[PATCH v1 2/3] SUNRPC: add bounds checking to svc_rqst_replace_page

[Chuck Lever <cel@xxxxxxxxxx>]

From: Chuck Lever <chuck.lever@xxxxxxxxxx>

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

Suggested-by: Jeff Layton <jlayton@xxxxxxxxxx>
Signed-off-by: Chuck Lever <chuck.lever@xxxxxxxxxx>
---
 include/linux/sunrpc/svc.h    |    2 +-
 include/trace/events/sunrpc.h |   25 +++++++++++++++++++++++++
 net/sunrpc/svc.c              |   15 ++++++++++++++-
 3 files changed, 40 insertions(+), 2 deletions(-)

diff --git a/include/linux/sunrpc/svc.h b/include/linux/sunrpc/svc.h
index 877891536c2f..f5af055280ff 100644
--- a/include/linux/sunrpc/svc.h
+++ b/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 *);
diff --git a/include/trace/events/sunrpc.h b/include/trace/events/sunrpc.h
index 3ca54536f8f7..5a3bb42e1f50 100644
--- a/include/trace/events/sunrpc.h
+++ b/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
diff --git a/net/sunrpc/svc.c b/net/sunrpc/svc.c
index fea7ce8fba14..633aa1eb476b 100644
--- a/net/sunrpc/svc.c
+++ b/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);