[syzbot] [media?] KASAN: slab-use-after-free Read in dvb_frontend_open

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 5cbb61bf4168 arm64/fpsimd: ptrace: zero target's fpsimd_st..
git tree: git://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git for-kernelci
console output: https://syzkaller.appspot.com/x/log.txt?x=14614fce580000
kernel config: https://syzkaller.appspot.com/x/.config?x=a834c6344141a58b
dashboard link: https://syzkaller.appspot.com/bug?extid=40339ea82afa8184ad5d
compiler: Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
userspace arch: arm64
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=127e5636580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/04156ec16593/disk-5cbb61bf.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/6bfa041e2c79/vmlinux-5cbb61bf.xz
kernel image: https://storage.googleapis.com/syzbot-assets/a92d82d8a79e/Image-5cbb61bf.gz.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+40339e...@syzkaller.appspotmail.com

i2c i2c-0: dvb_frontend_start: failed to start kthread (-4)
==================================================================
BUG: KASAN: slab-use-after-free in dvb_frontend_open+0xdac/0x105c drivers/media/dvb-core/dvb_frontend.c:2892
Read of size 4 at addr ffff0000cc9e9c3c by task syz.4.109/5364

CPU: 1 UID: 0 PID: 5364 Comm: syz.4.109 Not tainted syzkaller #0 PREEMPT
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 03/18/2026
Call trace:
show_stack+0x2c/0x3c arch/arm64/kernel/stacktrace.c:499 (C)
__dump_stack+0x30/0x40 lib/dump_stack.c:94
dump_stack_lvl+0xd8/0x12c lib/dump_stack.c:120
print_address_description+0xb0/0x238 mm/kasan/report.c:378
print_report+0x68/0x84 mm/kasan/report.c:482
kasan_report+0x8c/0xc4 mm/kasan/report.c:595
__asan_report_load4_noabort+0x20/0x2c mm/kasan/report_generic.c:380
dvb_frontend_open+0xdac/0x105c drivers/media/dvb-core/dvb_frontend.c:2892
dvb_device_open+0x1f4/0x250 drivers/media/dvb-core/dvbdev.c:109
chrdev_open+0x398/0x3e8 fs/char_dev.c:411
do_dentry_open+0x5c8/0x10dc fs/open.c:947
vfs_open+0x44/0x2d4 fs/open.c:1079
do_open fs/namei.c:4699 [inline]
path_openat+0x2234/0x2a6c fs/namei.c:4858
do_file_open+0x1c4/0x2e4 fs/namei.c:4887
do_sys_openat2+0x114/0x1e8 fs/open.c:1364
do_sys_open+0xac/0xdc fs/open.c:1370
__do_sys_openat fs/open.c:1386 [inline]
__se_sys_openat fs/open.c:1381 [inline]
__arm64_sys_openat+0x9c/0xb8 fs/open.c:1381
__invoke_syscall arch/arm64/kernel/syscall.c:35 [inline]
invoke_syscall+0x98/0x244 arch/arm64/kernel/syscall.c:49
el0_svc_common+0xe8/0x23c arch/arm64/kernel/syscall.c:121
do_el0_svc+0x48/0x58 arch/arm64/kernel/syscall.c:140
el0_svc+0x60/0x25c arch/arm64/kernel/entry-common.c:723
el0t_64_sync_handler+0x48/0x148 arch/arm64/kernel/entry-common.c:742
el0t_64_sync+0x198/0x19c arch/arm64/kernel/entry.S:594

