0: Adding interface: batadv_slave_1
[ 91.357166][ T5757] batman_adv: batadv0: The MTU of interface batadv_slave_1 is too small (1500) to handle the transport of batman-adv packets. Packets going over this interface will be fragmented on layer2 which could impact the performance. Setting the MTU to 1532 would solve the problem.
[ 91.384019][ T5757] batman_adv: batadv0: Not using interface batadv_slave_1 (retrying later): interface not active
[ 91.420336][ T5757] hsr_slave_0: entered promiscuous mode
[ 91.427350][ T5757] hsr_slave_1: entered promiscuous mode
[ 91.433876][ T5757] debugfs: 'hsr0' already exists in 'hsr'
[ 91.439785][ T5757] Cannot create hsr debugfs directory
[ 91.532434][ T5757] netdevsim netdevsim0 netdevsim0: renamed from eth0
[ 91.541647][ T5757] 8021q: adding VLAN 0 to HW filter on device netdevsim0
[ 91.550200][ T5757] netdevsim netdevsim0 netdevsim1: renamed from eth1
[ 91.559463][ T5757] 8021q: adding VLAN 0 to HW filter on device netdevsim1
[ 91.569779][ T5757] netdevsim netdevsim0 netdevsim2: renamed from eth2
[ 91.579911][ T5757] 8021q: adding VLAN 0 to HW filter on device netdevsim2
[ 91.588284][ T5757] netdevsim netdevsim0 netdevsim3: renamed from eth3
[ 91.597422][ T5757] 8021q: adding VLAN 0 to HW filter on device netdevsim3
[ 91.618154][ T5757] bridge0: port 2(bridge_slave_1) entered blocking state
[ 91.625451][ T5757] bridge0: port 2(bridge_slave_1) entered forwarding state
[ 91.633183][ T5757] bridge0: port 1(bridge_slave_0) entered blocking state
[ 91.640456][ T5757] bridge0: port 1(bridge_slave_0) entered forwarding state
[ 91.651809][ T58] bridge0: port 1(bridge_slave_0) entered disabled state
[ 91.659706][ T58] bridge0: port 2(bridge_slave_1) entered disabled state
[ 91.705781][ T5757] 8021q: adding VLAN 0 to HW filter on device bond0
[ 91.724491][ T5757] 8021q: adding VLAN 0 to HW filter on device team0
[ 91.736217][ T58] bridge0: port 1(bridge_slave_0) entered blocking state
[ 91.743525][ T58] bridge0: port 1(bridge_slave_0) entered forwarding state
[ 91.758884][ T58] bridge0: port 2(bridge_slave_1) entered blocking state
[ 91.766069][ T58] bridge0: port 2(bridge_slave_1) entered forwarding state
[ 92.054180][ T5757] 8021q: adding VLAN 0 to HW filter on device batadv0
[ 92.094726][ T5757] veth0_vlan: entered promiscuous mode
[ 92.105045][ T5757] veth1_vlan: entered promiscuous mode
[ 92.132923][ T5757] veth0_macvtap: entered promiscuous mode
[ 92.143175][ T5757] veth1_macvtap: entered promiscuous mode
[ 92.158019][ T5757] batman_adv: batadv0: Interface activated: batadv_slave_0
[ 92.171480][ T5757] batman_adv: batadv0: Interface activated: batadv_slave_1
[ 92.185832][ T12] netdevsim netdevsim0 netdevsim0: set [1, 0] type 2 family 0 port 6081 - 0
[ 92.198478][ T12] netdevsim netdevsim0 netdevsim1: set [1, 0] type 2 family 0 port 6081 - 0
[ 92.207627][ T12] netdevsim netdevsim0 netdevsim2: set [1, 0] type 2 family 0 port 6081 - 0
[ 92.217684][ T12] netdevsim netdevsim0 netdevsim3: set [1, 0] type 2 family 0 port 6081 - 0
[ 92.260657][ T12] wlan0: Created IBSS using preconfigured BSSID 50:50:50:50:50:50
[ 92.271752][ T12] wlan0: Creating new IBSS network, BSSID 50:50:50:50:50:50
[ 92.292048][ T12] wlan1: Created IBSS using preconfigured BSSID 50:50:50:50:50:50
[ 92.300386][ T12] wlan1: Creating new IBSS network, BSSID 50:50:50:50:50:50
[ 92.434520][ T36] ==================================================================
