Re: [syzbot] [ext4?] possible deadlock in ext4_xattr_set_handle (3)

[Jan Kara]

Hi!

On Tue 24-01-23 16:27:36, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    edc00350d205 Merge tag 'block-6.2-2023-01-20' of git://git..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=134b1441480000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=899d86a7610a0ea0
> dashboard link: https://syzkaller.appspot.com/bug?extid=edce54daffee36421b4c
> compiler:       gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2
> 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+edce54daffee36421b4c@xxxxxxxxxxxxxxxxxxxxxxxxx
> 
> ext4 filesystem being mounted at /syzkaller-testdir3627507797/syzkaller.9jT2hR/316/file0 supports timestamps until 2038 (0x7fffffff)
> ======================================================
> WARNING: possible circular locking dependency detected
> 6.2.0-rc4-syzkaller-00350-gedc00350d205 #0 Not tainted
> ------------------------------------------------------
> syz-executor.2/573 is trying to acquire lock:
> ffffffff8c8d4f60 (fs_reclaim){+.+.}-{0:0}, at: might_alloc include/linux/sched/mm.h:271 [inline]
> ffffffff8c8d4f60 (fs_reclaim){+.+.}-{0:0}, at: slab_pre_alloc_hook mm/slab.h:720 [inline]
> ffffffff8c8d4f60 (fs_reclaim){+.+.}-{0:0}, at: slab_alloc_node mm/slub.c:3434 [inline]
> ffffffff8c8d4f60 (fs_reclaim){+.+.}-{0:0}, at: __kmem_cache_alloc_node+0x41/0x430 mm/slub.c:3491
> 
> but task is already holding lock:
> ffff8880277eb2f0 (&ei->xattr_sem){++++}-{3:3}, at: ext4_write_lock_xattr fs/ext4/xattr.h:155 [inline]
> ffff8880277eb2f0 (&ei->xattr_sem){++++}-{3:3}, at: ext4_xattr_set_handle+0x160/0x1510 fs/ext4/xattr.c:2305
> 
> which lock already depends on the new lock.

So I don't see how the below is ever possible:

> -> #0 (fs_reclaim){+.+.}-{0:0}:
>        check_prev_add kernel/locking/lockdep.c:3097 [inline]
>        check_prevs_add kernel/locking/lockdep.c:3216 [inline]
>        validate_chain kernel/locking/lockdep.c:3831 [inline]
>        __lock_acquire+0x2a43/0x56d0 kernel/locking/lockdep.c:5055
>        lock_acquire kernel/locking/lockdep.c:5668 [inline]
>        lock_acquire+0x1e3/0x630 kernel/locking/lockdep.c:5633
>        __fs_reclaim_acquire mm/page_alloc.c:4674 [inline]

So we are acquiring fs_reclaim here which means that
current_gfp_context(gfp_mask) contained __GFP_FS...

>        fs_reclaim_acquire+0x11d/0x160 mm/page_alloc.c:4688
>        might_alloc include/linux/sched/mm.h:271 [inline]
>        slab_pre_alloc_hook mm/slab.h:720 [inline]
>        slab_alloc_node mm/slub.c:3434 [inline]
>        __kmem_cache_alloc_node+0x41/0x430 mm/slub.c:3491
>        __do_kmalloc_node mm/slab_common.c:967 [inline]
>        __kmalloc_node+0x4d/0xd0 mm/slab_common.c:975
>        kmalloc_node include/linux/slab.h:610 [inline]
>        kvmalloc_node+0x76/0x1a0 mm/util.c:581
>        kvmalloc include/linux/slab.h:737 [inline]
>        ext4_xattr_inode_cache_find fs/ext4/xattr.c:1484 [inline]
>        ext4_xattr_inode_lookup_create fs/ext4/xattr.c:1527 [inline]
>        ext4_xattr_set_entry+0x1d92/0x3a00 fs/ext4/xattr.c:1669
>        ext4_xattr_block_set+0x61b/0x3000 fs/ext4/xattr.c:1906
>        ext4_xattr_set_handle+0xd8a/0x1510 fs/ext4/xattr.c:2390
>        ext4_xattr_set+0x144/0x360 fs/ext4/xattr.c:2492

... however here we've started a transaction so we should have
PF_MEMALLOC_NOFS set?

The only good explanation I have is that lockdep is mixing dependencies
from an ext4 filesystem without a journal with dependencies created by
filesystem with a journal. Since we have no reproducer, it's hard to
tell.

>        __vfs_setxattr+0x173/0x1e0 fs/xattr.c:202
>        __vfs_setxattr_noperm+0x129/0x5f0 fs/xattr.c:236
>        __vfs_setxattr_locked+0x1d3/0x260 fs/xattr.c:297
>        vfs_setxattr+0x143/0x340 fs/xattr.c:323
>        do_setxattr+0x151/0x190 fs/xattr.c:608
>        setxattr+0x146/0x160 fs/xattr.c:631
>        path_setxattr+0x197/0x1c0 fs/xattr.c:650
>        __do_sys_setxattr fs/xattr.c:666 [inline]
>        __se_sys_setxattr fs/xattr.c:662 [inline]
>        __ia32_sys_setxattr+0xc0/0x160 fs/xattr.c:662
>        do_syscall_32_irqs_on arch/x86/entry/common.c:112 [inline]
>        __do_fast_syscall_32+0x65/0xf0 arch/x86/entry/common.c:178
>        do_fast_syscall_32+0x33/0x70 arch/x86/entry/common.c:203
>        entry_SYSENTER_compat_after_hwframe+0x70/0x82

								Honza
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR