On 2019-12-26, Tycho Andersen wrote: > On Fri, Dec 27, 2019 at 01:32:29AM +1100, Aleksa Sarai wrote: > > On 2019-12-26, Christian Brauner 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 > > > > Cc: Kees Cook > > > > > > 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 > > > > > > > --- > > > > 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