svcrdma: underflow issue in decode_write_list()

My static checker marks everything from ntohl() as untrusted and it
complains we could have an underflow problem doing:

	return (u32 *)&ary->wc_array[nchunks];

Also on 32 bit systems the upper bound check could overflow.

Cc: stable@vger.kernel.org
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

net/sunrpc/xprtrdma/svc_rdma_marshal.c

@@ -98,6 +98,7 @@ void svc_rdma_rcl_chunk_counts(struct rpcrdma_read_chunk *ch,
  */
 static u32 *decode_write_list(u32 *va, u32 *vaend)
 {
+	unsigned long start, end;
 	int nchunks;
 
 	struct rpcrdma_write_array *ary =
@@ -113,9 +114,12 @@ static u32 *decode_write_list(u32 *va, u32 *vaend)
 		return NULL;
 	}
 	nchunks = ntohl(ary->wc_nchunks);
-	if (((unsigned long)&ary->wc_array[0] +
-	     (sizeof(struct rpcrdma_write_chunk) * nchunks)) >
-	    (unsigned long)vaend) {
+
+	start = (unsigned long)&ary->wc_array[0];
+	end = (unsigned long)vaend;
+	if (nchunks < 0 ||
+	    nchunks > (SIZE_MAX - start) / sizeof(struct rpcrdma_write_chunk) ||
+	    (start + (sizeof(struct rpcrdma_write_chunk) * nchunks)) > end) {
 		dprintk("svcrdma: ary=%p, wc_nchunks=%d, vaend=%p\n",
 			ary, nchunks, vaend);
 		return NULL;
@@ -129,6 +133,7 @@ static u32 *decode_write_list(u32 *va, u32 *vaend)
 
 static u32 *decode_reply_array(u32 *va, u32 *vaend)
 {
+	unsigned long start, end;
 	int nchunks;
 	struct rpcrdma_write_array *ary =
 		(struct rpcrdma_write_array *)va;
@@ -143,9 +148,12 @@ static u32 *decode_reply_array(u32 *va, u32 *vaend)
 		return NULL;
 	}
 	nchunks = ntohl(ary->wc_nchunks);
-	if (((unsigned long)&ary->wc_array[0] +
-	     (sizeof(struct rpcrdma_write_chunk) * nchunks)) >
-	    (unsigned long)vaend) {
+
+	start = (unsigned long)&ary->wc_array[0];
+	end = (unsigned long)vaend;
+	if (nchunks < 0 ||
+	    nchunks > (SIZE_MAX - start) / sizeof(struct rpcrdma_write_chunk) ||
+	    (start + (sizeof(struct rpcrdma_write_chunk) * nchunks)) > end) {
 		dprintk("svcrdma: ary=%p, wc_nchunks=%d, vaend=%p\n",
 			ary, nchunks, vaend);
 		return NULL;