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

[Jan Kara]

On Tue 30-05-23 17:39:16, Baokun Li wrote:
> On 2023/5/30 15:42, Jan Kara wrote:
> > On Tue 30-05-23 10:00:44, Baokun Li wrote:
> > > On 2023/5/29 22:44, Jan Kara wrote:
> > > > On Mon 29-05-23 16:01:48, Baokun Li wrote:
> > > > > Syzbot reported a BUG_ON:
> > > > > ==================================================================
> > > > > EXT4-fs (loop0): mounted filesystem without journal. Quota mode: none.
> > > > > EXT4-fs error (device loop0): ext4_mb_generate_buddy:1098: group 0, block
> > > > >        bitmap and bg descriptor inconsistent: 25 vs 150994969 free clusters
> > > > > ------------[ cut here ]------------
> > > > > kernel BUG at fs/ext4/ext4_jbd2.c:53!
> > > > > invalid opcode: 0000 [#1] PREEMPT SMP KASAN
> > > > > CPU: 1 PID: 494 Comm: syz-executor.0 6.1.0-rc7-syzkaller-ga4412fdd49dc #0
> > > > > RIP: 0010:__ext4_journal_stop+0x1b3/0x1c0
> > > > >    [...]
> > > > > Call Trace:
> > > > >    ext4_write_inline_data_end+0xa39/0xdf0
> > > > >    ext4_da_write_end+0x1e2/0x950
> > > > >    generic_perform_write+0x401/0x5f0
> > > > >    ext4_buffered_write_iter+0x35f/0x640
> > > > >    ext4_file_write_iter+0x198/0x1cd0
> > > > >    vfs_write+0x8b5/0xef0
> > > > >    [...]
> > > > > ==================================================================
> > > > > 
> > > > > The above BUG_ON is triggered by the following race:
> > > > > 
> > > > >              cpu1                    cpu2
> > > > > ________________________|________________________
> > > > > ksys_write
> > > > >    vfs_write
> > > > >     new_sync_write
> > > > >      ext4_file_write_iter
> > > > >       ext4_buffered_write_iter
> > > > >        generic_perform_write
> > > > >         ext4_da_write_begin
> > > > >                             do_fault
> > > > >                              do_page_mkwrite
> > > > >                               ext4_page_mkwrite
> > > > >                                ext4_convert_inline_data
> > > > >                                 ext4_convert_inline_data_nolock
> > > > >                                  ext4_destroy_inline_data_nolock
> > > > >                                   //clear EXT4_STATE_MAY_INLINE_DATA
> > > > >                                  ext4_map_blocks --> return error
> > > > >          ext4_test_inode_state(inode, EXT4_STATE_MAY_INLINE_DATA)
> > > > >          ext4_block_write_begin
> > > > >                                  ext4_restore_inline_data
> > > > >                                   // set EXT4_STATE_MAY_INLINE_DATA
> > > > >         ext4_da_write_end
> > > > >          ext4_test_inode_state(inode, EXT4_STATE_MAY_INLINE_DATA)
> > > > >          ext4_write_inline_data_end
> > > > >           handle=NULL
> > > > >           ext4_journal_stop(handle)
> > > > >            __ext4_journal_stop
> > > > >             ext4_put_nojournal(handle)
> > > > >              ref_cnt = (unsigned long)handle
> > > > >              BUG_ON(ref_cnt == 0)  ---> BUG_ON
> > > > > 
> > > > > The root cause of this problem is that the ext4_convert_inline_data() in
> > > > > ext4_page_mkwrite() does not grab i_rwsem, so it may race with
> > > > > ext4_buffered_write_iter() and cause the write_begin() and write_end()
> > > > > functions to be inconsistent and trigger BUG_ON.
> > > > > 
> > > > > To solve the above issue, we cannot add inode_lock directly to
> > > > > ext4_page_mkwrite(), because this function is a hot path and frequent calls
> > > > > to inode_lock will cause performance degradation for multi-threaded reads
> > > > > and writes. Hence, we move ext4_convert_inline_data() to ext4_file_mmap(),
