[syzbot] [fs?] KASAN: slab-use-after-free Read in current_time

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: eb3f4b7426cf Merge tag 'nfsd-7.1-2' of git://git.kernel.or..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=155429ec580000
kernel config: https://syzkaller.appspot.com/x/.config?x=8118209836970b54
dashboard link: https://syzkaller.appspot.com/bug?extid=36e50496c8ac4bcde3f9
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=109bb673980000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=1299f856580000

Downloadable assets:
disk image (non-bootable): https://storage.googleapis.com/syzbot-assets/d900f083ada3/non_bootable_disk-eb3f4b74.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/1406171f5cf6/vmlinux-eb3f4b74.xz
kernel image: https://storage.googleapis.com/syzbot-assets/19b8bb9b727a/bzImage-eb3f4b74.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+36e504...@syzkaller.appspotmail.com

==================================================================
BUG: KASAN: slab-use-after-free in is_mgtime include/linux/fs.h:2313 [inline]
BUG: KASAN: slab-use-after-free in current_time+0x354/0x3b0 fs/inode.c:2378
Read of size 2 at addr ffff8880407e4282 by task syz.0.661/9127

CPU: 3 UID: 0 PID: 9127 Comm: syz.0.661 Not tainted syzkaller #0 PREEMPT(full)
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
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
is_mgtime include/linux/fs.h:2313 [inline]
current_time+0x354/0x3b0 fs/inode.c:2378
atime_needs_update+0x33f/0x6b0 fs/inode.c:2236
pick_link+0x3b0/0x13c0 fs/namei.c:2040
step_into_slowpath+0x9ba/0xf90 fs/namei.c:2123
step_into fs/namei.c:2148 [inline]
walk_component fs/namei.c:2284 [inline]
link_path_walk+0xf28/0x1cc0 fs/namei.c:2652
path_parentat fs/namei.c:2856 [inline]
__filename_parentat+0x213/0x740 fs/namei.c:2880
filename_parentat fs/namei.c:2898 [inline]
filename_unlinkat+0xf7/0x730 fs/namei.c:5537
__do_sys_unlinkat fs/namei.c:5597 [inline]
__se_sys_unlinkat fs/namei.c:5589 [inline]
__x64_sys_unlinkat+0xc0/0x130 fs/namei.c:5589
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x115/0x870 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f2baad9ce59
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:00007f2babd0b028 EFLAGS: 00000246 ORIG_RAX: 0000000000000107
RAX: ffffffffffffffda RBX: 00007f2bab015fa0 RCX: 00007f2baad9ce59
RDX: 0000000000000000 RSI: 00002000000001c0 RDI: 0000000000000004
RBP: 00007f2baae32d6f R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f2bab016038 R14: 00007f2bab015fa0 R15: 00007ffdc75513d8
</TASK>

Allocated by task 9127:
kasan_save_stack+0x30/0x50 mm/kasan/common.c:57
kasan_save_track+0x14/0x30 mm/kasan/common.c:78
unpoison_slab_object mm/kasan/common.c:340 [inline]
__kasan_slab_alloc+0x89/0x90 mm/kasan/common.c:366
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]
kmem_cache_alloc_lru_noprof+0x246/0x6e0 mm/slub.c:4918
alloc_inode+0x183/0x250 fs/inode.c:347
new_inode+0x22/0x1c0 fs/inode.c:1179
bpf_get_inode kernel/bpf/inode.c:117 [inline]
bpf_get_inode kernel/bpf/inode.c:102 [inline]
bpf_symlink+0x69/0x240 kernel/bpf/inode.c:391
vfs_symlink fs/namei.c:5643 [inline]
vfs_symlink+0x178/0x4d0 fs/namei.c:5622
filename_symlinkat+0x2a6/0x560 fs/namei.c:5668
__do_sys_symlinkat fs/namei.c:5688 [inline]
__se_sys_symlinkat fs/namei.c:5683 [inline]
__x64_sys_symlinkat+0x9c/0xe0 fs/namei.c:5683
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x115/0x870 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

Freed by task 9128:
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]
kmem_cache_free+0x127/0x6c0 mm/slub.c:6378
destroy_inode+0xcb/0x1c0 fs/inode.c:394
evict+0x599/0xad0 fs/inode.c:865
iput_final fs/inode.c:1960 [inline]
iput.part.0+0x605/0xf50 fs/inode.c:2009
iput+0x35/0x40 fs/inode.c:1975
filename_unlinkat+0x466/0x730 fs/namei.c:5572
__do_sys_unlinkat fs/namei.c:5597 [inline]
__se_sys_unlinkat fs/namei.c:5589 [inline]
__x64_sys_unlinkat+0xc0/0x130 fs/namei.c:5589
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x115/0x870 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

