[PATCH] knfsd: fix an NFSD bug with full sized, non-page-aligned reads

NFSd assumes that largest number of pages that will be needed for a
request+response is 2+N where N pages is the size of the largest permitted
read/write request.  The '2' are 1 for the non-data part of the request, and 1
for the non-data part of the reply.

However, when a read request is not page-aligned, and we choose to use
->sendfile to send it directly from the page cache, we may need N+1 pages to
hold the whole reply.  This can overflow and array and cause an Oops.

This patch increases size of the array for holding pages by one and makes sure
that entry is NULL when it is not in use.
Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/nfsd/vfs.c

@@ -822,7 +822,8 @@ nfsd_read_actor(read_descriptor_t *desc, struct page *page, unsigned long offset
 		rqstp->rq_res.page_len = size;
 	} else if (page != pp[-1]) {
 		get_page(page);
-		put_page(*pp);
+		if (*pp)
+			put_page(*pp);
 		*pp = page;
 		rqstp->rq_resused++;
 		rqstp->rq_res.page_len += size;

include/linux/sunrpc/svc.h

@@ -144,8 +144,11 @@ extern u32 svc_max_payload(const struct svc_rqst *rqstp);
  *
  * Each request/reply pair can have at most one "payload", plus two pages,
  * one for the request, and one for the reply.
+ * We using ->sendfile to return read data, we might need one extra page
+ * if the request is not page-aligned.  So add another '1'.
  */
-#define RPCSVC_MAXPAGES		((RPCSVC_MAXPAYLOAD+PAGE_SIZE-1)/PAGE_SIZE + 2)
+#define RPCSVC_MAXPAGES		((RPCSVC_MAXPAYLOAD+PAGE_SIZE-1)/PAGE_SIZE \
+				+ 2 + 1)
 
 static inline u32 svc_getnl(struct kvec *iov)
 {

net/sunrpc/svcsock.c

@@ -1278,6 +1278,8 @@ svc_recv(struct svc_rqst *rqstp, long timeout)
 				schedule_timeout_uninterruptible(msecs_to_jiffies(500));
 			rqstp->rq_pages[i] = p;
 		}
+	rqstp->rq_pages[i++] = NULL; /* this might be seen in nfs_read_actor */
+	BUG_ON(pages >= RPCSVC_MAXPAGES);
 
 	/* Make arg->head point to first page and arg->pages point to rest */
 	arg = &rqstp->rq_arg;