Re: [syzbot] WARNING: suspicious RCU usage in tomoyo_encode2

[Dmitry Vyukov]

On Sat, May 8, 2021 at 7:05 AM syzbot
<syzbot+dbea9c4c82135d4f5da3@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
>
> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit:    d2b6f8a1 Merge tag 'xfs-5.13-merge-3' of git://git.kernel...
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=152ab345d00000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=65c207250bba4efe
> dashboard link: https://syzkaller.appspot.com/bug?extid=dbea9c4c82135d4f5da3
>
> Unfortunately, I don't have any reproducer for this issue yet.
>
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+dbea9c4c82135d4f5da3@xxxxxxxxxxxxxxxxxxxxxxxxx

Looks like another false positive caused by concurrent lockdep reports.

#syz fix: rcu: Reject RCU_LOCKDEP_WARN() false positives

[  181.705340][T11603] =============================
[  181.705345][T11603] WARNING: suspicious RCU usage
[  181.705351][T11603] 5.12.0-syzkaller #0 Not tainted
[  181.707688][T11626] ======================================================
[  181.707696][T11626] WARNING: possible circular locking dependency detected
[  181.710012][T11603] -----------------------------
[  181.710018][T11603] kernel/sched/core.c:8304 Illegal context switch
in RCU-bh read-side critical section!
[  181.714836][T11626] 5.12.0-syzkaller #0 Not tainted
[  181.714847][T11626] ------------------------------------------------------
[  181.714852][T11626] syz-executor.5/11626 is trying to acquire lock:
[  181.714863][T11626] ffff88814224c5a0 (&bdev->bd_mutex){+.+.}-{3:3},
at: del_gendisk+0x250/0x9e0



> =============================
> WARNING: suspicious RCU usage
> 5.12.0-syzkaller #0 Not tainted
> -----------------------------
> kernel/sched/core.c:8304 Illegal context switch in RCU-bh read-side critical section!
>
> other info that might help us debug this:
>
>
> rcu_scheduler_active = 2, debug_locks = 0
> 1 lock held by systemd-udevd/11603:
>  #0:
> ffffffff8c712b18
>  (
> tomoyo_ss
> ){....}-{0:0}
> , at: tomoyo_path_perm+0x1c1/0x400 security/tomoyo/file.c:847
>
> stack backtrace:
> CPU: 1 PID: 11603 Comm: systemd-udevd Not tainted 5.12.0-syzkaller #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> Call Trace:
>  __dump_stack lib/dump_stack.c:79 [inline]
>  dump_stack+0x141/0x1d7 lib/dump_stack.c:120
>  ___might_sleep+0x229/0x2c0 kernel/sched/core.c:8304
>  might_alloc include/linux/sched/mm.h:197 [inline]
>  slab_pre_alloc_hook mm/slab.h:497 [inline]
>  slab_alloc_node mm/slub.c:2827 [inline]
>  slab_alloc mm/slub.c:2916 [inline]
>  __kmalloc+0x26f/0x300 mm/slub.c:4054
>  kmalloc include/linux/slab.h:561 [inline]
>  kzalloc include/linux/slab.h:686 [inline]
>  tomoyo_encode2.part.0+0xe9/0x3a0 security/tomoyo/realpath.c:45
>  tomoyo_encode2 security/tomoyo/realpath.c:31 [inline]
>  tomoyo_encode+0x28/0x50 security/tomoyo/realpath.c:80
>  tomoyo_realpath_from_path+0x186/0x620 security/tomoyo/realpath.c:288
>  tomoyo_get_realpath security/tomoyo/file.c:151 [inline]
>  tomoyo_path_perm+0x21b/0x400 security/tomoyo/file.c:822
>  security_inode_getattr+0xcf/0x140 security/security.c:1295
>  vfs_getattr fs/stat.c:131 [inline]
>  vfs_statx+0x164/0x390 fs/stat.c:199
>  vfs_fstatat fs/stat.c:217 [inline]
>  vfs_stat include/linux/fs.h:3380 [inline]
>  __do_sys_newstat+0x91/0x110 fs/stat.c:359
>  do_syscall_64+0x3a/0xb0 arch/x86/entry/common.c:47
>  entry_SYSCALL_64_after_hwframe+0x44/0xae
> RIP: 0033:0x7ffa23631295
> Code: 00 00 00 e8 5d 01 00 00 48 83 c4 18 c3 0f 1f 84 00 00 00 00 00 83 ff 01 48 89 f0 77 30 48 89 c7 48 89 d6 b8 04 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 03 f3 c3 90 48 8b 15 d1 db 2b 00 f7 d8 64 89
> RSP: 002b:00007fff554379b8 EFLAGS: 00000246
>  ORIG_RAX: 0000000000000004
> RAX: ffffffffffffffda RBX: 0000558bf63356b0 RCX: 00007ffa23631295
> RDX: 00007fff554379c0 RSI: 00007fff554379c0 RDI: 00007fff55437a70
> RBP: 00007fff55437ae0 R08: 000000000000c0c0 R09: 0000000000000000
> R10: 00007fff55437ac0 R11: 0000000000000246 R12: 00007fff55437af0
> R13: 0000000000000001 R14: 0000558bf63353b0 R15: 000000000000000e
>
>
> ---
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