Re: [PATCH v2] ext4: fix race condition between buffer write and page_mkwrite

Theodore Ts'o
2023-06-04T03:05:51Z

I tried testing to see if this fixed [1], and it appears to be
triggering a lockdep warning[2] at this line in the patch:

[1] https://syzkaller.appspot.com/bug?extid=f4582777a19ec422b517
[2] https://syzkaller.appspot.com/x/report.txt?x=17260843280000

> diff --git a/fs/ext4/file.c b/fs/ext4/file.c
> index d101b3b0c7da..9df82d72eb90 100644
> --- a/fs/ext4/file.c
> +++ b/fs/ext4/file.c
> @@ -808,6 +809,27 @@ static int ext4_file_mmap(struct file *file, struct vm_area_struct *vma)
> if (!daxdev_mapping_supported(vma, dax_dev))
> return -EOPNOTSUPP;
>
> + /*
> + * Writing via mmap has no logic to handle inline data, so we
> + * need to call ext4_convert_inline_data() to convert the inode
> + * to normal format before doing so, otherwise a BUG_ON will be
> + * triggered in ext4_writepages() due to the
> + * EXT4_STATE_MAY_INLINE_DATA flag. Moreover, we need to grab
> + * i_rwsem during conversion, since clearing and setting the
> + * inline data flag may race with ext4_buffered_write_iter()
> + * to trigger a BUG_ON.
> + */
> + if (ext4_has_feature_inline_data(sb) &&
> + vma->vm_flags & VM_SHARED && vma->vm_flags & VM_MAYWRITE) {
> + int err;
> +
> + inode_lock(inode); <=================== LOCKDEP warning
> + err = ext4_convert_inline_data(inode);
> + inode_unlock(inode);
> + if (err)
> + return err;
> + }

The details of the lockdep warning from [2], which appears to be a
mmap(2) racing with a buffered write(2) are below. Could you take a
look?

Thanks!

- Ted

======================================================
WARNING: possible circular locking dependency detected
6.4.0-rc4-syzkaller-geb1f822c76be-dirty #0 Not tainted
------------------------------------------------------
syz-executor.4/5589 is trying to acquire lock:
ffff888024228168 (&mm->mmap_lock){++++}-{3:3}, at: mmap_read_lock include/linux/mmap_lock.h:142 [inline]
ffff888024228168 (&mm->mmap_lock){++++}-{3:3}, at: do_user_addr_fault+0xb3d/0x1210 arch/x86/mm/fault.c:1391

but task is already holding lock:
ffff88806a066800 (&sb->s_type->i_mutex_key#8){++++}-{3:3}, at: inode_lock include/linux/fs.h:775 [inline]
ffff88806a066800 (&sb->s_type->i_mutex_key#8){++++}-{3:3}, at: ext4_buffered_write_iter+0xb0/0x460 fs/ext4/file.c:283

which lock already depends on the new lock.

the existing dependency chain (in reverse order) is:

-> #1 (&sb->s_type->i_mutex_key#8){++++}-{3:3}:
down_write+0x92/0x200 kernel/locking/rwsem.c:1573
inode_lock include/linux/fs.h:775 [inline]
ext4_file_mmap+0x62e/0x800 fs/ext4/file.c:826
call_mmap include/linux/fs.h:1873 [inline]
mmap_region+0x694/0x28d0 mm/mmap.c:2652
do_mmap+0x831/0xf60 mm/mmap.c:1394
vm_mmap_pgoff+0x1a2/0x3b0 mm/util.c:543
ksys_mmap_pgoff+0x41f/0x5a0 mm/mmap.c:1440
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x39/0xb0 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x63/0xcd

-> #0 (&mm->mmap_lock){++++}-{3:3}:
check_prev_add kernel/locking/lockdep.c:3113 [inline]
check_prevs_add kernel/locking/lockdep.c:3232 [inline]
validate_chain kernel/locking/lockdep.c:3847 [inline]
__lock_acquire+0x2fcd/0x5f30 kernel/locking/lockdep.c:5088
lock_acquire kernel/locking/lockdep.c:5705 [inline]
lock_acquire+0x1b1/0x520 kernel/locking/lockdep.c:5670
down_read+0x9c/0x480 kernel/locking/rwsem.c:1520
mmap_read_lock include/linux/mmap_lock.h:142 [inline]
do_user_addr_fault+0xb3d/0x1210 arch/x86/mm/fault.c:1391
handle_page_fault arch/x86/mm/fault.c:1534 [inline]
exc_page_fault+0x98/0x170 arch/x86/mm/fault.c:1590
asm_exc_page_fault+0x26/0x30 arch/x86/include/asm/idtentry.h:570
fault_in_readable+0x1a5/0x210 mm/gup.c:1856
fault_in_iov_iter_readable+0x252/0x2c0 lib/iov_iter.c:362
generic_perform_write+0x1ae/0x570 mm/filemap.c:3913
ext4_buffered_write_iter+0x15b/0x460 fs/ext4/file.c:289
ext4_file_write_iter+0xbe0/0x1740 fs/ext4/file.c:710
call_write_iter include/linux/fs.h:1868 [inline]
new_sync_write fs/read_write.c:491 [inline]
vfs_write+0x945/0xd50 fs/read_write.c:584
ksys_write+0x12b/0x250 fs/read_write.c:637
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x39/0xb0 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x63/0xcd

other info that might help us debug this:

Possible unsafe locking scenario:

