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

[Kees Cook <kees@kernel.org>]

On Thu, Oct 23, 2025 at 12:47:32PM +0200, Paolo Abeni wrote:
> On 10/20/25 11:26 PM, Kees Cook wrote:
> > 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