Re: [PATCH] ext4: avoid exposure of stale data in ext4_punch_hole()

[Jan Kara]

  Hi,

On Fri 27-09-13 17:05:18, Maxim Patlasov wrote:
> On 09/26/2013 10:53 PM, Jan Kara wrote:
> >   Hello,
> >
> >On Thu 26-09-13 21:32:07, Maxim Patlasov wrote:
> >>While handling punch-hole fallocate, it's useless to truncate page cache
> >>before removing the range from extent tree (or block map in indirect case)
> >>because page cache can be re-populated (by read-ahead or read(2) or mmap-ed
> >>read) immediately after truncating page cache, but before updating extent
> >>tree (or block map). In that case the user will see stale data even after
> >>fallocate is completed.
> >   Yes, this is a known problem. The trouble is there isn't a reliable fix
> >currently possible. If we don't truncate page cache before removing blocks,
> >we will have pages in memory being backed by already freed blocks - not
> >good as that can lead to data corruption. So you should't really remove the
> >truncation from before we remove the blocks.
> 
> I'd like to understand the problem better. Could you please provide
> any details about that data corruption? And if it was already
> discussed somewhere, please point me to there.
  It was discussed at linux-ext4 / linux-fsdevel sometime in spring but I'm
not sure to which extent we covered details of the races. Anyway if you
have blocks in pagecache which point to already freed blocks (lets say they
belong to file foo), the following
can happen:
  1) Someone reallocates freed blocks for another file bar. And writes new
     data to them. Writeback flushes the data to disk.
  2) Someone dirties pages with stale mapping data in file foo.
  3) Writeback writes dirty pages of foo, overwriting data in bar.

> >You are right that if punch hole races with page fault or read, we can
> >create again pages with block mapping which will become stale soon and the
> >same problem as I wrote above applies. Truncating pagecache after we
> >removed blocks only narrows the race window but doesn't really fix the
> >problem.
> 
> There seems to be two different problems: 1) pages backed by already
> freed blocks; 2) keeping page-cache populated by pages with stale
> data after fallocate completes. Your concerns refer to the first
> problem. My patch was intended to resolve the second. It seems to me
> that my patch really fixes the second problem and it doesn't make
> things worse w.r.t. the fisrt problem. Do I miss something?
  IMHO these are different aspects of the same problem but that's not
important. Your patch actually makes things worse because currently if the
file isn't written to via mmap while punch_hole is running, everything is
fine. After your patch writeback of old data could race with punch hole
freeing blocks resulting in data corruption I have described above. The
'no-harm' solution would be to add another truncation of pagecache after
punch hole is done. I think that would be a good way to reduce the race
window before the problem gets fixed properly.

								Honza

> >>Signed-off-by: Maxim Patlasov <mpatlasov@xxxxxxxxxxxxx>
> >>---
> >>  fs/ext4/inode.c |   17 +++++++++--------
> >>  1 file changed, 9 insertions(+), 8 deletions(-)
> >>
> >>diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> >>index 0d424d7..6b71116 100644
> >>--- a/fs/ext4/inode.c
> >>+++ b/fs/ext4/inode.c
> >>@@ -3564,14 +3564,6 @@ int ext4_punch_hole(struct inode *inode, loff_t offset, loff_t length)
> >>  	}
> >>-	first_block_offset = round_up(offset, sb->s_blocksize);
> >>-	last_block_offset = round_down((offset + length), sb->s_blocksize) - 1;
> >>-
> >>-	/* Now release the pages and zero block aligned part of pages*/
> >>-	if (last_block_offset > first_block_offset)
> >>-		truncate_pagecache_range(inode, first_block_offset,
> >>-					 last_block_offset);
> >>-
> >>  	/* Wait all existing dio workers, newcomers will block on i_mutex */
> >>  	ext4_inode_block_unlocked_dio(inode);
> >>  	inode_dio_wait(inode);
> >>@@ -3621,6 +3613,15 @@ int ext4_punch_hole(struct inode *inode, loff_t offset, loff_t length)
> >>  	up_write(&EXT4_I(inode)->i_data_sem);
> >>  	if (IS_SYNC(inode))
> >>  		ext4_handle_sync(handle);
> >>+
> >>+	first_block_offset = round_up(offset, sb->s_blocksize);
> >>+	last_block_offset = round_down((offset + length), sb->s_blocksize) - 1;
> >>+
> >>+	/* Now release the pages and zero block aligned part of pages */
> >>+	if (last_block_offset > first_block_offset)
> >>+		truncate_pagecache_range(inode, first_block_offset,
> >>+					 last_block_offset);
> >>+
> >>  	inode->i_mtime = inode->i_ctime = ext4_current_time(inode);
> >>  	ext4_mark_inode_dirty(handle, inode);
> >>  out_stop:
> >>
> >>--
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> 
-- 
Jan Kara <jack@xxxxxxx>
SUSE Labs, CR
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