Re: [syzbot] WARNING in mntput_no_expire (2)

[Christian Brauner]

On Thu, Apr 01, 2021 at 02:09:20AM -0700, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    d19cc4bf Merge tag 'trace-v5.12-rc5' of git://git.kernel.o..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=1018f281d00000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=d1a3d65a48dbd1bc
> dashboard link: https://syzkaller.appspot.com/bug?extid=c88a7030da47945a3cc3
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=12f50d11d00000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=137694a1d00000
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+c88a7030da47945a3cc3@xxxxxxxxxxxxxxxxxxxxxxxxx
> 
> ------------[ cut here ]------------
> WARNING: CPU: 1 PID: 8409 at fs/namespace.c:1186 mntput_no_expire+0xaca/0xcb0 fs/namespace.c:1186
> Modules linked in:
> CPU: 1 PID: 8409 Comm: syz-executor035 Not tainted 5.12.0-rc5-syzkaller #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> RIP: 0010:mntput_no_expire+0xaca/0xcb0 fs/namespace.c:1186
> Code: ff 48 c7 c2 e0 cb 78 89 be c2 02 00 00 48 c7 c7 a0 cb 78 89 c6 05 e5 6d e5 0b 01 e8 ff e1 f6 06 e9 3f fd ff ff e8 c6 a5 a8 ff <0f> 0b e9 fc fc ff ff e8 ba a5 a8 ff e8 55 dc 94 ff 31 ff 89 c5 89
> RSP: 0018:ffffc9000165fc78 EFLAGS: 00010293
> RAX: 0000000000000000 RBX: 1ffff920002cbf95 RCX: 0000000000000000
> RDX: ffff88802072d4c0 RSI: ffffffff81cb4b8a RDI: 0000000000000003
> RBP: ffff888011656900 R08: 0000000000000000 R09: ffffffff8fa978af
> R10: ffffffff81cb4884 R11: 0000000000000000 R12: 0000000000000008
> R13: ffffc9000165fcc8 R14: dffffc0000000000 R15: 00000000ffffffff
> FS:  0000000000000000(0000) GS:ffff8880b9d00000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 000055a722053160 CR3: 000000000bc8e000 CR4: 00000000001506e0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> Call Trace:
>  mntput fs/namespace.c:1232 [inline]
>  cleanup_mnt+0x523/0x530 fs/namespace.c:1132
>  task_work_run+0xdd/0x1a0 kernel/task_work.c:140
>  exit_task_work include/linux/task_work.h:30 [inline]
>  do_exit+0xbfc/0x2a60 kernel/exit.c:825
>  do_group_exit+0x125/0x310 kernel/exit.c:922
>  __do_sys_exit_group kernel/exit.c:933 [inline]
>  __se_sys_exit_group kernel/exit.c:931 [inline]
>  __x64_sys_exit_group+0x3a/0x50 kernel/exit.c:931
>  do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46
>  entry_SYSCALL_64_after_hwframe+0x44/0xae
> RIP: 0033:0x446af9
> Code: Unable to access opcode bytes at RIP 0x446acf.
> RSP: 002b:00000000005dfe48 EFLAGS: 00000246 ORIG_RAX: 00000000000000e7
> RAX: ffffffffffffffda RBX: 00000000004ce450 RCX: 0000000000446af9
> RDX: 000000000000003c RSI: 00000000000000e7 RDI: 0000000000000001
> RBP: 0000000000000001 R08: ffffffffffffffbc R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000246 R12: 00000000004ce450
> R13: 0000000000000001 R14: 0000000000000000 R15: 0000000000000001

[+Cc Jens + io_uring]

Hm, this reproducer uses io_uring and it's the io_uring_enter() that
triggers this reliably. With this reproducer I've managed to reproduce
the issue on v5.12-rc4, and v5.12-rc3, v5.12-rc2 and v5.12-rc1.
It's not reproducible at
9820b4dca0f9c6b7ab8b4307286cdace171b724d
which is the commit immediately before the first v5.12 io_uring merge.
It's first reproducible with the first io_uring merge for v5.12, i.e.
5bbb336ba75d95611a7b9456355b48705016bdb1

Christian