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 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
> > 


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

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