On Mar 17, 2023, at 9:06 AM, Jeff Layton [off-list ref] wrote:

On Fri, 2023-03-17 at 06:56 -0400, Jeff Layton wrote:

quoted

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 <redacted>
Tested-by: David Critch <redacted>
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!

quoted

+		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 [off-list ref]

--
Chuck Lever