SUNRPC: fix copying of multiple pages in gss_read_proxy_verf()

When the passed token is longer than 4032 bytes, the remaining part
of the token must be copied from the rqstp->rq_arg.pages. But the
copy must make sure it happens in a consecutive way.

With the existing code, the first memcpy copies 'length' bytes from
argv->iobase, but since the header is in front, this never fills the
whole first page of in_token->pages.

The mecpy in the loop copies the following bytes, but starts writing at
the next page of in_token->pages.  This leaves the last bytes of page 0
unwritten.

Symptoms were that users with many groups were not able to access NFS
exports, when using Active Directory as the KDC.
Signed-off-by: Martijn de Gouw <martijn.de.gouw@prodrive-technologies.com>
Fixes: 5866efa8 "SUNRPC: Fix svcauth_gss_proxy_init()"
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

net/sunrpc/auth_gss/svcauth_gss.c

@@ -1147,9 +1147,9 @@ static int gss_read_proxy_verf(struct svc_rqst *rqstp,
 			       struct gssp_in_token *in_token)
 {
 	struct kvec *argv = &rqstp->rq_arg.head[0];
-	unsigned int page_base, length;
-	int pages, i, res;
-	size_t inlen;
+	unsigned int length, pgto_offs, pgfrom_offs;
+	int pages, i, res, pgto, pgfrom;
+	size_t inlen, to_offs, from_offs;
 
 	res = gss_read_common_verf(gc, argv, authp, in_handle);
 	if (res)
@@ -1177,17 +1177,24 @@ static int gss_read_proxy_verf(struct svc_rqst *rqstp,
 	memcpy(page_address(in_token->pages[0]), argv->iov_base, length);
 	inlen -= length;
 
-	i = 1;
-	page_base = rqstp->rq_arg.page_base;
+	to_offs = length;
+	from_offs = rqstp->rq_arg.page_base;
 	while (inlen) {
-		length = min_t(unsigned int, inlen, PAGE_SIZE);
-		memcpy(page_address(in_token->pages[i]),
-		       page_address(rqstp->rq_arg.pages[i]) + page_base,
+		pgto = to_offs >> PAGE_SHIFT;
+		pgfrom = from_offs >> PAGE_SHIFT;
+		pgto_offs = to_offs & ~PAGE_MASK;
+		pgfrom_offs = from_offs & ~PAGE_MASK;
+
+		length = min_t(unsigned int, inlen,
+			 min_t(unsigned int, PAGE_SIZE - pgto_offs,
+			       PAGE_SIZE - pgfrom_offs));
+		memcpy(page_address(in_token->pages[pgto]) + pgto_offs,
+		       page_address(rqstp->rq_arg.pages[pgfrom]) + pgfrom_offs,
 		       length);
 
+		to_offs += length;
+		from_offs += length;
 		inlen -= length;
-		page_base = 0;
-		i++;
 	}
 	return 0;
 }