Re: [PATCH v7 1/6] seccomp: add a return code to trap to userspace

From: Tycho Andersen <tycho@tycho.ws>
To: Kees Cook <keescook@chromium.org>
Cc: LKML <linux-kernel@vger.kernel.org>,
	Linux Containers <containers@lists.linux-foundation.org>,
	Linux API <linux-api@vger.kernel.org>,
	Andy Lutomirski <luto@amacapital.net>,
	Oleg Nesterov <oleg@redhat.com>,
	"Eric W . Biederman" <ebiederm@xmission.com>,
	"Serge E . Hallyn" <serge@hallyn.com>,
	Christian Brauner <christian.brauner@ubuntu.com>,
	Tyler Hicks <tyhicks@canonical.com>,
	Akihiro Suda <suda.akihiro@lab.ntt.co.jp>,
	Jann Horn <jannh@google.com>,
	"linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>
Subject: Re: [PATCH v7 1/6] seccomp: add a return code to trap to userspace
Date: Wed, 17 Oct 2018 16:33:47 -0600	[thread overview]
Message-ID: <20181017223347.GC14047@cisco> (raw)
In-Reply-To: <CAGXu5jK5yN7zcPUMzFYnUbRrVkLNE0YDCioYdNC3P6mF437+5g@mail.gmail.com>

On Wed, Oct 17, 2018 at 03:21:02PM -0700, Kees Cook wrote:
> On Wed, Oct 17, 2018 at 1:29 PM, Tycho Andersen <tycho@tycho.ws> wrote:
> > On Thu, Sep 27, 2018 at 02:31:24PM -0700, Kees Cook wrote:
> >> On Thu, Sep 27, 2018 at 8:11 AM, Tycho Andersen <tycho@tycho.ws> wrote:
> >> > @@ -60,4 +62,29 @@ struct seccomp_data {
> >> >         __u64 args[6];
> >> >  };
> >> >
> >> > +struct seccomp_notif {
> >> > +       __u16 len;
> >> > +       __u64 id;
> >> > +       __u32 pid;
> >> > +       __u8 signaled;
> >> > +       struct seccomp_data data;
> >> > +};
> >> > +
> >> > +struct seccomp_notif_resp {
> >> > +       __u16 len;
> >> > +       __u64 id;
> >> > +       __s32 error;
> >> > +       __s64 val;
> >> > +};
> >>
> >> So, len has to come first, for versioning. However, since it's ahead
> >> of a u64, this leaves a struct padding hole. pahole output:
> >>
> >> struct seccomp_notif {
> >>         __u16                      len;                  /*     0     2 */
> >>
> >>         /* XXX 6 bytes hole, try to pack */
> >>
> >>         __u64                      id;                   /*     8     8 */
> >>         __u32                      pid;                  /*    16     4 */
> >>         __u8                       signaled;             /*    20     1 */
> >>
> >>         /* XXX 3 bytes hole, try to pack */
> >>
> >>         struct seccomp_data        data;                 /*    24    64 */
> >>         /* --- cacheline 1 boundary (64 bytes) was 24 bytes ago --- */
> >>
> >>         /* size: 88, cachelines: 2, members: 5 */
> >>         /* sum members: 79, holes: 2, sum holes: 9 */
> >>         /* last cacheline: 24 bytes */
> >> };
> >> struct seccomp_notif_resp {
> >>         __u16                      len;                  /*     0     2 */
> >>
> >>         /* XXX 6 bytes hole, try to pack */
> >>
> >>         __u64                      id;                   /*     8     8 */
> >>         __s32                      error;                /*    16     4 */
> >>
> >>         /* XXX 4 bytes hole, try to pack */
> >>
> >>         __s64                      val;                  /*    24     8 */
> >>
> >>         /* size: 32, cachelines: 1, members: 4 */
> >>         /* sum members: 22, holes: 2, sum holes: 10 */
> >>         /* last cacheline: 32 bytes */
> >> };
> >>
> >> How about making len u32, and moving pid and error above "id"? This
> >> leaves a hole after signaled, so changing "len" won't be sufficient
> >> for versioning here. Perhaps move it after data?
> >
> > Just to confirm my understanding; I've got these as:
> >
> > struct seccomp_notif {
> >         __u32                      len;                  /*     0     4 */
> >         __u32                      pid;                  /*     4     4 */
> >         __u64                      id;                   /*     8     8 */
> >         __u8                       signaled;             /*    16     1 */
> >
> >         /* XXX 7 bytes hole, try to pack */
> >
> >         struct seccomp_data        data;                 /*    24    64 */
> >         /* --- cacheline 1 boundary (64 bytes) was 24 bytes ago --- */
> >
> >         /* size: 88, cachelines: 2, members: 5 */
> >         /* sum members: 81, holes: 1, sum holes: 7 */
> >         /* last cacheline: 24 bytes */
> > };
> > struct seccomp_notif_resp {
> >         __u32                      len;                  /*     0     4 */
> >         __s32                      error;                /*     4     4 */
> >         __u64                      id;                   /*     8     8 */
> >         __s64                      val;                  /*    16     8 */
> >
> >         /* size: 24, cachelines: 1, members: 4 */
> >         /* last cacheline: 24 bytes */
> > };
> >
> > in the next version. Since the structure has no padding at the end of
> > it, I think the Right Thing will happen. Note that this is slightly
> > different than what Kees suggested, if I add signaled after data, then
> > I end up with:
> >
> > struct seccomp_notif {
> >         __u32                      len;                  /*     0     4 */
> >         __u32                      pid;                  /*     4     4 */
> >         __u64                      id;                   /*     8     8 */
> >         struct seccomp_data        data;                 /*    16    64 */
> >         /* --- cacheline 1 boundary (64 bytes) was 16 bytes ago --- */
> >         __u8                       signaled;             /*    80     1 */
> >
> >         /* size: 88, cachelines: 2, members: 5 */
> >         /* padding: 7 */
> >         /* last cacheline: 24 bytes */
> > };
> >
> > which I think will have the versioning problem if the next member
> > introduces is < 7 bytes.
> 
> It'll be a problem in either place. What I was thinking was that
> specific versioning is required instead of just length.

Oh, if we decide to use the padded space? Yes, that makes sense. Ok,
I'll switch it to a version.

Tycho