NFSD: Watch for rq_pages bounds checking errors in nfsd_splice_actor()

There have been several bugs over the years where the NFSD splice
actor has attempted to write outside the rq_pages array.

This is a "should never happen" condition, but if for some reason
the pipe splice actor should attempt to walk past the end of
rq_pages, it needs to terminate the READ operation to prevent
corruption of the pointer addresses in the fields just beyond the
array.

A server crash is thus prevented. Since the code is not behaving,
the READ operation returns -EIO to the client. None of the READ
payload data can be trusted if the splice actor isn't operating as
expected.
Suggested-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>

fs/nfsd/vfs.c

@@ -930,6 +930,9 @@ nfsd_open_verified(struct svc_rqst *rqstp, struct svc_fh *fhp, int may_flags,
  * Grab and keep cached pages associated with a file in the svc_rqst
  * so that they can be passed to the network sendmsg/sendpage routines
  * directly. They will be released after the sending has completed.
+ *
+ * Return values: Number of bytes consumed, or -EIO if there are no
+ * remaining pages in rqstp->rq_pages.
  */
 static int
 nfsd_splice_actor(struct pipe_inode_info *pipe, struct pipe_buffer *buf,
@@ -948,7 +951,8 @@ nfsd_splice_actor(struct pipe_inode_info *pipe, struct pipe_buffer *buf,
 		 */
 		if (page == *(rqstp->rq_next_page - 1))
 			continue;
-		svc_rqst_replace_page(rqstp, page);
+		if (unlikely(!svc_rqst_replace_page(rqstp, page)))
+			return -EIO;
 	}
 	if (rqstp->rq_res.page_len == 0)	// first call
 		rqstp->rq_res.page_base = offset % PAGE_SIZE;

include/linux/sunrpc/svc.h

@@ -422,7 +422,7 @@ struct svc_serv *svc_create(struct svc_program *, unsigned int,
 			    int (*threadfn)(void *data));
 struct svc_rqst *svc_rqst_alloc(struct svc_serv *serv,
 					struct svc_pool *pool, int node);
-void		   svc_rqst_replace_page(struct svc_rqst *rqstp,
+bool		   svc_rqst_replace_page(struct svc_rqst *rqstp,
 					 struct page *page);
 void		   svc_rqst_free(struct svc_rqst *);
 void		   svc_exit_thread(struct svc_rqst *);

include/trace/events/sunrpc.h

@@ -1790,6 +1790,31 @@ DEFINE_EVENT(svc_rqst_status, svc_send,
 	TP_PROTO(const struct svc_rqst *rqst, int status),
 	TP_ARGS(rqst, status));
 
+TRACE_EVENT(svc_replace_page_err,
+	TP_PROTO(const struct svc_rqst *rqst),
+
+	TP_ARGS(rqst),
+	TP_STRUCT__entry(
+		SVC_RQST_ENDPOINT_FIELDS(rqst)
+
+		__field(const void *, begin)
+		__field(const void *, respages)
+		__field(const void *, nextpage)
+	),
+
+	TP_fast_assign(
+		SVC_RQST_ENDPOINT_ASSIGNMENTS(rqst);
+
+		__entry->begin = rqst->rq_pages;
+		__entry->respages = rqst->rq_respages;
+		__entry->nextpage = rqst->rq_next_page;
+	),
+
+	TP_printk(SVC_RQST_ENDPOINT_FORMAT " begin=%p respages=%p nextpage=%p",
+		SVC_RQST_ENDPOINT_VARARGS,
+		__entry->begin, __entry->respages, __entry->nextpage)
+);
+
 TRACE_EVENT(svc_stats_latency,
 	TP_PROTO(
 		const struct svc_rqst *rqst

net/sunrpc/svc.c

@@ -842,9 +842,21 @@ EXPORT_SYMBOL_GPL(svc_set_num_threads);
  *
  * When replacing a page in rq_pages, batch the release of the
  * replaced pages to avoid hammering the page allocator.
+ *
+ * Return values:
+ *   %true: page replaced
+ *   %false: array bounds checking failed
  */
-void svc_rqst_replace_page(struct svc_rqst *rqstp, struct page *page)
+bool svc_rqst_replace_page(struct svc_rqst *rqstp, struct page *page)
 {
+	struct page **begin = rqstp->rq_pages;
+	struct page **end = &rqstp->rq_pages[RPCSVC_MAXPAGES];
+
+	if (unlikely(rqstp->rq_next_page < begin || rqstp->rq_next_page > end)) {
+		trace_svc_replace_page_err(rqstp);
+		return false;
+	}
+
 	if (*rqstp->rq_next_page) {
 		if (!pagevec_space(&rqstp->rq_pvec))
 			__pagevec_release(&rqstp->rq_pvec);
@@ -853,6 +865,7 @@ void svc_rqst_replace_page(struct svc_rqst *rqstp, struct page *page)
 
 	get_page(page);
 	*(rqstp->rq_next_page++) = page;
+	return true;
 }
 EXPORT_SYMBOL_GPL(svc_rqst_replace_page);