[PATCH 6.1.y] SUNRPC: svcauth_gss: avoid NULL deref on zero length gss_token in gss_read_proxy_verf

[Sasha Levin <sashal@kernel.org>]

From: Joshua Rogers <linux@joshua.hu>

[ Upstream commit d4b69a6186b215d2dc1ebcab965ed88e8d41768d ]

A zero length gss_token results in pages == 0 and in_token->pages[0]
is NULL. The code unconditionally evaluates
page_address(in_token->pages[0]) for the initial memcpy, which can
dereference NULL even when the copy length is 0. Guard the first
memcpy so it only runs when length > 0.

Fixes: 5866efa8cbfb ("SUNRPC: Fix svcauth_gss_proxy_init()")
Cc: stable@vger.kernel.org
Signed-off-by: Joshua Rogers <linux@joshua.hu>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
[ adapted xdr buffer pointer API to older argv iov_base/iov_len API ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 net/sunrpc/auth_gss/svcauth_gss.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/sunrpc/auth_gss/svcauth_gss.c b/net/sunrpc/auth_gss/svcauth_gss.c
index d0575747ff0e..df0b8b1cdfc4 100644
--- a/net/sunrpc/auth_gss/svcauth_gss.c
+++ b/net/sunrpc/auth_gss/svcauth_gss.c
@@ -1179,7 +1179,8 @@ static int gss_read_proxy_verf(struct svc_rqst *rqstp,
 	}
 
 	length = min_t(unsigned int, inlen, argv->iov_len);
-	memcpy(page_address(in_token->pages[0]), argv->iov_base, length);
+	if (length)
+		memcpy(page_address(in_token->pages[0]), argv->iov_base, length);
 	inlen -= length;
 
 	to_offs = length;
-- 
2.51.0