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

Theodore Ts'o
2023-06-04T21:09:21Z

On Sat, Jun 03, 2023 at 11:04:45PM -0400, Theodore Ts'o wrote:
> 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

Looking at this more closely, the fundamental problem is by the time
ext4_file_mmap() is called, the mm layer has already taken
current->mm->mmap_lock, and when we try to take the inode_lock, this
causes locking ordering problems with how buffered write path works,
which take the inode_lock first, and then in some cases, may end up
taking the mmap_lock if there is a page fault for the buffer used for
the buffered write.

If we're going to stick with the approach in this patch, I think what
we would need is to add a pre_mmap() function to file_operations
struct, which would get called by the mmap path *before* taking
current->mm->mmap_lock, so we can do the inline conversion before we
take the mmap_lock.

I'm not sure how the mm folks would react to such a proposal, though.
I could be seen as a bit hacky, and it's not clear that any file
system other than ext4 would need something like this. Willy, as
someone who does a lot of work in both mm and fs worlds --- I'm
curious what you think about this idea?

Thanks,

- Ted

> > 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
>