> > > > > and only when inline_data is enabled and mmap a file in shared write mode,
> > > > > we hold the lock to convert, which can reduce the impact on performance.
> > > > > 
> > > > > Reported-by: Jun Nie <jun.nie@xxxxxxxxxx>
> > > > > Closes: https://lore.kernel.org/lkml/63903521.5040307@xxxxxxxxxx/t/
> > > > > Reported-by: syzbot+a158d886ca08a3fecca4@xxxxxxxxxxxxxxxxxxxxxxxxx
> > > > > Closes: https://syzkaller.appspot.com/bug?id=899b37f20ce4072bcdfecfe1647b39602e956e36
> > > > > Fixes: 7b4cc9787fe3 ("ext4: evict inline data when writing to memory map")
> > > > > CC: stable@xxxxxxxxxxxxxxx # 4.12+
> > > > > Signed-off-by: Baokun Li <libaokun1@xxxxxxxxxx>
> > > > Thanks for the patch! The problem with i_rwsem in ext4_page_mkwrite() is
> > > > not so much about performance as about lock ordering. In
> > > > ext4_page_mkwrite() we are called with mmap_sem held and so we cannot
> > > > acquire i_rwsem because it ranks about it.
> > > Thank you for your review!
> > > 
> > > I'm sorry I didn't make myself clear here.
> > > 
> > > Yes, we can't get i_rwsem after holding mmap_sem at any time, otherwise
> > > ABBA deadlock may occur. The "add inode_lock directly" in my patch
> > > description actually looks like this:
> > > ```
> > > diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> > > index b98d2d58b900..c9318dc2a613 100644
> > > --- a/fs/ext4/inode.c
> > > +++ b/fs/ext4/inode.c
> > > @@ -6025,12 +6025,14 @@ vm_fault_t ext4_page_mkwrite(struct vm_fault *vmf)
> > >          sb_start_pagefault(inode->i_sb);
> > >          file_update_time(vma->vm_file);
> > > 
> > > -       filemap_invalidate_lock_shared(mapping);
> > > -
> > > +       inode_lock(inode);
> > >          err = ext4_convert_inline_data(inode);
> > > +       inode_unlock(inode);
> > >          if (err)
> > >                  goto out_ret;
> > > 
> > > +       filemap_invalidate_lock_shared(mapping);
> > > +
> > >          /*
> > >           * On data journalling we skip straight to the transaction handle:
> > >           * there's no delalloc; page truncated will be checked later; the
> > > ```
> > Yes, but even this could deadlock. The ABBA deadlock I'm speaking about
> > would not be created with mapping->invalidate_lock but rather with
> > task->mm->mmap_lock which is acquired at the beginning of page fault in the
> > arch code.
> 
> Thanks for the explanation!
> 
> I thought the mmap_sem said was "&EXT4_I(inode)->i_mmap_sem".

We don't have that one anymore :). But yes, there are too many mmap locks so
I should have been more precise from the start. I'm sorry for the
confusion.

> But here are some more questions:
> 
> 1) In the arch code page fault is handled by mmap_read_lock(mm) to get the
>    shared lock, why would this lead to ABBA deadlock?
> 
> 2) Why would page fault be triggered in the write process?
> 
> Could you explain it in more detail?

OK, let me draw here the exact deadlock graph:

CPU0				CPU1			CPU2
write(2)			page fault		mmap(2)
				  do_user_addr_fault()
				    mmap_read_lock()      ...
				    ...                   vm_mmap_pgoff()
							    mmap_write_lock_killable(mm)
							      - blocks on CPU1
								holding the lock
  ext4_buffered_write_iter()
    inode_lock(inode);
    ...
    generic_perform_write(iocb, from);
      fault_in_iov_iter_readable(i, bytes)
        - hits pagefault
	do_user_addr_fault()
	  mmap_read_lock()
	    - blocks on CPU2 as writer queued first
				    ext4_page_mkwrite()
				      - would deadlock here if we
				        try to take inode lock

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