[ 92.443019][ T36] BUG: KASAN: null-ptr-deref in down_write_nested+0xb4/0x200
[ 92.451123][ T36] Write of size 8 at addr 00000000000000d8 by task kworker/u8:2/36
[ 92.459336][ T36]
[ 92.461731][ T36] CPU: 0 UID: 0 PID: 36 Comm: kworker/u8:2 Not tainted syzkaller #0 PREEMPT(full)
[ 92.461766][ T36] Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/09/2026
[ 92.461789][ T36] Workqueue: netns cleanup_net
[ 92.461838][ T36] Call Trace:
[ 92.461849][ T36] <TASK>
[ 92.461860][ T36] dump_stack_lvl+0x100/0x190
[ 92.461910][ T36] kasan_report+0xdf/0x1c0
[ 92.461955][ T36] ? down_write_nested+0xb4/0x200
[ 92.462001][ T36] kasan_check_range+0x10f/0x1e0
[ 92.462031][ T36] down_write_nested+0xb4/0x200
[ 92.462071][ T36] ? __simple_recursive_removal+0x7f/0x5c0
[ 92.462115][ T36] ? __pfx_down_write_nested+0x10/0x10
[ 92.462157][ T36] ? lock_release+0x24d/0x310
[ 92.462193][ T36] ? do_raw_spin_unlock+0x145/0x1e0
[ 92.462235][ T36] __simple_recursive_removal+0x7f/0x5c0
[ 92.462279][ T36] ? __pfx_remove_one+0x10/0x10
[ 92.462319][ T36] ? mntput+0x70/0xa0
[ 92.462360][ T36] debugfs_remove+0x5d/0x80
[ 92.462397][ T36] nsim_destroy+0x6a/0x970
[ 92.462434][ T36] ? mntput_no_expire+0x202/0x220
[ 92.462477][ T36] __nsim_dev_port_del+0x1c2/0x2b0
[ 92.462520][ T36] nsim_dev_reload_destroy+0x10a/0x4a0
[ 92.462545][ T36] ? xa_get_mark+0x19c/0x750
[ 92.462570][ T36] ? lock_release+0x24d/0x310
[ 92.462606][ T36] nsim_dev_reload_down+0x66/0xd0
[ 92.462631][ T36] devlink_reload+0x1ae/0x7a0
[ 92.462659][ T36] ? __pfx_devlink_reload+0x10/0x10
[ 92.462691][ T36] devlink_pernet_pre_exit+0x247/0x370
[ 92.462730][ T36] ? iput.part.0+0x1a0/0x1050
[ 92.462762][ T36] ? __pfx_devlink_pernet_pre_exit+0x10/0x10
[ 92.462846][ T36] ? iput+0x3a/0x40
[ 92.462875][ T36] ? sock_release+0x14d/0x1c0
[ 92.462910][ T36] ? __pfx_devlink_pernet_pre_exit+0x10/0x10
[ 92.462950][ T36] ops_undo_list+0x187/0xab0
[ 92.462981][ T36] ? __pfx_ops_undo_list+0x10/0x10
[ 92.463009][ T36] ? rcu_is_watching+0x12/0xc0
[ 92.463073][ T36] ? cleanup_net+0x332/0x920
[ 92.463102][ T36] ? lock_release+0x24d/0x310
[ 92.463136][ T36] ? idr_destroy+0x62/0x2e0
[ 92.463167][ T36] cleanup_net+0x499/0x920
[ 92.463199][ T36] ? __pfx_cleanup_net+0x10/0x10
[ 92.463228][ T36] ? rcu_is_watching+0x12/0xc0
[ 92.463253][ T36] ? lock_acquire+0x301/0x370
[ 92.463288][ T36] ? rcu_is_watching+0x12/0xc0
[ 92.463368][ T36] process_one_work+0xa23/0x1940
[ 92.463500][ T36] ? __pfx_process_one_work+0x10/0x10
[ 92.463540][ T36] ? __pfx_cleanup_net+0x10/0x10
[ 92.463574][ T36] worker_thread+0x5ef/0xe50
[ 92.463618][ T36] ? __pfx_worker_thread+0x10/0x10
[ 92.463657][ T36] ? kthread+0x13a/0x450
[ 92.463685][ T36] ? __pfx_worker_thread+0x10/0x10
[ 92.463717][ T36] kthread+0x370/0x450
[ 92.463747][ T36] ? __pfx_kthread+0x10/0x10
[ 92.463779][ T36] ret_from_fork+0x72b/0xd50
[ 92.463809][ T36] ? __pfx_ret_from_fork+0x10/0x10
[ 92.463840][ T36] ? __switch_to+0x800/0x10f0
[ 92.463880][ T36] ? __switch_to_asm+0x39/0x70
[ 92.463915][ T36] ? __pfx_kthread+0x10/0x10
[ 92.463946][ T36] ret_from_fork_asm+0x1a/0x30
[ 92.463991][ T36] </TASK>
[ 92.464012][ T36] ==================================================================
[ 92.772868][ T36] Kernel panic - not syncing: KASAN: panic_on_warn set ...
[ 92.780396][ T36] CPU: 0 UID: 0 PID: 36 Comm: kworker/u8:2 Not tainted syzkaller #0 PREEMPT(full)
