nfsd: don't replace page in rq_pages if it's a continuation of last page

jira LE-1907
Rebuild_History Non-Buildable kernel-rt-5.14.0-284.30.1.rt14.315.el9_2
commit-author Jeff Layton <jlayton@kernel.org>
commit 27c934d

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
91e23b1 ("NFSD: Clean up nfsd_splice_actor()") removed the check
for it.

Fixes: 91e23b1 ("NFSD: Clean up nfsd_splice_actor()")
	Cc: Al Viro <viro@zeniv.linux.org.uk>
	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>
	Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
(cherry picked from commit 27c934d)
	Signed-off-by: Jonathan Maple <jmaple@ciq.com>

Author: PlaidCat

fs/nfsd/vfs.c

@@ -850,8 +850,15 @@ 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++)
+	for (page += offset / PAGE_SIZE; page <= last_page; page++) {
+		/*
+		 * Skip page replacement when extending the contents
+		 * of the current page.
+		 */
+		if (page == *(rqstp->rq_next_page - 1))
+			continue;
 		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;