SUNRPC: Fix READ_PLUS crasher

stable inclusion
from stable-4.19.251
commit 917d77f59a77cab142a6758984d506addb9d414b
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I5I4FP
CVE: NA

--------------------------------

commit a23dd544 upstream.

Looks like there are still cases when "space_left - frag1bytes" can
legitimately exceed PAGE_SIZE. Ensure that xdr->end always remains
within the current encode buffer.
Reported-by: NBruce Fields <bfields@fieldses.org>
Reported-by: NZorro Lang <zlang@redhat.com>
Link: https://bugzilla.kernel.org/show_bug.cgi?id=216151
Fixes: 6c254bf3 ("SUNRPC: Fix the calculation of xdr->end in xdr_get_next_encode_buffer()")
Signed-off-by: NChuck Lever <chuck.lever@oracle.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: NYongqiang Liu <liuyongqiang13@huawei.com>

net/sunrpc/xdr.c

@@ -544,7 +544,7 @@ static __be32 *xdr_get_next_encode_buffer(struct xdr_stream *xdr,
 	 */
 	xdr->p = (void *)p + frag2bytes;
 	space_left = xdr->buf->buflen - xdr->buf->len;
-	if (space_left - nbytes >= PAGE_SIZE)
+	if (space_left - frag1bytes >= PAGE_SIZE)
 		xdr->end = (void *)p + PAGE_SIZE;
 	else
 		xdr->end = (void *)p + space_left - frag1bytes;