The buggy address belongs to the object at ffff8880407e4280
which belongs to the cache inode_cache of size 1088
The buggy address is located 2 bytes inside of
freed 1088-byte region [ffff8880407e4280, ffff8880407e46c0)

The buggy address belongs to the physical page:
page: refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x407e0
head: order:3 mapcount:0 entire_mapcount:0 nr_pages_mapped:0 pincount:0
memcg:ffff8880407e7b81
flags: 0xfff00000000040(head|node=0|zone=1|lastcpupid=0x7ff)
page_type: f5(slab)
raw: 00fff00000000040 ffff88801d6bc000 dead000000000100 dead000000000122
raw: 0000000000000000 00000008001a001a 00000000f5000000 ffff8880407e7b81
head: 00fff00000000040 ffff88801d6bc000 dead000000000100 dead000000000122
head: 0000000000000000 00000008001a001a 00000000f5000000 ffff8880407e7b81
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 Reclaimable, gfp_mask 0xd20d0(__GFP_RECLAIMABLE|__GFP_IO|__GFP_FS|__GFP_NOWARN|__GFP_NORETRY|__GFP_COMP|__GFP_NOMEMALLOC), pid 5773, tgid 5773 (syz-executor), ts 108002716269, free_ts 0
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]
kmem_cache_alloc_lru_noprof+0x485/0x6e0 mm/slub.c:4918
alloc_inode+0x183/0x250 fs/inode.c:347
iget_locked+0x1d9/0x6d0 fs/inode.c:1474
kernfs_get_inode+0x46/0x470 fs/kernfs/inode.c:252
kernfs_iop_lookup+0x1a7/0x2d0 fs/kernfs/dir.c:1274
lookup_open.isra.0+0x631/0x11b0 fs/namei.c:4484
open_last_lookups fs/namei.c:4611 [inline]
path_openat+0xa98/0x31a0 fs/namei.c:4855
do_file_open+0x20e/0x430 fs/namei.c:4887
do_sys_openat2+0x10d/0x1e0 fs/open.c:1364
do_sys_open fs/open.c:1370 [inline]
__do_sys_openat fs/open.c:1386 [inline]
__se_sys_openat fs/open.c:1381 [inline]
__x64_sys_openat+0x12d/0x210 fs/open.c:1381
page_owner free stack trace missing

Memory state around the buggy address:
ffff8880407e4180: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
ffff8880407e4200: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
>ffff8880407e4280: fa fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
^
ffff8880407e4300: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
ffff8880407e4380: 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 you want syzbot to run the reproducer, reply with:
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[syzbot]

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

***

Subject: [PATCH] fs/namei: fix UAF in pick_link() by unlazying before atime check
Author: karti...@gmail.com

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


In pick_link(), atime_needs_update() is called while the path walk
may still be in RCU mode (LOOKUP_RCU set), meaning no reference is
held on the inode. A concurrent iput() from another task can free
the inode while atime_needs_update() reads inode->i_opflags (offset
+2) inside current_time() -> is_mgtime(), causing a use-after-free.

KASAN reports:

BUG: KASAN: slab-use-after-free in is_mgtime include/linux/fs.h:2313

Read of size 2 at addr ffff8880407e4282 (offset +2 = i_opflags)

The race:
Task A (pick_link) Task B (unlinkat)
------------------ -----------------
atime_needs_update(inode) iput(inode)
current_time(inode) evict()
is_mgtime(inode) destroy_inode()
inode->i_opflags UAF inode FREED

Fix: call try_to_unlazy() before atime_needs_update() so that
a proper inode reference is held before any inode fields are read.
If try_to_unlazy() detects the inode is already gone, it returns
false and we exit safely with -ECHILD without touching freed memory.

