rpc: fix xdr_truncate_encode to handle buffer ending on page boundary

A struct xdr_stream at a page boundary might point to the end of one
page or the beginning of the next, but xdr_truncate_encode isn't
prepared to handle the former.

This can cause corruption of NFSv4 READDIR replies in the case that a
readdir entry that would have exceeded the client's dircount/maxcount
limit would have ended exactly on a 4k page boundary.  You're more
likely to hit this case on large directories.

Other xdr_truncate_encode callers are probably also affected.
Reported-by: NHolger Hoffstätte <holger.hoffstaette@googlemail.com>
Tested-by: NHolger Hoffstätte <holger.hoffstaette@googlemail.com>
Fixes: 3e19ce76 "rpc: xdr_truncate_encode"
Cc: stable@vger.kernel.org
Signed-off-by: NJ. Bruce Fields <bfields@redhat.com>

net/sunrpc/xdr.c

@@ -606,7 +606,7 @@ void xdr_truncate_encode(struct xdr_stream *xdr, size_t len)
 	struct kvec *head = buf->head;
 	struct kvec *tail = buf->tail;
 	int fraglen;
-	int new, old;
+	int new;
 
 	if (len > buf->len) {
 		WARN_ON_ONCE(1);
@@ -629,8 +629,8 @@ void xdr_truncate_encode(struct xdr_stream *xdr, size_t len)
 	buf->len -= fraglen;
 
 	new = buf->page_base + buf->page_len;
-	old = new + fraglen;
-	xdr->page_ptr -= (old >> PAGE_SHIFT) - (new >> PAGE_SHIFT);
+
+	xdr->page_ptr = buf->pages + (new >> PAGE_SHIFT);
 
 	if (buf->page_len) {
 		xdr->p = page_address(*xdr->page_ptr);