sunrpc: Avoid a KASAN slab-out-of-bounds bug in xdr_set_page_base()

This seems to happen fairly easily during READ_PLUS testing on NFS v4.2.
I found that we could end up accessing xdr->buf->pages[pgnr] with a pgnr
greater than the number of pages in the array. So let's just return
early if we're setting base to a point at the end of the page data and
let xdr_set_tail_base() handle setting up the buffer pointers instead.
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
Fixes: 8d86e373 ("SUNRPC: Clean up helpers xdr_set_iov() and xdr_set_page_base()")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

net/sunrpc/xdr.c

@@ -1230,10 +1230,9 @@ static unsigned int xdr_set_page_base(struct xdr_stream *xdr,
 	void *kaddr;
 
 	maxlen = xdr->buf->page_len;
-	if (base >= maxlen) {
-		base = maxlen;
-		maxlen = 0;
-	} else
+	if (base >= maxlen)
+		return 0;
+	else
 		maxlen -= base;
 	if (len > maxlen)
 		len = maxlen;