[ 92.790031][ T36] Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/09/2026
[ 92.800267][ T36] Workqueue: netns cleanup_net
[ 92.805109][ T36] Call Trace:
[ 92.808440][ T36] <TASK>
[ 92.811417][ T36] dump_stack_lvl+0x100/0x190
[ 92.816229][ T36] vpanic+0x552/0x970
[ 92.820274][ T36] ? __pfx_vpanic+0x10/0x10
[ 92.824842][ T36] ? rcu_is_watching+0x12/0xc0
[ 92.829669][ T36] ? trace_irq_enable.constprop.0+0x122/0x160
[ 92.835913][ T36] ? down_write_nested+0xb4/0x200
[ 92.841014][ T36] panic+0xd1/0xe0
[ 92.844800][ T36] ? __pfx_panic+0x10/0x10
[ 92.849264][ T36] ? check_panic_on_warn+0x1f/0x90
[ 92.854429][ T36] check_panic_on_warn.cold+0x19/0x34
[ 92.859848][ T36] end_report.part.0+0x3a/0x90
[ 92.864760][ T36] kasan_report.cold+0xe/0x18
[ 92.869508][ T36] ? down_write_nested+0xb4/0x200
[ 92.874628][ T36] kasan_check_range+0x10f/0x1e0
[ 92.879623][ T36] down_write_nested+0xb4/0x200
[ 92.884625][ T36] ? __simple_recursive_removal+0x7f/0x5c0
[ 92.890505][ T36] ? __pfx_down_write_nested+0x10/0x10
[ 92.896033][ T36] ? lock_release+0x24d/0x310
[ 92.900788][ T36] ? do_raw_spin_unlock+0x145/0x1e0
[ 92.906176][ T36] __simple_recursive_removal+0x7f/0x5c0
[ 92.911965][ T36] ? __pfx_remove_one+0x10/0x10
[ 92.916896][ T36] ? mntput+0x70/0xa0
[ 92.920939][ T36] debugfs_remove+0x5d/0x80
[ 92.925495][ T36] nsim_destroy+0x6a/0x970
[ 92.929962][ T36] ? mntput_no_expire+0x202/0x220
[ 92.935044][ T36] __nsim_dev_port_del+0x1c2/0x2b0
[ 92.940226][ T36] nsim_dev_reload_destroy+0x10a/0x4a0
[ 92.945728][ T36] ? xa_get_mark+0x19c/0x750
[ 92.950360][ T36] ? lock_release+0x24d/0x310
[ 92.955078][ T36] nsim_dev_reload_down+0x66/0xd0
[ 92.960223][ T36] devlink_reload+0x1ae/0x7a0
[ 92.964932][ T36] ? __pfx_devlink_reload+0x10/0x10
[ 92.970170][ T36] devlink_pernet_pre_exit+0x247/0x370
[ 92.975679][ T36] ? iput.part.0+0x1a0/0x1050
[ 92.980407][ T36] ? __pfx_devlink_pernet_pre_exit+0x10/0x10
[ 92.986708][ T36] ? iput+0x3a/0x40
[ 92.990650][ T36] ? sock_release+0x14d/0x1c0
[ 92.995410][ T36] ? __pfx_devlink_pernet_pre_exit+0x10/0x10
[ 93.001440][ T36] ops_undo_list+0x187/0xab0
[ 93.006072][ T36] ? __pfx_ops_undo_list+0x10/0x10
[ 93.011244][ T36] ? rcu_is_watching+0x12/0xc0
[ 93.016081][ T36] ? cleanup_net+0x332/0x920
[ 93.020714][ T36] ? lock_release+0x24d/0x310
[ 93.025452][ T36] ? idr_destroy+0x62/0x2e0
[ 93.030007][ T36] cleanup_net+0x499/0x920
[ 93.034648][ T36] ? __pfx_cleanup_net+0x10/0x10
[ 93.039645][ T36] ? rcu_is_watching+0x12/0xc0
[ 93.044549][ T36] ? lock_acquire+0x301/0x370
[ 93.049372][ T36] ? rcu_is_watching+0x12/0xc0
[ 93.054634][ T36] process_one_work+0xa23/0x1940
[ 93.059802][ T36] ? __pfx_process_one_work+0x10/0x10
[ 93.065227][ T36] ? __pfx_cleanup_net+0x10/0x10
[ 93.070244][ T36] worker_thread+0x5ef/0xe50
[ 93.074965][ T36] ? __pfx_worker_thread+0x10/0x10
[ 93.080203][ T36] ? kthread+0x13a/0x450
[ 93.084484][ T36] ? __pfx_worker_thread+0x10/0x10
[ 93.089720][ T36] kthread+0x370/0x450
[ 93.093825][ T36] ? __pfx_kthread+0x10/0x10
[ 93.098662][ T36] ret_from_fork+0x72b/0xd50
[ 93.103320][ T36] ? __pfx_ret_from_fork+0x10/0x10
[ 93.108847][ T36] ? __switch_to+0x800/0x10f0
[ 93.113675][ T36] ? __switch_to_asm+0x39/0x70
[ 93.118505][ T36] ? __pfx_kthread+0x10/0x10
[ 93.123162][ T36] ret_from_fork_asm+0x1a/0x30
[ 93.127988][ T36] </TASK>
[ 93.131657][ T36] Kernel Offset: disabled
[ 93.136033][ T36] Rebooting in 86400 seconds..


