Re: Splice & iomap dio problems

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Wed, Nov 13, 2019 at 07:00:32PM +0100, Jan Kara wrote:
> Hello,
> 
> I've spent today tracking down the syzkaller report of a WARN_ON hit in
> iov_iter_pipe() [1]. The immediate problem is that syzkaller reproducer
> (calling sendfile(2) from different threads at the same time a file to the
> same file in rather evil way) results in splice code leaking pipe pages
> (nrbufs doesn't return to 0 after read+write in the splice) and eventually
> we run out of pipe pages and hit the warning in iov_iter_pipe(). The
> problem is not specific to ext4, I can see in my tracing that when the
> underlying filesystem is XFS, we can leak the pipe pages in the same way
> (but for XFS somehow the problem doesn't happen as often).  Rather the
> problem seems to be in how iomap direct IO code, pipe iter code, and splice
> code interact.
> 
> So the problematic situation is when we do direct IO read into pipe pages
> and the read hits EOF which is not on page boundary. Say the file has 4608
> (4096+512) bytes, block size == page size == 4096. What happens is that iomap
> code maps the extent, gets that the extent size is 8192 (mapping ignores

I wonder, would this work properly if the read side returns a 4608-byte
mapping instead of an 8192-byte mapping?  It doesn't make a lot of sense
(to me, anyway) for a read mapping to go beyond EOF.

> i_size). Then we call iomap_dio_bio_actor(), which creates its private
> iter, truncates it to 8192, and calls bio_iov_iter_get_pages(). That
> eventually results in preparing two pipe buffers with length 4096 to accept
> the read. Then read completes, in iomap_dio_complete() we truncate the return
> value from 8192 (which was the real amount of IO we performed) to 4608. Now
> this amount (4608) gets passed through splice code to
> iter_file_splice_write(), we write out that amount, but then when cleaning
> up pipe buffers, the last pipe buffer has still 3584 unused so we leave
> the pipe buffer allocated and effectively leak it.
> 
> Now I was also investigating why the old direct IO code doesn't leak pipe
> buffers like this and the trick is done by the iov_iter_revert() call
> generic_file_read_iter(). This results in setting iter position right to
> the position where direct IO read reported it ended (4608) and truncating
> pipe buffers after this point. So splice code then sees the second pipe
> buffer has length only 512 which matches the amount it was asked to write
> and so the pipe buffer gets freed after the write in
> iter_file_splice_write().
> 
> The question is how to best fix this. The quick fix is to add
> iov_iter_revert() call to iomap_dio_rw() so that in case of sync IO (we
> always do only sync IO to pipes), we properly set iter position in case of
> short read / write. But it looks somewhat hacky to me and this whole
> interaction of iter and pipes looks fragile to me.
> 
> Another option I can see is to truncate the iter to min(i_size-pos, length) in
> iomap_dio_bio_actor() which *should* do the trick AFAICT. But I'm not sure
> if it won't break something else.

Do the truncation in ->iomap_begin on the read side, as I suggested above?

> Any other ideas?
> 
> As a side note the logic copying iter in iomap_dio_bio_actor() looks
> suspicious. We copy 'dio->submit.iter' to 'iter' but then in the loop we call
> iov_iter_advance() on dio->submit.iter. So if bio_iov_iter_get_pages()
> didn't return enough pages and we loop again, 'iter' will have stale
> contents and things go sideways from there? What am I missing? And why do
> we do that strange copying of iter instead of using iov_iter_truncate() and
> iov_iter_reexpand() on the 'dio->submit.iter' directly?

I'm similarly puzzled; I would've thought that we'd need to advance the
private @iter too.  Or just truncate and reexpand the dio->submit.iter
and not have the private one.

With any luck hch will have some ideas? :/

--D

> 
> 								Honza
> 
> [1] https://lore.kernel.org/lkml/000000000000d60aa50596c63063@google.com
> 
> -- 
> Jan Kara <jack@suse.com>
> SUSE Labs, CR