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

[Jan Kara <jack@suse.cz>]

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@linaro.org>
> > > Closes: https://lore.kernel.org/lkml/63903521.5040307@huawei.com/t/
> > > Reported-by: syzbot+a158d886ca08a3fecca4@syzkaller.appspotmail.com
> > > Closes: https://syzkaller.appspot.com/bug?id=899b37f20ce4072bcdfecfe1647b39602e956e36
> > > Fixes: 7b4cc9787fe3 ("ext4: evict inline data when writing to memory map")
> > > CC: stable@vger.kernel.org # 4.12+
> > > Signed-off-by: Baokun Li <libaokun1@huawei.com>
> > 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. And when you do write(2), you hold inode_lock() and then you
copy data from the use provided buffer to the pagecache pages and that can
cause a page fault on the user provided buffer which will try to grab
task->mm->mmap_lock.

This lock inversion is the main reason why inode lock cannot be used
anywhere in the page fault path.

								Honza

-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR