Re: [syzbot] [media?] [usb?] KASAN: slab-use-after-free Read in v4l2_open

[syzbot]

Hello,

syzbot has tested the proposed patch but the reproducer is still triggering an issue:
possible deadlock in em28xx_v4l2_open

======================================================
WARNING: possible circular locking dependency detected
syzkaller #0 Not tainted
------------------------------------------------------
v4l_id/6669 is trying to acquire lock:
ffff888030ac9cd0 (&dev->lock#3){+.+.}-{4:4}
, at: em28xx_v4l2_open+0xf0/0xa20 drivers/media/usb/em28xx/em28xx-video.c:2146

but task is already holding lock:
ffffffff8f6a0520 (videodev_lock){+.+.}-{4:4}, at: v4l2_open+0x32/0x3a0 drivers/media/v4l2-core/v4l2-dev.c:417

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #1 (videodev_lock
){+.+.}-{4:4}:
       __mutex_lock_common kernel/locking/mutex.c:632 [inline]
       __mutex_lock+0x19e/0x1420 kernel/locking/mutex.c:794
       __video_register_device+0x54e/0x4d20 drivers/media/v4l2-core/v4l2-dev.c:1008
       video_register_device include/media/v4l2-dev.h:390 [inline]
       em28xx_v4l2_init+0x1d32/0x2e70 drivers/media/usb/em28xx/em28xx-video.c:2788
       em28xx_init_extension+0x120/0x1c0 drivers/media/usb/em28xx/em28xx-core.c:1117
       process_one_work+0x9ab/0x1780 kernel/workqueue.c:3288
       process_scheduled_works kernel/workqueue.c:3379 [inline]
       worker_thread+0xba8/0x11e0 kernel/workqueue.c:3465
       kthread+0x388/0x470 kernel/kthread.c:436
       ret_from_fork+0x51e/0xb90 arch/x86/kernel/process.c:158
       ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

-> #0 (&dev->lock#3){+.+.}-{4:4}:
       check_prev_add kernel/locking/lockdep.c:3165 [inline]
       check_prevs_add kernel/locking/lockdep.c:3284 [inline]
       validate_chain kernel/locking/lockdep.c:3908 [inline]
       __lock_acquire+0x15a5/0x2cf0 kernel/locking/lockdep.c:5237
       lock_acquire+0xf0/0x2e0 kernel/locking/lockdep.c:5868
       __mutex_lock_common kernel/locking/mutex.c:632 [inline]
       __mutex_lock+0x19e/0x1420 kernel/locking/mutex.c:794
       em28xx_v4l2_open+0xf0/0xa20 drivers/media/usb/em28xx/em28xx-video.c:2146
       v4l2_open+0x1b1/0x3a0 drivers/media/v4l2-core/v4l2-dev.c:432
       chrdev_open+0x4cd/0x5e0 fs/char_dev.c:411
       do_dentry_open+0x785/0x14e0 fs/open.c:949
       vfs_open+0x3b/0x340 fs/open.c:1081
       do_open fs/namei.c:4693 [inline]
       path_openat+0x2e08/0x3860 fs/namei.c:4852
       do_file_open+0x23e/0x4a0 fs/namei.c:4881
       do_sys_openat2+0x113/0x200 fs/open.c:1366
       do_sys_open fs/open.c:1372 [inline]
       __do_sys_openat fs/open.c:1388 [inline]
       __se_sys_openat fs/open.c:1383 [inline]
       __x64_sys_openat+0x138/0x170 fs/open.c:1383
       do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
       do_syscall_64+0x14d/0xf80 arch/x86/entry/syscall_64.c:94
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

other info that might help us debug this:

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(videodev_lock);
                               lock(&dev->lock#3);
                               lock(videodev_lock);
  lock(&dev->lock#3
);

 *** DEADLOCK ***

1 lock held by v4l_id/6669:
 #0: 
ffffffff8f6a0520 (videodev_lock){+.+.}-{4:4}, at: v4l2_open+0x32/0x3a0 drivers/media/v4l2-core/v4l2-dev.c:417

stack backtrace:
CPU: 0 UID: 0 PID: 6669 Comm: v4l_id Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/12/2026
Call Trace:
 <TASK>
 dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120
 print_circular_bug+0x2e1/0x300 kernel/locking/lockdep.c:2043
 check_noncircular+0x12e/0x150 kernel/locking/lockdep.c:2175
 check_prev_add kernel/locking/lockdep.c:3165 [inline]
 check_prevs_add kernel/locking/lockdep.c:3284 [inline]
 validate_chain kernel/locking/lockdep.c:3908 [inline]
 __lock_acquire+0x15a5/0x2cf0 kernel/locking/lockdep.c:5237
 lock_acquire+0xf0/0x2e0 kernel/locking/lockdep.c:5868
 __mutex_lock_common kernel/locking/mutex.c:632 [inline]
 __mutex_lock+0x19e/0x1420 kernel/locking/mutex.c:794
 em28xx_v4l2_open+0xf0/0xa20 drivers/media/usb/em28xx/em28xx-video.c:2146
 v4l2_open+0x1b1/0x3a0 drivers/media/v4l2-core/v4l2-dev.c:432
 chrdev_open+0x4cd/0x5e0 fs/char_dev.c:411
 do_dentry_open+0x785/0x14e0 fs/open.c:949
 vfs_open+0x3b/0x340 fs/open.c:1081
 do_open fs/namei.c:4693 [inline]
 path_openat+0x2e08/0x3860 fs/namei.c:4852
 do_file_open+0x23e/0x4a0 fs/namei.c:4881
 do_sys_openat2+0x113/0x200 fs/open.c:1366
 do_sys_open fs/open.c:1372 [inline]
 __do_sys_openat fs/open.c:1388 [inline]
 __se_sys_openat fs/open.c:1383 [inline]
 __x64_sys_openat+0x138/0x170 fs/open.c:1383
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0x14d/0xf80 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f80328a7407
Code: 48 89 fa 4c 89 df e8 38 aa 00 00 8b 93 08 03 00 00 59 5e 48 83 f8 fc 74 1a 5b c3 0f 1f 84 00 00 00 00 00 48 8b 44 24 10 0f 05 <5b> c3 0f 1f 80 00 00 00 00 83 e2 39 83 fa 08 75 de e8 23 ff ff ff
RSP: 002b:00007ffdfce9d3b0 EFLAGS: 00000202 ORIG_RAX: 0000000000000101
RAX: ffffffffffffffda RBX: 00007f8033087880 RCX: 00007f80328a7407
RDX: 0000000000000000 RSI: 00007ffdfce9df1d RDI: ffffffffffffff9c
RBP: 0000000000000002 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000202 R12: 0000000000000000
R13: 00007ffdfce9d600 R14: 00007f80331ee000 R15: 0000556e3f2c74d8
 </TASK>


Tested on:

commit:         8e42d251 Add linux-next specific files for 20260318
git tree:       linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=178e4a06580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=7f5b21566f9d8af6
dashboard link: https://syzkaller.appspot.com/bug?extid=1a7507a194fff09e5c44
compiler:       Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
patch:          https://syzkaller.appspot.com/x/patch.diff?x=12e57352580000