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

[Jan Kara]

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.

> ---
>  fs/ext4/file.c  | 24 +++++++++++++++++++++++-
>  fs/ext4/inode.c |  4 ----
>  2 files changed, 23 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/ext4/file.c b/fs/ext4/file.c
> index d101b3b0c7da..7a04376c33f2 100644
> --- a/fs/ext4/file.c
> +++ b/fs/ext4/file.c
> @@ -795,7 +795,8 @@ static const struct vm_operations_struct ext4_file_vm_ops = {
>  static int ext4_file_mmap(struct file *file, struct vm_area_struct *vma)
>  {
>  	struct inode *inode = file->f_mapping->host;
> -	struct ext4_sb_info *sbi = EXT4_SB(inode->i_sb);
> +	struct super_block *sb = inode->i_sb;
> +	struct ext4_sb_info *sbi = EXT4_SB(sb);
>  	struct dax_device *dax_dev = sbi->s_daxdev;
>  
>  	if (unlikely(ext4_forced_shutdown(sbi)))
> @@ -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_WRITE) {

Sadly this does not work because we can mmap(2) the file read-only and then
use mprotect(2) to make file writeable. But we can test for VM_MAYWRITE
which gets set when mapping can be made writeable (basically anytime when
the file descriptor itself is writeable).

Otherwise the patch looks good.

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