syzkaller build log:
go env (err=<nil>)
AR='ar'
CC='gcc'
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_ENABLED='1'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
CXX='g++'
GCCGO='gccgo'
GO111MODULE='auto'
GOAMD64='v1'
GOARCH='amd64'
GOAUTH='netrc'
GOBIN=''
GOCACHE='/syzkaller/.cache/go-build'
GOCACHEPROG=''
GODEBUG=''
GOENV='/syzkaller/.config/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFIPS140='off'
GOFLAGS=''
GOGCCFLAGS='-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build3995708942=/tmp/go-build -gno-record-gcc-switches'
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMOD='/syzkaller/jobs/linux/gopath/src/github.com/google/syzkaller/go.mod'
GOMODCACHE='/syzkaller/jobs/linux/gopath/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='linux'
GOPATH='/syzkaller/jobs/linux/gopath'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/usr/local/go'
GOSUMDB='sum.golang.org'
GOTELEMETRY='local'
GOTELEMETRYDIR='/syzkaller/.config/go/telemetry'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/usr/local/go/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='go1.26.0'
GOWORK=''
PKG_CONFIG='pkg-config'

git status (err=<nil>)
HEAD detached at 4f06757f6
nothing to commit, working tree clean


tput: No value for $TERM and no -T specified
tput: No value for $TERM and no -T specified
Makefile:31: run command via tools/syz-env for best compatibility, see:
Makefile:32: https://github.com/google/syzkaller/blob/master/docs/contributing.md#using-syz-env
go list -f '{{.Stale}}' -ldflags="-s -w -X github.com/google/syzkaller/prog.GitRevision=4f06757f683a802254a4c8edbfe67ff801904c5e -X github.com/google/syzkaller/prog.gitRevisionDate=20260612-145234" ./sys/syz-sysgen | grep -q false || go install -ldflags="-s -w -X github.com/google/syzkaller/prog.GitRevision=4f06757f683a802254a4c8edbfe67ff801904c5e -X github.com/google/syzkaller/prog.gitRevisionDate=20260612-145234" ./sys/syz-sysgen
make .descriptions
tput: No value for $TERM and no -T specified
tput: No value for $TERM and no -T specified
Makefile:31: run command via tools/syz-env for best compatibility, see:
Makefile:32: https://github.com/google/syzkaller/blob/master/docs/contributing.md#using-syz-env
bin/syz-sysgen
touch .descriptions
GOOS=linux GOARCH=amd64 go build -ldflags="-s -w -X github.com/google/syzkaller/prog.GitRevision=4f06757f683a802254a4c8edbfe67ff801904c5e -X github.com/google/syzkaller/prog.gitRevisionDate=20260612-145234" -o ./bin/linux_amd64/syz-execprog github.com/google/syzkaller/tools/syz-execprog
mkdir -p ./bin/linux_amd64
g++ -o ./bin/linux_amd64/syz-executor executor/executor.cc \
-m64 -O2 -pthread -Wall -Werror -Wparentheses -Wunused-const-variable -Wframe-larger-than=16384 -Wno-stringop-overflow -Wno-array-bounds -Wno-format-overflow -Wno-unused-but-set-variable -Wno-unused-command-line-argument -static-pie -std=c++17 -I. -Iexecutor/_include -DGOOS_linux=1 -DGOARCH_amd64=1 \
-DHOSTGOOS_linux=1 -DGIT_REVISION=\"4f06757f683a802254a4c8edbfe67ff801904c5e\"
/usr/bin/ld: /tmp/ccxfdxNm.o: in function `Connection::Connect(char const*, char const*)':
executor.cc:(.text._ZN10Connection7ConnectEPKcS1_[_ZN10Connection7ConnectEPKcS1_]+0x386): warning: Using 'gethostbyname' in statically linked applications requires at runtime the shared libraries from the glibc version used for linking
./tools/check-syzos.sh 2>/dev/null


Error text is too large and was truncated, full error text is at:
https://syzkaller.appspot.com/x/error.txt?x=160368ea580000

Tested on:

commit: e771677c Merge tag 'for-linus-iommufd' of git://git.ke..
git tree: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master

kernel config: https://syzkaller.appspot.com/x/.config?x=ea349aa18d06ec5e
dashboard link: https://syzkaller.appspot.com/bug?extid=6c25f4750230faf70be9

compiler: gcc (Debian 14.2.0-19) 14.2.0, GNU ld (GNU Binutils for Debian) 2.44

patch: https://syzkaller.appspot.com/x/patch.diff?x=177a74fe580000

[syzbot]

For archival purposes, forwarding an incoming command email to
linux-...@vger.kernel.org, syzkall...@googlegroups.com.

***

Subject: [PATCH] netdevsim: fix use-after-free in __nsim_dev_port_del
Author: hrushi...@gmail.com

#syz test: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master

debugfs files created under a port's ddir (ethtool/get_err,
ethtool/set_err, ring params, bpf_offloaded_id, udp_ports/inject_error,
etc.) store raw pointers directly into the netdevsim struct, which lives
in the net_device private data kmalloc slab.

If these files outlive the netdevsim struct, a concurrent reader can
trigger a slab-use-after-free by passing debugfs_file_get() (which only
checks dentry lifetime) and then dereferencing the freed data pointer
in debugfs_u32_get().

In __nsim_dev_port_del(), nsim_destroy() is called before
nsim_dev_port_debugfs_exit(). However, nsim_destroy() calls free_netdev()
at its end, while nsim_dev_port_debugfs_exit() removes the port's
debugfs directory. This means the slab is freed before the debugfs
files are removed.