CPU0 CPU1
---- ----
lock(&sb->s_type->i_mutex_key#8);
lock(&mm->mmap_lock);
lock(&sb->s_type->i_mutex_key#8);
rlock(&mm->mmap_lock);

*** DEADLOCK ***

3 locks held by syz-executor.4/5589:
#0: ffff88802a7fe0e8 (&f->f_pos_lock){+.+.}-{3:3}, at: __fdget_pos+0xe7/0x100 fs/file.c:1047
#1: ffff888021fe0460 (sb_writers#4){.+.+}-{0:0}, at: ksys_write+0x12b/0x250 fs/read_write.c:637
#2: ffff88806a066800 (&sb->s_type->i_mutex_key#8){++++}-{3:3}, at: inode_lock include/linux/fs.h:775 [inline]
#2: ffff88806a066800 (&sb->s_type->i_mutex_key#8){++++}-{3:3}, at: ext4_buffered_write_iter+0xb0/0x460 fs/ext4/file.c:283

stack backtrace:
CPU: 0 PID: 5589 Comm: syz-executor.4 Not tainted 6.4.0-rc4-syzkaller-geb1f822c76be-dirty #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/25/2023
Call Trace:
<TASK>
__dump_stack lib/dump_stack.c:88 [inline]
dump_stack_lvl+0xd9/0x150 lib/dump_stack.c:106
check_noncircular+0x25f/0x2e0 kernel/locking/lockdep.c:2188
check_prev_add kernel/locking/lockdep.c:3113 [inline]
check_prevs_add kernel/locking/lockdep.c:3232 [inline]
validate_chain kernel/locking/lockdep.c:3847 [inline]
__lock_acquire+0x2fcd/0x5f30 kernel/locking/lockdep.c:5088
lock_acquire kernel/locking/lockdep.c:5705 [inline]
lock_acquire+0x1b1/0x520 kernel/locking/lockdep.c:5670
down_read+0x9c/0x480 kernel/locking/rwsem.c:1520
mmap_read_lock include/linux/mmap_lock.h:142 [inline]
do_user_addr_fault+0xb3d/0x1210 arch/x86/mm/fault.c:1391
handle_page_fault arch/x86/mm/fault.c:1534 [inline]
exc_page_fault+0x98/0x170 arch/x86/mm/fault.c:1590
asm_exc_page_fault+0x26/0x30 arch/x86/include/asm/idtentry.h:570
RIP: 0010:fault_in_readable+0x1a5/0x210 mm/gup.c:1856
Code: fc ff df 48 c7 04 02 00 00 00 00 48 83 c4 48 4c 89 e0 5b 5d 41 5c 41 5d 41 5e 41 5f c3 45 31 e4 eb ce e8 ae 51 c4 ff 45 31 f6 <41> 8a 45 00 31 ff 44 89 f6 88 44 24 28 e8 b9 4d c4 ff 45 85 f6 75
RSP: 0018:ffffc90006187a38 EFLAGS: 00050246
RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
RDX: ffff888026905940 RSI: ffffffff81bff672 RDI: 0000000000000007
RBP: 00000000200002cc R08: 0000000000000007 R09: 0000000000000000
R10: 00000000000002c0 R11: 1ffffffff219cbe3 R12: 000000000000000c
R13: 00000000200002c0 R14: 0000000000000000 R15: 1ffff92000c30f48
fault_in_iov_iter_readable+0x252/0x2c0 lib/iov_iter.c:362
generic_perform_write+0x1ae/0x570 mm/filemap.c:3913
ext4_buffered_write_iter+0x15b/0x460 fs/ext4/file.c:289
ext4_file_write_iter+0xbe0/0x1740 fs/ext4/file.c:710
call_write_iter include/linux/fs.h:1868 [inline]
new_sync_write fs/read_write.c:491 [inline]
vfs_write+0x945/0xd50 fs/read_write.c:584
ksys_write+0x12b/0x250 fs/read_write.c:637
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x39/0xb0 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x63/0xcd
RIP: 0033:0x7f2359a8c169
Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 f1 19 00 00 90 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 73 01 c3 48 c7 c1 b8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007f235a721168 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 00007f2359bac050 RCX: 00007f2359a8c169
RDX: 000000000000000c RSI: 00000000200002c0 RDI: 0000000000000004
RBP: 00007f2359ae7ca1 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007ffea89659df R14: 00007f235a721300 R15: 0000000000022000
</TASK>
----------------
Code disassembly (best guess), 2 bytes skipped:
0: df 48 c7 fisttps -0x39(%rax)
3: 04 02 add $0x2,%al
5: 00 00 add %al,(%rax)
7: 00 00 add %al,(%rax)
9: 48 83 c4 48 add $0x48,%rsp
d: 4c 89 e0 mov %r12,%rax
10: 5b pop %rbx
11: 5d pop %rbp
12: 41 5c pop %r12
14: 41 5d pop %r13
16: 41 5e pop %r14
18: 41 5f pop %r15
1a: c3 retq
1b: 45 31 e4 xor %r12d,%r12d
1e: eb ce jmp 0xffffffee
20: e8 ae 51 c4 ff callq 0xffc451d3
25: 45 31 f6 xor %r14d,%r14d
* 28: 41 8a 45 00 mov 0x0(%r13),%al <-- trapping instruction
2c: 31 ff xor %edi,%edi
2e: 44 89 f6 mov %r14d,%esi
31: 88 44 24 28 mov %al,0x28(%rsp)
35: e8 b9 4d c4 ff callq 0xffc44df3
3a: 45 85 f6 test %r14d,%r14d
3d: 75 .byte 0x75