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

["Theodore Ts'o" <tytso@mit.edu>]

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
>