SUNRPC: svcauth_gss: avoid NULL deref on zero length gss_token in gss_read_proxy_verf

SUNRPC: svcauth_gss: avoid NULL deref on zero length gss_token in gss_read_proxy_verf

[ 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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>