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Date: Sun, 21 Oct 2018 17:04:37 +0100
From: Tycho Andersen <tycho@...ho.ws>
To: Kees Cook <keescook@...omium.org>
Cc: LKML <linux-kernel@...r.kernel.org>,
Linux Containers <containers@...ts.linux-foundation.org>,
Linux API <linux-api@...r.kernel.org>,
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.brauner@...ntu.com>,
Tyler Hicks <tyhicks@...onical.com>,
Akihiro Suda <suda.akihiro@....ntt.co.jp>,
Jann Horn <jannh@...gle.com>,
"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH v7 1/6] seccomp: add a return code to trap to userspace
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@...ho.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@...ho.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.
Euh, so I implemented this, and it sucks :). It's ugly, and generally
feels bad.
What if instead we just get rid of versioning all together, and
instead introduce a u32 flags? We could have one flag right now
(SECCOMP_NOTIF_FLAG_SIGNALED), and use introduce others as we add more
information to the response. Then we can add
SECCOMP_NOTIF_FLAG_EXTRA_FOO, and add another SECCOMP_IOCTL_GET_FOO to
grab the info?
FWIW, it's not really clear to me that we'll ever add anything to the
response since hopefully we'll land PUT_FD, so maybe this is all moot
anyway.
Tycho
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