linux-kernel - Re: possible deadlock in ovl_write

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Date:   Tue, 27 Nov 2018 16:48:33 +0100
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Amir Goldstein <amir73il@...il.com>
Cc:     syzbot+695726bc473f9c36a4b6@...kaller.appspotmail.com,
        Miklos Szeredi <miklos@...redi.hu>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        overlayfs <linux-unionfs@...r.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>
Subject: Re: possible deadlock in ovl_write_iter

On Tue, Nov 27, 2018 at 4:07 PM, Amir Goldstein <amir73il@...il.com> wrote:
> On Tue, Nov 27, 2018 at 9:06 AM syzbot
> <syzbot+695726bc473f9c36a4b6@...kaller.appspotmail.com> wrote:
>>
>> syzbot has found a reproducer for the following crash on:
>>
>> HEAD commit:    6f8b52ba442c Merge tag 'hwmon-for-v4.20-rc5' of git://git...
>> git tree:       upstream
>> console output: https://syzkaller.appspot.com/x/log.txt?x=120f3905400000
>> kernel config:  https://syzkaller.appspot.com/x/.config?x=c94f9f0c0363db4b
>> dashboard link: https://syzkaller.appspot.com/bug?extid=695726bc473f9c36a4b6
>> compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
>> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=10cad225400000
>> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=13813093400000
>>
>> IMPORTANT: if you fix the bug, please add the following tag to the commit:
>> Reported-by: syzbot+695726bc473f9c36a4b6@...kaller.appspotmail.com
>>
>> overlayfs: filesystem on './file0' not supported as upperdir
>>
>> ======================================================
>> WARNING: possible circular locking dependency detected
>> 4.20.0-rc4+ #351 Not tainted
>> ------------------------------------------------------
>> syz-executor338/5996 is trying to acquire lock:
>> 00000000b59bb66d (&ovl_i_mutex_key[depth]){+.+.}, at: inode_lock
>> include/linux/fs.h:757 [inline]
>> 00000000b59bb66d (&ovl_i_mutex_key[depth]){+.+.}, at:
>> ovl_write_iter+0x151/0xd10 fs/overlayfs/file.c:231
>>
>> but task is already holding lock:
>> 00000000e0274330 (&pipe->mutex/1){+.+.}, at: pipe_lock_nested fs/pipe.c:62
>> [inline]
>> 00000000e0274330 (&pipe->mutex/1){+.+.}, at: pipe_lock+0x6e/0x80
>> fs/pipe.c:70
>>
>> which lock already depends on the new lock.
>>
>>
>> the existing dependency chain (in reverse order) is:
>>
>> -> #2 (&pipe->mutex/1){+.+.}:
>>         __mutex_lock_common kernel/locking/mutex.c:925 [inline]
>>         __mutex_lock+0x166/0x16f0 kernel/locking/mutex.c:1072
>>         mutex_lock_nested+0x16/0x20 kernel/locking/mutex.c:1087
>>         pipe_lock_nested fs/pipe.c:62 [inline]
>>         pipe_lock+0x6e/0x80 fs/pipe.c:70
>>         iter_file_splice_write+0x27d/0x1050 fs/splice.c:700
>>         do_splice_from fs/splice.c:851 [inline]
>>         do_splice+0x64a/0x1430 fs/splice.c:1147
>>         __do_sys_splice fs/splice.c:1414 [inline]
>>         __se_sys_splice fs/splice.c:1394 [inline]
>>         __x64_sys_splice+0x2c1/0x330 fs/splice.c:1394
>>         do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
>>         entry_SYSCALL_64_after_hwframe+0x49/0xbe
>>
>> -> #1 (sb_writers#3){.+.+}:
>>         percpu_down_read_preempt_disable include/linux/percpu-rwsem.h:36
>> [inline]
>>         percpu_down_read include/linux/percpu-rwsem.h:59 [inline]
>>         __sb_start_write+0x214/0x370 fs/super.c:1387
>>         sb_start_write include/linux/fs.h:1597 [inline]
>>         mnt_want_write+0x3f/0xc0 fs/namespace.c:360
>>         ovl_want_write+0x76/0xa0 fs/overlayfs/util.c:24
>>         ovl_setattr+0x10b/0xaf0 fs/overlayfs/inode.c:30
>>         notify_change+0xbde/0x1110 fs/attr.c:334
>>         do_truncate+0x1bd/0x2d0 fs/open.c:63
>>         handle_truncate fs/namei.c:3008 [inline]
>>         do_last fs/namei.c:3424 [inline]
>>         path_openat+0x375f/0x5150 fs/namei.c:3534
>>         do_filp_open+0x255/0x380 fs/namei.c:3564
>>         do_sys_open+0x568/0x700 fs/open.c:1063
>>         __do_sys_openat fs/open.c:1090 [inline]
>>         __se_sys_openat fs/open.c:1084 [inline]
>>         __x64_sys_openat+0x9d/0x100 fs/open.c:1084
>>         do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
>>         entry_SYSCALL_64_after_hwframe+0x49/0xbe
>>
>> -> #0 (&ovl_i_mutex_key[depth]){+.+.}:
>>         lock_acquire+0x1ed/0x520 kernel/locking/lockdep.c:3844
>>         down_write+0x8a/0x130 kernel/locking/rwsem.c:70
>>         inode_lock include/linux/fs.h:757 [inline]
>>         ovl_write_iter+0x151/0xd10 fs/overlayfs/file.c:231
>>         call_write_iter include/linux/fs.h:1857 [inline]
>>         new_sync_write fs/read_write.c:474 [inline]
