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

[Aleksa Sarai]

On 2019-12-27, Aleksa Sarai <cyphar@xxxxxxxxxx> wrote:
> On 2019-12-26, Christian Brauner <christian.brauner@xxxxxxxxxx> wrote:
> > On December 26, 2019 3:32:29 PM GMT+01:00, Aleksa Sarai <cyphar@xxxxxxxxxx> 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 = {};
> > >
> > >> >  	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).

Scratch that -- as Tycho just mentioned, there is un-named padding in
the struct so check_zeroed_user() is the wrong thing to do. But this
also will make extensions harder to deal with because (presumably) they
will also have un-named padding, making copy_struct_from_user() the
wrong thing to do as well.

So while there's not much to be done to fix the current struct layout, I
humbly suggest that any future struct extensions should not have any
un-named padding (so that at the very least you could use
copy_struct_from_user() in some form).

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

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