Fix by calling debugfs_remove_recursive(ns->nsim_dev_port->ddir) in
nsim_destroy() right before free_netdev(). This ensures all per-port
debugfs files are destroyed synchronously before the backing memory is
freed. The subsequent call to nsim_dev_port_debugfs_exit() in
__nsim_dev_port_del() becomes a harmless no-op.

Reported-by: syzbot+6c25f4...@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=6c25f4750230faf70be9
Fixes: e05b2d141fef ("netdevsim: move netdev creation/destruction to dev probe")
Signed-off-by: Hrushiraj Gandhi <hrushi...@gmail.com>
---

drivers/net/netdevsim/netdev.c | 16 ++++++++++++++++
1 file changed, 16 insertions(+)

diff --git a/drivers/net/netdevsim/netdev.c b/drivers/net/netdevsim/netdev.c
index a750768912b5..511139d65d69 100644
--- a/drivers/net/netdevsim/netdev.c
+++ b/drivers/net/netdevsim/netdev.c
@@ -1213,6 +1213,22 @@ void nsim_destroy(struct netdevsim *ns)
ns->page = NULL;
}

+ /*
+ * Remove all per-port debugfs files before free_netdev() releases
+ * the netdevsim struct. Files like ethtool/get_err, bpf_offloaded_id,
+ * etc. store raw pointers into ns (netdev_priv data inside the
+ * net_device slab). A concurrent reader with the file open could pass
+ * debugfs_file_get() and then dereference freed memory if these files
+ * outlive free_netdev(). debugfs_remove_recursive() on the parent ddir
+ * handles this atomically; the subsequent nsim_dev_port_debugfs_exit()
+ * call in __nsim_dev_port_del() becomes a harmless no-op.
+ *
+ * The three explicitly tracked dentries (vlan_dfs, qr_dfs, pp_dfs)
+ * are children of ddir and are covered by the recursive removal below,
+ * so their individual debugfs_remove() calls become no-ops too.
+ */
+ debugfs_remove_recursive(ns->nsim_dev_port->ddir);
+
free_netdev(dev);
}

--
2.47.3

[syzbot]

Hello,

syzbot tried to test the proposed patch but the build/boot failed:

failed to copy syz-execprog to VM: scp failed: timedout after 1m0s ["scp" "-P" "22" "-F" "/dev/null" "-o" "UserKnownHostsFile=/dev/null" "-o" "IdentitiesOnly=yes" "-o" "BatchMode=yes" "-o" "StrictHostKeyChecking=no" "-o" "ConnectTimeout=10" "-v" "-O" "/syzkaller/jobs/linux/gopath/src/github.com/google/syzkaller/bin/linux_amd64/syz-execprog" "ro...@10.128.1.113:ci-qemu-gce-upstream-auto-test-job/syz-execprog"]

GOGCCFLAGS='-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build368877244=/tmp/go-build -gno-record-gcc-switches'

/usr/bin/ld: /tmp/ccyi0Yf5.o: in function `Connection::Connect(char const*, char const*)':

executor.cc:(.text._ZN10Connection7ConnectEPKcS1_[_ZN10Connection7ConnectEPKcS1_]+0x386): warning: Using 'gethostbyname' in statically linked applications requires at runtime the shared libraries from the glibc version used for linking
./tools/check-syzos.sh 2>/dev/null

Tested on:

commit: e771677c Merge tag 'for-linus-iommufd' of git://git.ke..
git tree: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
kernel config: https://syzkaller.appspot.com/x/.config?x=ea349aa18d06ec5e
dashboard link: https://syzkaller.appspot.com/bug?extid=6c25f4750230faf70be9
compiler: gcc (Debian 14.2.0-19) 14.2.0, GNU ld (GNU Binutils for Debian) 2.44

patch: https://syzkaller.appspot.com/x/patch.diff?x=17b693b6580000

[syzbot]

drivers/net/netdevsim/netdev.c | 17 +++++++++++++++++
1 file changed, 17 insertions(+)

diff --git a/drivers/net/netdevsim/netdev.c b/drivers/net/netdevsim/netdev.c
index a750768912b5..e2d714673c2d 100644
--- a/drivers/net/netdevsim/netdev.c
+++ b/drivers/net/netdevsim/netdev.c
@@ -1213,6 +1213,23 @@ void nsim_destroy(struct netdevsim *ns)