>>         __vfs_write+0x6b8/0x9f0 fs/read_write.c:487
>>         __kernel_write+0x10c/0x370 fs/read_write.c:506
>>         write_pipe_buf+0x180/0x240 fs/splice.c:797
>>         splice_from_pipe_feed fs/splice.c:503 [inline]
>>         __splice_from_pipe+0x38b/0x7c0 fs/splice.c:627
>>         splice_from_pipe+0x1ec/0x340 fs/splice.c:662
>>         default_file_splice_write+0x3c/0x90 fs/splice.c:809
>>         do_splice_from fs/splice.c:851 [inline]
>>         do_splice+0x64a/0x1430 fs/splice.c:1147
>>         __do_sys_splice fs/splice.c:1414 [inline]
>>         __se_sys_splice fs/splice.c:1394 [inline]
>>         __x64_sys_splice+0x2c1/0x330 fs/splice.c:1394
>>         do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
>>         entry_SYSCALL_64_after_hwframe+0x49/0xbe
>>
>> other info that might help us debug this:
>>
>> Chain exists of:
>>    &ovl_i_mutex_key[depth] --> sb_writers#3 --> &pipe->mutex/1
>>
>>   Possible unsafe locking scenario:
>>
>>         CPU0                    CPU1
>>         ----                    ----
>>    lock(&pipe->mutex/1);
>>                                 lock(sb_writers#3);
>>                                 lock(&pipe->mutex/1);
>>    lock(&ovl_i_mutex_key[depth]);
>>
>>   *** DEADLOCK ***
>>
>> 2 locks held by syz-executor338/5996:
>>   #0: 00000000024e7b73 (sb_writers#8){.+.+}, at: file_start_write
>> include/linux/fs.h:2810 [inline]
>>   #0: 00000000024e7b73 (sb_writers#8){.+.+}, at: do_splice+0xd2e/0x1430
>> fs/splice.c:1146
>>   #1: 00000000e0274330 (&pipe->mutex/1){+.+.}, at: pipe_lock_nested
>> fs/pipe.c:62 [inline]
>>   #1: 00000000e0274330 (&pipe->mutex/1){+.+.}, at: pipe_lock+0x6e/0x80
>> fs/pipe.c:70
>>
>> stack backtrace:
>> CPU: 0 PID: 5996 Comm: syz-executor338 Not tainted 4.20.0-rc4+ #351
>> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
>> Google 01/01/2011
>> Call Trace:
>>   __dump_stack lib/dump_stack.c:77 [inline]
>>   dump_stack+0x244/0x39d lib/dump_stack.c:113
>>   print_circular_bug.isra.35.cold.54+0x1bd/0x27d
>> kernel/locking/lockdep.c:1221
>>   check_prev_add kernel/locking/lockdep.c:1863 [inline]
>>   check_prevs_add kernel/locking/lockdep.c:1976 [inline]
>>   validate_chain kernel/locking/lockdep.c:2347 [inline]
>>   __lock_acquire+0x3399/0x4c20 kernel/locking/lockdep.c:3341
>>   lock_acquire+0x1ed/0x520 kernel/locking/lockdep.c:3844
>>   down_write+0x8a/0x130 kernel/locking/rwsem.c:70
>>   inode_lock include/linux/fs.h:757 [inline]
>>   ovl_write_iter+0x151/0xd10 fs/overlayfs/file.c:231
>>   call_write_iter include/linux/fs.h:1857 [inline]
>>   new_sync_write fs/read_write.c:474 [inline]
>>   __vfs_write+0x6b8/0x9f0 fs/read_write.c:487
>>   __kernel_write+0x10c/0x370 fs/read_write.c:506
>>   write_pipe_buf+0x180/0x240 fs/splice.c:797
>>   splice_from_pipe_feed fs/splice.c:503 [inline]
>>   __splice_from_pipe+0x38b/0x7c0 fs/splice.c:627
>>   splice_from_pipe+0x1ec/0x340 fs/splice.c:662
>>   default_file_splice_write+0x3c/0x90 fs/splice.c:809
>>   do_splice_from fs/splice.c:851 [inline]
>>   do_splice+0x64a/0x1430 fs/splice.c:1147
>>   __do_sys_splice fs/splice.c:1414 [inline]
>>   __se_sys_splice fs/splice.c:1394 [inline]
>>   __x64_sys_splice+0x2c1/0x330 fs/splice.c:1394
>>   do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
>>   entry_SYSCALL_64_after_hwframe+0x49/0xbe
>> RIP: 0033:0x445ad9
>> Code: e8 5c b7 02 00 48 83 c4 18 c3 0f 1f 80 00 00 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 0f 83 2b 12 fc ff c3 66 2e 0f 1f 84 00 00 00 00
>> RSP: 002b:00007f18e3f71cd8 EFLAGS: 00000216 ORIG_RAX: 0000000000000113
>> RAX: ffffffffffffffda RBX: 00000000006dac78 RCX: 0000000000445ad9
>> RDX: 000000000000000a RSI: 0000000000000000 RDI: 0000000000000007
>> RBP: 00000000006dac70 R08: 000100000000000a R09: 0000000000000007
>> R10: 0000000000000000 R11: 0000000000000216 R12: 00000000006dac7c
>> R13: 00007ffde0706e9f R14: 00007f18e3f729c0 R15: 00000000006dad4c
>>
>
> This looks like a false positive because lockdep is not aware of
> s_stack_depth of the file (fs) associated with the pipe.

There must be some annotation to tell lockdep about this.


>
> HOWEVER, because overlayfs calls do_splice_direct() on copy up,
> it is important to make sure that higher layer do_splice_direct() cannot
> recursively trigger copy up.
>
> At this time, overlayfs does the copy up on open for write, so any
> higher layer do_splice_direct() will already get an out file that has been
> copied up, but with future plans for "lazy copy up on first write", we need
> to be careful.
>
> Thanks,
> Amir.
>
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