Re: [PATCH v3 2/9] net/l2tp: Add missing sa_family validation in pppol2tp_sockaddr_get_info

On Thu, Oct 23, 2025 at 12:47:32PM +0200, Paolo Abeni wrote:

On 10/20/25 11:26 PM, Kees Cook wrote:

quoted

While reviewing the struct proto_ops connect() and bind() callback
implementations, I noticed that there doesn't appear to be any
validation that AF_PPPOX sockaddr structures actually have sa_family set
to AF_PPPOX. The pppol2tp_sockaddr_get_info() checks only look at the
sizes.

I don't see any way that this might actually cause problems as specific
info fields are being populated, for which the existing size checks are
correct, but it stood out as a missing address family check.

Add the check and return -EAFNOSUPPORT on mismatch.

Signed-off-by: Kees Cook <kees@kernel.org>
---
 net/l2tp/l2tp_ppp.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/net/l2tp/l2tp_ppp.c b/net/l2tp/l2tp_ppp.c
index 5e12e7ce17d8..b7a9c224520f 100644
--- a/net/l2tp/l2tp_ppp.c
+++ b/net/l2tp/l2tp_ppp.c

@@ -535,6 +535,13 @@ struct l2tp_connect_info {
 static int pppol2tp_sockaddr_get_info(const void *sa, int sa_len,
 				      struct l2tp_connect_info *info)
 {
+	const struct sockaddr_unspec *sockaddr = sa;
+
+	if (sa_len < offsetofend(struct sockaddr, sa_family))
+		return -EINVAL;
+	if (sockaddr->sa_family != AF_PPPOX)
+		return -EAFNOSUPPORT;

I fear we can't introduce this check, as it could break existing
user-space application currently passing random data into sa_family but
still able to connect successfully.

Isn't sa_family kind of the critical determining factor on how the
network stack decides to handle sockaddr stuff? I'll drop it for now,
I guess, but that's surprising to me.

-Kees

-- 
Kees Cook