linux-kernel - Re: [PATCH v8 1/2] seccomp: add a return code to trap to userspace

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Date:   Tue, 30 Oct 2018 18:29:17 -0600
From:   Tycho Andersen <tycho@...ho.ws>
To:     Kees Cook <keescook@...omium.org>
Cc:     Andy Lutomirski <luto@...capital.net>,
        Oleg Nesterov <oleg@...hat.com>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        "Serge E . Hallyn" <serge@...lyn.com>,
        Christian Brauner <christian@...uner.io>,
        Tyler Hicks <tyhicks@...onical.com>,
        Akihiro Suda <suda.akihiro@....ntt.co.jp>,
        Aleksa Sarai <asarai@...e.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux Containers <containers@...ts.linux-foundation.org>,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH v8 1/2] seccomp: add a return code to trap to userspace

On Tue, Oct 30, 2018 at 03:34:54PM -0700, Kees Cook wrote:
> On Tue, Oct 30, 2018 at 3:32 PM, Tycho Andersen <tycho@...ho.ws> wrote:
> > On Tue, Oct 30, 2018 at 03:00:17PM -0700, Kees Cook wrote:
> >> On Tue, Oct 30, 2018 at 2:54 PM, Tycho Andersen <tycho@...ho.ws> wrote:
> >> > On Tue, Oct 30, 2018 at 02:49:21PM -0700, Kees Cook wrote:
> >> >> On Mon, Oct 29, 2018 at 3:40 PM, Tycho Andersen <tycho@...ho.ws> wrote:
> >> >> >     * switch to a flags based future-proofing mechanism for struct
> >> >> >       seccomp_notif and seccomp_notif_resp, thus avoiding version issues
> >> >> >       with structure length (Kees)
> >> >> [...]
> >> >> >
> >> >> > +struct seccomp_notif {
> >> >> > +       __u64 id;
> >> >> > +       __u32 pid;
> >> >> > +       __u32 flags;
> >> >> > +       struct seccomp_data data;
> >> >> > +};
> >> >> > +
> >> >> > +struct seccomp_notif_resp {
> >> >> > +       __u64 id;
> >> >> > +       __s64 val;
> >> >> > +       __s32 error;
> >> >> > +       __u32 flags;
> >> >> > +};
> >> >>
> >> >> Hrm, so, what's the plan for when struct seccomp_data changes size?
> >> >
> >> > I guess my plan was don't ever change the size again, just use flags
> >> > and have extra state available via ioctl().
> >> >
> >> >> I'm realizing that it might be "too late" for userspace to discover
> >> >> it's running on a newer kernel. i.e. it gets a user notification, and
> >> >> discovers flags it doesn't know how to handle. Do we actually need
> >> >> both flags AND a length? Designing UAPI is frustrating! :)
> >> >
> >> > :). I don't see this as such a big problem -- in fact it's better than
> >> > the length mode, where you don't know what you don't know, because it
> >> > only copied as much info as you could handle. Older userspace would
> >> > simply not use information it didn't know how to use.
> >> >
> >> >> Do we need another ioctl to discover the seccomp_data size maybe?
> >> >
> >> > That could be an option as well, assuming we agree that size would
> >> > work, which I thought we didn't?
> >>
> >> Size alone wasn't able to determine the layout of the seccomp_notif
> >> structure since it had holes (in the prior version). seccomp_data
> >> doesn't have holes and is likely to change in size (see the recent
> >> thread on adding the MPK register to it...)
> >
> > Oh, sorry, I misread this as seccomp_notif, not seccomp_data.
> >
> >> I'm trying to imagine the right API for this. A portable user of
> >> seccomp_notif expects the id/pid/flags/data to always be in the same
> >> place, but it's the size of seccomp_data that may change. So it wants
> >> to allocate space for seccomp_notif header and "everything else", of
> >> which is may only understand the start of seccomp_data (and ignore any
> >> new trailing fields).
> >>
> >> So... perhaps the "how big are things?" ioctl would report the header
> >> size and the seccomp_data size. Then both are flexible. And flags
> >> would be left as a way to "version" the header?
> >>
> >> Any Linux API list members want to chime in here?
> >
> > So:
> >
> > struct seccomp_notify_sizes {
> >     u16 seccomp_notify;
> >     u16 seccomp_data;
> > };
> >
> > ioctl(fd, SECCOMP_IOCTL_GET_SIZE, &sizes);
> >
> > This would be only one extra syscall over the lifetime of the listener
> > process, which doesn't seem too bad. One thing that's slightly
> > annoying is that you can't do it until you actually get an event, so
> > maybe it could be a command on the seccomp syscall instead:
> >
> > seccomp(SECCOMP_GET_NOTIF_SIZES, 0, &sizes);
> 
> Yeah, top-level makes more sense. u16 seems fine too.

So one problem is this is that the third argument of the seccomp
syscall is declared as const char, so I get:

kernel/seccomp.c: In function ‘seccomp_get_notif_sizes’:
kernel/seccomp.c:1401:19: warning: passing argument 1 of ‘copy_to_user’ discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
  if (copy_to_user(usizes, &sizes, sizeof(sizes)))
                   ^~~~~~
In file included from ./include/linux/compat.h:19:0,
                 from kernel/seccomp.c:19:
./include/linux/uaccess.h:152:1: note: expected ‘void *’ but argument is of type ‘const char *’
 copy_to_user(void __user *to, const void *from, unsigned long n)
 ^~~~~~~~~~~~

If I drop the const it doesn't complain, but I'm not sure what the protocol is
for changing the types of syscall declarations. In principle it doesn't really
mean anything, but...

Tycho