Re: [PATCH 1/2] nfsd: don't replace page in rq_pages if it's a continuation of last page

[Chuck Lever III <chuck.lever@oracle.com>]

> On Mar 17, 2023, at 9:06 AM, Jeff Layton <jlayton@kernel.org> wrote:
> 
> On Fri, 2023-03-17 at 06:56 -0400, Jeff Layton wrote:
>> The splice read calls nfsd_splice_actor to put the pages containing file
>> data into the svc_rqst->rq_pages array. It's possible however to get a
>> splice result that only has a partial page at the end, if (e.g.) the
>> filesystem hands back a short read that doesn't cover the whole page.
>> 
>> nfsd_splice_actor will plop the partial page into its rq_pages array and
>> return. Then later, when nfsd_splice_actor is called again, the
>> remainder of the page may end up being filled out. At this point,
>> nfsd_splice_actor will put the page into the array _again_ corrupting
>> the reply. If this is done enough times, rq_next_page will overrun the
>> array and corrupt the trailing fields -- the rq_respages and
>> rq_next_page pointers themselves.
>> 
>> If we've already added the page to the array in the last pass, don't add
>> it to the array a second time when dealing with a splice continuation.
>> This was originally handled properly in nfsd_splice_actor, but commit
>> 91e23b1c3982 removed the check for it, and started universally replacing
>> pages.
>> 
>> Fixes: 91e23b1c3982 ("NFSD: Clean up nfsd_splice_actor()")
>> Reported-by: Dario Lesca <d.lesca@solinos.it>
>> Tested-by: David Critch <dcritch@redhat.com>
>> Link: https://bugzilla.redhat.com/show_bug.cgi?id=2150630
>> Signed-off-by: Jeff Layton <jlayton@kernel.org>
>> ---
>> fs/nfsd/vfs.c | 7 +++++--
>> 1 file changed, 5 insertions(+), 2 deletions(-)
>> 
>> diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
>> index 502e1b7742db..3709ef57d96e 100644
>> --- a/fs/nfsd/vfs.c
>> +++ b/fs/nfsd/vfs.c
>> @@ -941,8 +941,11 @@ nfsd_splice_actor(struct pipe_inode_info *pipe, struct pipe_buffer *buf,
>> 	struct page *last_page;
>> 
>> 	last_page = page + (offset + sd->len - 1) / PAGE_SIZE;
>> -	for (page += offset / PAGE_SIZE; page <= last_page; page++)
>> -		svc_rqst_replace_page(rqstp, page);
>> +	for (page += offset / PAGE_SIZE; page <= last_page; page++) {
>> +		/* Only replace page if we haven't already done so */
> 
> Note that I think that this was probably the real rationale for the pp[-
> 1] check that 91e23b1c3982 removed. Given that, maybe we should flesh
> this comment out a bit more for posterity?
> 
> 		/*
> 		 * When we're splicing from a pipe, it's possible that
> 		 * we'll get an incomplete page that may be updated on
> 		 * a later call. Only splice it into rq_pages once.
> 		 */

The "real" bug here is that the API contract for pipe splicing
isn't well defined, so I agree that it's very likely the pp[-1]
check was because a splice can call the actor repeatedly for the
same page. No one could remember why that check was there.

To be clear, if the passed-in page matches the current page in
the rqst, we're "extending the current page" rather than avoiding
replacement... maybe:

	/*
	 * Skip page replacement when extending the contents
	 * of the current page.
	 */

In the patch description, would you mention that this case
arises if the READ request is not page-aligned?

If you resend this patch, please Cc: viro@ . Thanks for chasing
this down!


>> +		if (page != *(rqstp->rq_next_page - 1))
>> +			svc_rqst_replace_page(rqstp, page);
>> +	}
>> 	if (rqstp->rq_res.page_len == 0)	// first call
>> 		rqstp->rq_res.page_base = offset % PAGE_SIZE;
>> 	rqstp->rq_res.page_len += sd->len;
> 
> -- 
> Jeff Layton <jlayton@kernel.org>

--
Chuck Lever