Allocated by task 1:
kasan_save_stack mm/kasan/common.c:57 [inline]
kasan_save_track+0x40/0x78 mm/kasan/common.c:78
kasan_save_alloc_info+0x44/0x54 mm/kasan/generic.c:570
poison_kmalloc_redzone mm/kasan/common.c:398 [inline]
__kasan_kmalloc+0x9c/0xb4 mm/kasan/common.c:415
kasan_kmalloc include/linux/kasan.h:263 [inline]
__kmalloc_cache_noprof+0x284/0x56c mm/slub.c:5415
kmalloc_noprof include/linux/slab.h:950 [inline]
kzalloc_noprof include/linux/slab.h:1188 [inline]
dvb_register_device+0x1ac/0x16ec drivers/media/dvb-core/dvbdev.c:472
dvb_register_frontend+0x464/0x698 drivers/media/dvb-core/dvb_frontend.c:3051
vidtv_bridge_dvb_init drivers/media/test-drivers/vidtv/vidtv_bridge.c:438 [inline]
vidtv_bridge_probe+0x57c/0xa24 drivers/media/test-drivers/vidtv/vidtv_bridge.c:510
platform_probe+0xfc/0x198 drivers/base/platform.c:1418
call_driver_probe drivers/base/dd.c:-1 [inline]
really_probe+0x2a8/0x7e8 drivers/base/dd.c:709
__driver_probe_device+0x1e0/0x33c drivers/base/dd.c:871
driver_probe_device+0x6c/0x19c drivers/base/dd.c:901
__driver_attach+0x164/0x374 drivers/base/dd.c:1295
bus_for_each_dev+0x128/0x1b4 drivers/base/bus.c:383
driver_attach+0x4c/0x5c drivers/base/dd.c:1313
bus_add_driver+0x208/0x4fc drivers/base/bus.c:756
driver_register+0x220/0x30c drivers/base/driver.c:249
__platform_driver_register+0x6c/0x80 drivers/base/platform.c:910
vidtv_bridge_init+0x34/0x5c drivers/media/test-drivers/vidtv/vidtv_bridge.c:600
do_one_initcall+0x274/0xc20 init/main.c:1392
do_initcall_level+0x128/0x1c4 init/main.c:1454
do_initcalls+0x70/0xd0 init/main.c:1470
do_basic_setup+0x7c/0x90 init/main.c:1490
kernel_init_freeable+0x268/0x3a8 init/main.c:1703
kernel_init+0x24/0x1dc init/main.c:1593
ret_from_fork+0x10/0x20 arch/arm64/kernel/entry.S:842

Freed by task 5364:
kasan_save_stack mm/kasan/common.c:57 [inline]
kasan_save_track+0x40/0x78 mm/kasan/common.c:78
kasan_save_free_info+0x58/0x70 mm/kasan/generic.c:584
poison_slab_object mm/kasan/common.c:253 [inline]
__kasan_slab_free+0x74/0xa4 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:6246 [inline]
kfree+0x188/0x5e4 mm/slub.c:6561
dvb_free_device drivers/media/dvb-core/dvbdev.c:616 [inline]
kref_put include/linux/kref.h:65 [inline]
dvb_device_put+0x64/0xd0 drivers/media/dvb-core/dvbdev.c:629
dvb_generic_release+0xec/0x154 drivers/media/dvb-core/dvbdev.c:169
dvb_frontend_open+0x9b8/0x105c drivers/media/dvb-core/dvb_frontend.c:2890
dvb_device_open+0x1f4/0x250 drivers/media/dvb-core/dvbdev.c:109
chrdev_open+0x398/0x3e8 fs/char_dev.c:411
do_dentry_open+0x5c8/0x10dc fs/open.c:947
vfs_open+0x44/0x2d4 fs/open.c:1079
do_open fs/namei.c:4699 [inline]
path_openat+0x2234/0x2a6c fs/namei.c:4858
do_file_open+0x1c4/0x2e4 fs/namei.c:4887
do_sys_openat2+0x114/0x1e8 fs/open.c:1364
do_sys_open+0xac/0xdc fs/open.c:1370
__do_sys_openat fs/open.c:1386 [inline]
__se_sys_openat fs/open.c:1381 [inline]
__arm64_sys_openat+0x9c/0xb8 fs/open.c:1381
__invoke_syscall arch/arm64/kernel/syscall.c:35 [inline]
invoke_syscall+0x98/0x244 arch/arm64/kernel/syscall.c:49
el0_svc_common+0xe8/0x23c arch/arm64/kernel/syscall.c:121
do_el0_svc+0x48/0x58 arch/arm64/kernel/syscall.c:140
el0_svc+0x60/0x25c arch/arm64/kernel/entry-common.c:723
el0t_64_sync_handler+0x48/0x148 arch/arm64/kernel/entry-common.c:742
el0t_64_sync+0x198/0x19c arch/arm64/kernel/entry.S:594

