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Date:   Thu, 11 Feb 2021 12:22:39 +0100
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Jan Kara <jack@...e.cz>
Cc:     syzbot <syzbot+bfdded10ab7dcd7507ae@...kaller.appspotmail.com>,
        Jan Kara <jack@...e.com>, linux-ext4@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        "Theodore Ts'o" <tytso@....edu>, Michal Hocko <mhocko@...e.cz>,
        Linux-MM <linux-mm@...ck.org>
Subject: Re: possible deadlock in start_this_handle (2)

On Thu, Feb 11, 2021 at 11:49 AM Jan Kara <jack@...e.cz> wrote:
>
> Hello,
>
> added mm guys to CC.
>
> On Wed 10-02-21 05:35:18, syzbot wrote:
> > HEAD commit:    1e0d27fc Merge branch 'akpm' (patches from Andrew)
> > git tree:       upstream
> > console output: https://syzkaller.appspot.com/x/log.txt?x=15cbce90d00000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=bd1f72220b2e57eb
> > dashboard link: https://syzkaller.appspot.com/bug?extid=bfdded10ab7dcd7507ae
> > userspace arch: i386
> >
> > 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+bfdded10ab7dcd7507ae@...kaller.appspotmail.com
> >
> > ======================================================
> > WARNING: possible circular locking dependency detected
> > 5.11.0-rc6-syzkaller #0 Not tainted
> > ------------------------------------------------------
> > kswapd0/2246 is trying to acquire lock:
> > ffff888041a988e0 (jbd2_handle){++++}-{0:0}, at: start_this_handle+0xf81/0x1380 fs/jbd2/transaction.c:444
> >
> > but task is already holding lock:
> > ffffffff8be892c0 (fs_reclaim){+.+.}-{0:0}, at: __fs_reclaim_acquire+0x0/0x30 mm/page_alloc.c:5195
> >
> > which lock already depends on the new lock.
> >
> > the existing dependency chain (in reverse order) is:
> >
> > -> #2 (fs_reclaim){+.+.}-{0:0}:
> >        __fs_reclaim_acquire mm/page_alloc.c:4326 [inline]
> >        fs_reclaim_acquire+0x117/0x150 mm/page_alloc.c:4340
> >        might_alloc include/linux/sched/mm.h:193 [inline]
> >        slab_pre_alloc_hook mm/slab.h:493 [inline]
> >        slab_alloc_node mm/slub.c:2817 [inline]
> >        __kmalloc_node+0x5f/0x430 mm/slub.c:4015
> >        kmalloc_node include/linux/slab.h:575 [inline]
> >        kvmalloc_node+0x61/0xf0 mm/util.c:587
> >        kvmalloc include/linux/mm.h:781 [inline]
> >        ext4_xattr_inode_cache_find fs/ext4/xattr.c:1465 [inline]
> >        ext4_xattr_inode_lookup_create fs/ext4/xattr.c:1508 [inline]
> >        ext4_xattr_set_entry+0x1ce6/0x3780 fs/ext4/xattr.c:1649
> >        ext4_xattr_ibody_set+0x78/0x2b0 fs/ext4/xattr.c:2224
> >        ext4_xattr_set_handle+0x8f4/0x13e0 fs/ext4/xattr.c:2380
> >        ext4_xattr_set+0x13a/0x340 fs/ext4/xattr.c:2493
> >        ext4_xattr_user_set+0xbc/0x100 fs/ext4/xattr_user.c:40
> >        __vfs_setxattr+0x10e/0x170 fs/xattr.c:177
> >        __vfs_setxattr_noperm+0x11a/0x4c0 fs/xattr.c:208
> >        __vfs_setxattr_locked+0x1bf/0x250 fs/xattr.c:266
> >        vfs_setxattr+0x135/0x320 fs/xattr.c:291
> >        setxattr+0x1ff/0x290 fs/xattr.c:553
> >        path_setxattr+0x170/0x190 fs/xattr.c:572
> >        __do_sys_setxattr fs/xattr.c:587 [inline]
> >        __se_sys_setxattr fs/xattr.c:583 [inline]
> >        __ia32_sys_setxattr+0xbc/0x150 fs/xattr.c:583
> >        do_syscall_32_irqs_on arch/x86/entry/common.c:77 [inline]
> >        __do_fast_syscall_32+0x56/0x80 arch/x86/entry/common.c:139
> >        do_fast_syscall_32+0x2f/0x70 arch/x86/entry/common.c:164
> >        entry_SYSENTER_compat_after_hwframe+0x4d/0x5c
>
> This stacktrace should never happen. ext4_xattr_set() starts a transaction.
> That internally goes through start_this_handle() which calls:
>
>         handle->saved_alloc_context = memalloc_nofs_save();
>
> and we restore the allocation context only in stop_this_handle() when
> stopping the handle. And with this fs_reclaim_acquire() should remove
> __GFP_FS from the mask and not call __fs_reclaim_acquire().
>
> Now I have no idea why something here didn't work out. Given we don't have
> a reproducer it will be probably difficult to debug this. I'd note that
> about year and half ago similar report happened (got autoclosed) so it may
> be something real somewhere but it may also be just some HW glitch or
> something like that.

HW glitch is theoretically possible. But if we are considering such
causes, I would say a kernel memory corruption is way more likely, we
have hundreds of known memory-corruption-capable bugs open. In most
cases they are caught by KASAN before doing silent damage. But KASAN
can miss some cases.

I see at least 4 existing bugs with similar stack:
https://syzkaller.appspot.com/bug?extid=bfdded10ab7dcd7507ae
https://syzkaller.appspot.com/bug?extid=a7ab8df042baaf42ae3c
https://syzkaller.appspot.com/bug?id=c814a55a728493959328551c769ede4c8ff72aab
https://syzkaller.appspot.com/bug?id=426ad9adca053dafcd698f3a48ad5406dccc972b

All in all, I would not assume it's a memory corruption. When we had
bugs that actually caused silent memory corruption, that caused a
spike of random one-time crashes all over the kernel. This does not
look like it.

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