ns->page = NULL;
}

+ /*
+ * Remove all per-port debugfs files before free_netdev() releases
+ * the netdevsim struct. Files like ethtool/get_err, bpf_offloaded_id,
+ * etc. store raw pointers into ns (netdev_priv data inside the
+ * net_device slab). A concurrent reader with the file open could pass
+ * debugfs_file_get() and then dereference freed memory if these files
+ * outlive free_netdev(). debugfs_remove_recursive() on the parent ddir
+ * handles this atomically; the subsequent nsim_dev_port_debugfs_exit()
+ * call in __nsim_dev_port_del() becomes a harmless no-op.
+ *
+ * The three explicitly tracked dentries (vlan_dfs, qr_dfs, pp_dfs)
+ * are children of ddir and are covered by the recursive removal below,
+ * so their individual debugfs_remove() calls become no-ops too.
+ */
+ debugfs_remove_recursive(ns->nsim_dev_port->ddir);

+ ns->nsim_dev_port->ddir = NULL;
+
free_netdev(dev);
}

--
2.47.3

[syzbot]

Hello,

syzbot has tested the proposed patch and the reproducer did not trigger any issue:

Reported-by: syzbot+6c25f4...@syzkaller.appspotmail.com
Tested-by: syzbot+6c25f4...@syzkaller.appspotmail.com

Tested on:

commit: e771677c Merge tag 'for-linus-iommufd' of git://git.ke..
git tree: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master

console output: https://syzkaller.appspot.com/x/log.txt?x=14ce93b6580000

kernel config: https://syzkaller.appspot.com/x/.config?x=ea349aa18d06ec5e
dashboard link: https://syzkaller.appspot.com/bug?extid=6c25f4750230faf70be9
compiler: gcc (Debian 14.2.0-19) 14.2.0, GNU ld (GNU Binutils for Debian) 2.44

patch: https://syzkaller.appspot.com/x/patch.diff?x=110e93b6580000

Note: testing is done by a robot and is best-effort only.

[syzbot]

For archival purposes, forwarding an incoming command email to
linux-...@vger.kernel.org, syzkall...@googlegroups.com.

***

Subject: [PATCH v3] netdevsim: fix use-after-free of ethtool debugfs data
Author: hrushi...@gmail.com

From: Hrushiraj Gandhi <hrushi...@gmail.com>

#syz test: git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net.git main

debugfs files created by nsim_ethtool_init() store raw pointers

directly into the netdevsim struct, which lives in the net_device
private data kmalloc slab.

If the ethtool subdirectory outlives the netdevsim struct, a concurrent

reader can trigger a slab-use-after-free by passing debugfs_file_get()
(which only checks dentry lifetime) and then dereferencing the freed

data pointer in e.g. debugfs_u32_get().

In __nsim_dev_port_del(), nsim_destroy() is called before

nsim_dev_port_debugfs_exit(). nsim_destroy() ends with free_netdev(),

while nsim_dev_port_debugfs_exit() removes the port's debugfs directory

afterwards. This means the slab is freed before the ethtool debugfs
files that point into it are removed.

The same window exists on nsim_create()'s error path: nsim_ethtool_init()
creates debugfs files under ddir with pointers into ns before
nsim_init_netdevsim()/nsim_init_netdevsim_vf(), which can fail. The
err_free_netdev label then calls free_netdev() while those ethtool
debugfs entries are still live.

All other features that create per-port debugfs files (pp_hold,
queue_reset, vlan) already remove their own entries explicitly in
nsim_destroy() before free_netdev(). ethtool is the only one that does
not. Fix this by saving the ethtool dentry in struct nsim_ethtool and
calling debugfs_remove_recursive() on it before free_netdev() in both
nsim_destroy() and the nsim_create() error path. The port ddir teardown
is left to nsim_dev_port_debugfs_exit() as before.

