On 2019-12-27, Aleksa Sarai wrote: > On 2019-12-26, Christian Brauner wrote: > > On December 26, 2019 3:32:29 PM GMT+01:00, 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 = {}; > > > > > >> > 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