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

[Kees Cook]

On Wed, Oct 17, 2018 at 1:29 PM, Tycho Andersen <tycho@xxxxxxxx> 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@xxxxxxxx> 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.

-Kees

-- 
Kees Cook
Pixel Security