The buggy address belongs to the object at ffff0000cc9e9c00
which belongs to the cache kmalloc-256 of size 256
The buggy address is located 60 bytes inside of
freed 256-byte region [ffff0000cc9e9c00, ffff0000cc9e9d00)

The buggy address belongs to the physical page:
page: refcount:0 mapcount:0 mapping:0000000000000000 index:0xffff0000cc9e9200 pfn:0x10c9e8
head: order:1 mapcount:0 entire_mapcount:0 nr_pages_mapped:0 pincount:0
flags: 0x5ffc00000000240(workingset|head|node=0|zone=2|lastcpupid=0x7ff)
page_type: f5(slab)
raw: 05ffc00000000240 ffff0000c0001b40 fffffdffc333d590 fffffdffc3064090
raw: ffff0000cc9e9200 000000080010000f 00000000f5000000 0000000000000000
head: 05ffc00000000240 ffff0000c0001b40 fffffdffc333d590 fffffdffc3064090
head: ffff0000cc9e9200 000000080010000f 00000000f5000000 0000000000000000
head: 05ffc00000000001 fffffdffc3327a01 00000000ffffffff 00000000ffffffff
head: ffffffffffffffff 0000000000000000 00000000ffffffff 0000000000000002
page dumped because: kasan: bad access detected

