[PATCH 5.15.y] SUNRPC: fix oversized GSS proxy token copy

[Sasha Levin <sashal@kernel.org>]

From: Jérémy Jean <Jeremy.Jean@oss.cyber.gouv.fr>

[ Upstream commit 62319c2534b603b416fa2aa2f3ef18c311f75010 ]

gss_read_proxy_verf() allocates one page per token chunk, but it copies the
entire linear head of an RPC request into in_token->pages[0]. A UDP datagram
can leave more than PAGE_SIZE bytes in the linear head, so a large
RPCSEC_GSS INIT token overwrites following pages.

Copy the linear head in page-sized chunks before switching to the request
page array. The linear head itself can span multiple pages.

Fixes: 5866efa8cbfb ("SUNRPC: Fix svcauth_gss_proxy_init()")
Cc: stable@vger.kernel.org
Signed-off-by: Jérémy Jean <Jeremy.Jean@oss.cyber.gouv.fr>
Assisted-by: Codex:gpt-5
Link: https://patch.msgid.link/20260910125120.3798995-1-Jeremy.Jean@oss.cyber.gouv.fr
Signed-off-by: Chuck Lever <cel@kernel.org>
[ sashal: adapted xdr stream pointer and length access to argv->iov_base and argv->iov_len ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 net/sunrpc/auth_gss/svcauth_gss.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/net/sunrpc/auth_gss/svcauth_gss.c b/net/sunrpc/auth_gss/svcauth_gss.c
index 8beca18863907..0468b9f0163fb 100644
--- a/net/sunrpc/auth_gss/svcauth_gss.c
+++ b/net/sunrpc/auth_gss/svcauth_gss.c
@@ -1185,10 +1185,11 @@ static int gss_read_proxy_verf(struct svc_rqst *rqstp,
 	}
 
 	length = min_t(unsigned int, inlen, argv->iov_len);
-	if (length)
-		memcpy(page_address(in_token->pages[0]), argv->iov_base, length);
+	for (to_offs = 0; to_offs < length; to_offs += PAGE_SIZE)
+		memcpy(page_address(in_token->pages[to_offs >> PAGE_SHIFT]),
+		       (char *)argv->iov_base + to_offs,
+		       min_t(unsigned int, length - to_offs, PAGE_SIZE));
 	inlen -= length;
-
 	to_offs = length;
 	from_offs = rqstp->rq_arg.page_base;
 	while (inlen) {
-- 
2.53.0