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

[Aleksa Sarai <cyphar@cyphar.com>]

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On 2019-12-26, Christian Brauner <christian.brauner@ubuntu.com> wrote:
> On December 26, 2019 3:32:29 PM GMT+01:00, Aleksa Sarai <cyphar@cyphar.com> 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 = {};
> >
> >> >  	memset(&unotif, 0, sizeof(unotif));
> >> >  
> >> >  	ret = down_interruptible(&filter->notif->request);
> >> > -- 
> >> > 2.20.1
> >> > 
> 
> It is an extensible struct. That's why we have notifier size checking built in.

Ah right, NOTIF_GET_SIZES. I reckon check_zeroed_user() is still a bit
simpler since none of the fields are used right now (and really, this
patch should be checking all of them, not just ->flags, if we want to
use any of them in the future).

But sure, copy_struct_from_user() also makes sense since it is
extensible (though I personally do find the whole NOTIF_GET_SIZES thing
a bit scary -- but that's water under the bridge at this point, and as
long as userspace is clever enough it shouldn't be a problem).

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

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