Reported-by: syzbot+6c25f4...@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=6c25f4750230faf70be9
Fixes: e05b2d141fef ("netdevsim: move netdev creation/destruction to dev probe")
Signed-off-by: Hrushiraj Gandhi <hrushi...@gmail.com>
---

v3:
- Instead of removing the entire port ddir before free_netdev(), save
and remove only the ethtool dentry. All other features (pp_hold,
queue_reset, vlan) already clean up after themselves; ethtool is the
only exception. This aligns ethtool with how the other features
behave, as suggested by Jakub Kicinski.
v2:
- Also fix the same use-after-free window on the error path of
nsim_create() as suggested by Simon Horman.
- Shorten the code comment in nsim_destroy() to be more concise.
---
drivers/net/netdevsim/ethtool.c | 1 +
drivers/net/netdevsim/netdev.c | 9 +++++++++
drivers/net/netdevsim/netdevsim.h | 1 +
3 files changed, 11 insertions(+)

diff --git a/drivers/net/netdevsim/ethtool.c b/drivers/net/netdevsim/ethtool.c
index 9350ba48eb81..a465f3220a7c 100644
--- a/drivers/net/netdevsim/ethtool.c
+++ b/drivers/net/netdevsim/ethtool.c
@@ -252,6 +252,7 @@ void nsim_ethtool_init(struct netdevsim *ns)
ns->ethtool.channels = ns->nsim_bus_dev->num_queues;

ethtool = debugfs_create_dir("ethtool", ns->nsim_dev_port->ddir);
+ ns->ethtool.ddir = ethtool;

debugfs_create_u32("get_err", 0600, ethtool, &ns->ethtool.get_err);
debugfs_create_u32("set_err", 0600, ethtool, &ns->ethtool.set_err);
diff --git a/drivers/net/netdevsim/netdev.c b/drivers/net/netdevsim/netdev.c
index 27e5f109f933..09d86a591fac 100644
--- a/drivers/net/netdevsim/netdev.c
+++ b/drivers/net/netdevsim/netdev.c
@@ -1165,6 +1165,7 @@ struct netdevsim *nsim_create(struct nsim_dev *nsim_dev,
return ns;

err_free_netdev:
+ debugfs_remove_recursive(ns->ethtool.ddir);
free_netdev(dev);
return ERR_PTR(err);
}
@@ -1214,6 +1215,14 @@ void nsim_destroy(struct netdevsim *ns)

ns->page = NULL;
}

+ /*

+ * Remove the ethtool debugfs directory before free_netdev() releases
+ * the netdevsim struct to prevent use-after-free in concurrent readers.
+ * Other per-port debugfs files are already removed above, and the port
+ * ddir itself is cleaned up by nsim_dev_port_debugfs_exit().
+ */
+ debugfs_remove_recursive(ns->ethtool.ddir);
+
free_netdev(dev);
}

diff --git a/drivers/net/netdevsim/netdevsim.h b/drivers/net/netdevsim/netdevsim.h
index 4c9cc96dcec3..c08a01af31e9 100644
--- a/drivers/net/netdevsim/netdevsim.h
+++ b/drivers/net/netdevsim/netdevsim.h
@@ -95,6 +95,7 @@ struct nsim_ethtool {
struct ethtool_coalesce coalesce;
struct ethtool_ringparam ring;
struct ethtool_fecparam fec;
+ struct dentry *ddir;
};

struct nsim_rq {
--
2.47.3

[syzbot]

Hello,

syzbot has tested the proposed patch and the reproducer did not trigger any issue:

Reported-by: syzbot+6c25f4...@syzkaller.appspotmail.com
Tested-by: syzbot+6c25f4...@syzkaller.appspotmail.com

Tested on:

commit: 6c00e85c Merge branch 'net-sched-finish-the-qdisc_dequ..
git tree: git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net.git main
console output: https://syzkaller.appspot.com/x/log.txt?x=13cff1fe580000
kernel config: https://syzkaller.appspot.com/x/.config?x=eb3cf873145f98c4

dashboard link: https://syzkaller.appspot.com/bug?extid=6c25f4750230faf70be9
compiler: gcc (Debian 14.2.0-19) 14.2.0, GNU ld (GNU Binutils for Debian) 2.44

patch: https://syzkaller.appspot.com/x/patch.diff?x=13ec661c580000