Re: [PATCH] seccomp: Check flags on seccomp_notif is unset

[Tycho Andersen]

On Fri, Dec 27, 2019 at 01:32:29AM +1100, Aleksa Sarai wrote:
> On 2019-12-26, Christian Brauner <christian.brauner@xxxxxxxxxx> wrote:
> > On Wed, Dec 25, 2019 at 09:45:33PM +0000, Sargun Dhillon wrote:
> > > This patch is a small change in enforcement of the uapi for
> > > SECCOMP_IOCTL_NOTIF_RECV ioctl. Specificaly, the datastructure which is
> > > passed (seccomp_notif), has a flags member. Previously that could be
> > > set to a nonsense value, and we would ignore it. This ensures that
> > > no flags are set.
> > > 
> > > Signed-off-by: Sargun Dhillon <sargun@xxxxxxxxx>
> > > Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> > 
> > I'm fine with this since we soon want to make use of the flag argument
> > when we add a flag to get a pidfd from the seccomp notifier on receive.
> > The major users I could identify already pass in seccomp_notif with all
> > fields set to 0. If we really break users we can always revert; this
> > seems very unlikely to me though.
> > 
> > One more question below, otherwise:
> > 
> > Reviewed-by: Christian Brauner <christian.brauner@xxxxxxxxxx>
> > 
> > > ---
> > >  kernel/seccomp.c | 7 +++++++
> > >  1 file changed, 7 insertions(+)
> > > 
> > > diff --git a/kernel/seccomp.c b/kernel/seccomp.c
> > > index 12d2227e5786..455925557490 100644
> > > --- a/kernel/seccomp.c
> > > +++ b/kernel/seccomp.c
> > > @@ -1026,6 +1026,13 @@ static long seccomp_notify_recv(struct seccomp_filter *filter,
> > >  	struct seccomp_notif unotif;
> > >  	ssize_t ret;
> > >  
> > > +	if (copy_from_user(&unotif, buf, sizeof(unotif)))
> > > +		return -EFAULT;
> > > +
> > > +	/* flags is reserved right now, make sure it's unset */
> > > +	if (unotif.flags)
> > > +		return -EINVAL;
> > > +
> > 
> > Might it make sense to use
> > 
> > 	err = copy_struct_from_user(&unotif, sizeof(unotif), buf, sizeof(unotif));
> > 	if (err)
> > 		return err;
> > 
> > This way we check that the whole struct is 0 and report an error as soon
> > as one of the members is non-zero. That's more drastic but it'd ensure
> > that other fields can be used in the future for whatever purposes.
> > It would also let us get rid of the memset() below. 
> 
> Given that this isn't an extensible struct, it would be simpler to just do
> check_zeroed_user() -- copy_struct_from_user() is overkill. That would
> also remove the need for any copy_from_user()s and the memset can be
> dropped by just doing
> 
>   struct seccomp_notif unotif = {};

This doesn't zero the padding according to the C standard, so no, you
can't drop the memset, or you may leak kernel stack bits.

As for the rest of it, while it is an ABI change I think all of the
users are CC'd on this thread, and it's an obvious goof on my part :).
So:

Acked-by: Tycho Andersen <tycho@xxxxxxxx>

Tycho