Reported-by: syzbot+36e504...@syzkaller.appspotmail.com
Link: https://syzkaller.appspot.com/bug?extid=36e50496c8ac4bcde3f9
Signed-off-by: Deepanshu Kartikey <karti...@gmail.com>"
---
fs/namei.c | 8 ++++----
1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/fs/namei.c b/fs/namei.c
index c7fac83c9a85..eac0ec35be37 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -2037,11 +2037,11 @@ static noinline const char *pick_link(struct nameidata *nd, struct path *link,
unlikely(link->mnt->mnt_flags & MNT_NOSYMFOLLOW))
return ERR_PTR(-ELOOP);

+ if (nd->flags & LOOKUP_RCU) {
+ if (!try_to_unlazy(nd))
+ return ERR_PTR(-ECHILD);
+ }
if (unlikely(atime_needs_update(&last->link, inode))) {
- if (nd->flags & LOOKUP_RCU) {
- if (!try_to_unlazy(nd))
- return ERR_PTR(-ECHILD);
- }
touch_atime(&last->link);
cond_resched();
}
--
2.43.0

[syzbot]

Hello,

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

Reported-by: syzbot+36e504...@syzkaller.appspotmail.com
Tested-by: syzbot+36e504...@syzkaller.appspotmail.com

Tested on:

commit: e43ffb69 Linux 7.1-rc6
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=11deefa6580000

kernel config: https://syzkaller.appspot.com/x/.config?x=8118209836970b54
dashboard link: https://syzkaller.appspot.com/bug?extid=36e50496c8ac4bcde3f9
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=12c8aab6580000

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] bpf: fix UAF by restoring RCU-delayed inode freeing in bpffs

Author: karti...@gmail.com

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

commit 4f375ade6aa9 moved all inode cleanup from ->free_inode() into
->destroy_inode() to avoid a sleeping-in-RCU-context warning when
calling bpf_any_put(). However this removed the RCU delay on freeing
the inode itself and the cached symlink body (i_link), both of which
can be accessed by RCU pathwalk (pick_link, may_lookup etc.).

This causes a use-after-free when a concurrent unlinkat() drops the
last inode reference and destroy_inode() frees the inode immediately,
while another task is still walking the path in RCU mode and reads
inode->i_opflags (offset +2) inside current_time() -> is_mgtime().

KASAN reports:

BUG: KASAN: slab-use-after-free in is_mgtime include/linux/fs.h:2313

Read of size 2 at addr ffff8880407e4282 (offset +2 = i_opflags)

The rules (per Al Viro):
->destroy_inode() called immediately, can sleep, use for blocking
cleanup e.g. bpf_any_put()
->free_inode() called after RCU grace period, use for freeing
inode and anything RCU-accessible e.g. i_link

Fix: keep bpf_any_put() in destroy_inode() since it is blocking, but
move kfree(i_link) and free_inode_nonrcu() into a new bpf_free_inode()
so they happen after RCU delay, preventing the UAF.

Reported-by: syzbot+36e504...@syzkaller.appspotmail.com
Link: https://syzkaller.appspot.com/bug?extid=36e50496c8ac4bcde3f9

Link: https://lore.kernel.org/all/20260423043906.GN3518998@ZenIV/

Signed-off-by: Deepanshu Kartikey <karti...@gmail.com>
---

kernel/bpf/inode.c | 13 +++++++++++--
1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/kernel/bpf/inode.c b/kernel/bpf/inode.c
index 25c06a011825..188c774a469c 100644
--- a/kernel/bpf/inode.c
+++ b/kernel/bpf/inode.c
@@ -766,10 +766,18 @@ static void bpf_destroy_inode(struct inode *inode)
{
enum bpf_type type;

- if (S_ISLNK(inode->i_mode))
- kfree(inode->i_link);
if (!bpf_inode_type(inode, &type))
bpf_any_put(inode->i_private, type);
+}
+
+/*
+ * Called after RCU grace period - safe to free inode and anything
+ * that might be accessed by RCU pathwalk (inode fields, i_link).
+ */
+static void bpf_free_inode(struct inode *inode)
+{
+ if (S_ISLNK(inode->i_mode))
+ kfree(inode->i_link);
free_inode_nonrcu(inode);
}

@@ -778,6 +786,7 @@ const struct super_operations bpf_super_ops = {
.drop_inode = inode_just_drop,
.show_options = bpf_show_options,
.destroy_inode = bpf_destroy_inode,
+ .free_inode = bpf_free_inode,
};

enum {
--
2.43.0

[syzbot]

Hello,

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

Reported-by: syzbot+36e504...@syzkaller.appspotmail.com
Tested-by: syzbot+36e504...@syzkaller.appspotmail.com

Tested on:

commit: e43ffb69 Linux 7.1-rc6
git tree: upstream

console output: https://syzkaller.appspot.com/x/log.txt?x=107ee6b6580000

kernel config: https://syzkaller.appspot.com/x/.config?x=8118209836970b54
dashboard link: https://syzkaller.appspot.com/bug?extid=36e50496c8ac4bcde3f9
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=168b557e580000