Memory state around the buggy address:
ffff0000cc9e9b00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
ffff0000cc9e9b80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
>ffff0000cc9e9c00: fa fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
^
ffff0000cc9e9c80: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
ffff0000cc9e9d00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
==================================================================
------------[ cut here ]------------
refcount_t: underflow; use-after-free.
WARNING: lib/refcount.c:28 at refcount_warn_saturate+0x154/0x1f8 lib/refcount.c:28, CPU#1: syz.4.109/5364
Modules linked in:
CPU: 1 UID: 0 PID: 5364 Comm: syz.4.109 Tainted: G B syzkaller #0 PREEMPT
Tainted: [B]=BAD_PAGE
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 03/18/2026
pstate: 63400005 (nZCv daif +PAN -UAO +TCO +DIT -SSBS BTYPE=--)
pc : refcount_warn_saturate+0x154/0x1f8 lib/refcount.c:28
lr : refcount_warn_saturate+0x154/0x1f8 lib/refcount.c:28
sp : ffff80009bdd7540
x29: ffff80009bdd7540 x28: ffff0000dbdcc748 x27: dfff800000000000
x26: 1fffe0001b7b98e9 x25: dfff800000000000 x24: ffff0000cc649068
x23: ffff0000dbdcc748 x22: ffff800083949be4 x21: 0000000000000000
x20: ffff0000cc9e9c10 x19: ffff800089f06000 x18: 0000000000000000
x17: 3d3d3d3d3d3d3d3d x16: 3d3d3d3d3d3d3d3d x15: 3d3d3d3d3d3d3d3d
x14: 3d3d3d3d3d3d3d3d x13: 0000000000000001 x12: 0000000000000000
x11: 0000000000000b4e x10: 0000000000ff0100 x9 : 9ef5470fdd42bb00
x8 : 9ef5470fdd42bb00 x7 : 0000000000000000 x6 : ffff8000804886d0
x5 : 0000000000000000 x4 : 0000000000000000 x3 : ffff8000802f13b0
x2 : 0000000100000000 x1 : ffff0000d7dc8000 x0 : 0000000000000000
Call trace:
refcount_warn_saturate+0x154/0x1f8 lib/refcount.c:28 (P)
__refcount_sub_and_test include/linux/refcount.h:400 [inline]
__refcount_dec_and_test include/linux/refcount.h:432 [inline]
refcount_dec_and_test include/linux/refcount.h:450 [inline]
kref_put include/linux/kref.h:64 [inline]
dvb_device_put+0xac/0xd0 drivers/media/dvb-core/dvbdev.c:629
dvb_device_open+0x238/0x250 drivers/media/dvb-core/dvbdev.c:113
chrdev_open+0x398/0x3e8 fs/char_dev.c:411
do_dentry_open+0x5c8/0x10dc fs/open.c:947
vfs_open+0x44/0x2d4 fs/open.c:1079
do_open fs/namei.c:4699 [inline]
path_openat+0x2234/0x2a6c fs/namei.c:4858
do_file_open+0x1c4/0x2e4 fs/namei.c:4887
do_sys_openat2+0x114/0x1e8 fs/open.c:1364
do_sys_open+0xac/0xdc fs/open.c:1370
__do_sys_openat fs/open.c:1386 [inline]
__se_sys_openat fs/open.c:1381 [inline]
__arm64_sys_openat+0x9c/0xb8 fs/open.c:1381
__invoke_syscall arch/arm64/kernel/syscall.c:35 [inline]
invoke_syscall+0x98/0x244 arch/arm64/kernel/syscall.c:49
el0_svc_common+0xe8/0x23c arch/arm64/kernel/syscall.c:121
do_el0_svc+0x48/0x58 arch/arm64/kernel/syscall.c:140
el0_svc+0x60/0x25c arch/arm64/kernel/entry-common.c:723
el0t_64_sync_handler+0x48/0x148 arch/arm64/kernel/entry-common.c:742
el0t_64_sync+0x198/0x19c arch/arm64/kernel/entry.S:594
irq event stamp: 562843
hardirqs last enabled at (562843): [<ffff8000867248cc>] irqentry_exit_to_kernel_mode_after_preempt include/linux/irq-entry-common.h:515 [inline]
hardirqs last enabled at (562843): [<ffff8000867248cc>] arm64_exit_to_kernel_mode+0x7c/0x90 arch/arm64/kernel/entry-common.c:62
hardirqs last disabled at (562842): [<ffff800086720b00>] __el1_irq arch/arm64/kernel/entry-common.c:493 [inline]
hardirqs last disabled at (562842): [<ffff800086720b00>] el1_interrupt+0x28/0x60 arch/arm64/kernel/entry-common.c:509
softirqs last enabled at (562784): [<ffff800080139e6c>] local_bh_enable+0x10/0x34 include/linux/bottom_half.h:32
softirqs last disabled at (562782): [<ffff800080139e38>] local_bh_disable+0x10/0x34 include/linux/bottom_half.h:19
---[ end trace 0000000000000000 ]---


---
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***

Subject: test
Author: yun....@windriver.com

#syz test

[syzbot]

Hello,

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

Reported-by: syzbot+40339e...@syzkaller.appspotmail.com
Tested-by: syzbot+40339e...@syzkaller.appspotmail.com

Tested on:

commit: 5cbb61bf arm64/fpsimd: ptrace: zero target's fpsimd_st..

git tree: git://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git for-kernelci

console output: https://syzkaller.appspot.com/x/log.txt?x=1783102e580000

kernel config: https://syzkaller.appspot.com/x/.config?x=a834c6344141a58b
dashboard link: https://syzkaller.appspot.com/bug?extid=40339ea82afa8184ad5d
compiler: Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
userspace arch: arm64

patch: https://syzkaller.appspot.com/x/patch.diff?x=1061a950580000

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