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

Tycho Andersen
2018-10-17T22:33:57Z

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