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

[Aleksa Sarai <cyphar@cyphar.com>]

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On 2019-12-26, Tycho Andersen <tycho@tycho.ws> wrote:
> On Fri, Dec 27, 2019 at 01:32:29AM +1100, Aleksa Sarai wrote:
> > On 2019-12-26, Christian Brauner <christian.brauner@ubuntu.com> 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@sargun.me>
> > > > Cc: Kees Cook <keescook@chromium.org>
> > > 
> > > 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@ubuntu.com>
> > > 
> > > > ---
> > > >  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.

Ah right, I didn't double-check if there was any un-named. IMHO, It's a
bit odd to have un-named padding in a struct intended for extensions
(specifically to avoid these problems -- because it means userspace will
pass garbage by accident and there's nothing we can do about it). But
it's a bit late to worry about that now. :P

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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