[PATCH 5.15.y] smb: client: harden POSIX SID length parsing

[Sasha Levin <sashal@kernel.org>]

From: Zihan Xi <xizh2024@lzu.edu.cn>

[ Upstream commit 7ad2bcf2441430bb2e918fb3ef9a90d775a6e422 ]

posix_info_sid_size() reads sid[1] to obtain the subauthority count,
but its existing boundary check still accepts buffers with only one
remaining byte. Require two bytes before reading sid[1] so all client
paths that reuse the helper reject truncated POSIX SIDs safely.

Fixes: 349e13ad30b4 ("cifs: add smb2 POSIX info level")
Cc: stable@vger.kernel.org
Reported-by: Yuan Tan <yuantan098@gmail.com>
Reported-by: Yifan Wu <yifanwucs@gmail.com>
Reported-by: Juefei Pu <tomapufckgml@gmail.com>
Reported-by: Xin Liu <bird@lzu.edu.cn>
Assisted-by: Codex:gpt-5.4
Signed-off-by: Zihan Xi <xizh2024@lzu.edu.cn>
Signed-off-by: Ren Wei <n05ec@lzu.edu.cn>
Signed-off-by: Steve French <stfrench@microsoft.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 fs/cifs/smb2pdu.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/cifs/smb2pdu.c b/fs/cifs/smb2pdu.c
index 302c08dfb686dc..afa7197413db9a 100644
--- a/fs/cifs/smb2pdu.c
+++ b/fs/cifs/smb2pdu.c
@@ -4592,7 +4592,7 @@ int posix_info_sid_size(const void *beg, const void *end)
 	size_t subauth;
 	int total;
 
-	if (beg + 1 > end)
+	if (beg + 2 > end)
 		return -1;
 
 	subauth = *(u8 *)(beg+1);
-- 
2.53.0