Re: [PATCH] seccomp: Add group_leader pid to seccomp_notif

Kees Cook
2020-05-17T21:31:03Z

On Sun, May 17, 2020 at 09:02:15AM -0600, Tycho Andersen wrote:
> On Sun, May 17, 2020 at 08:46:03AM -0600, Tycho Andersen wrote:
> > On Sun, May 17, 2020 at 04:33:11PM +0200, Christian Brauner wrote:
> > > struct seccomp_notif2 {
> > > __u32 notif_size;
> > > __u64 id;
> > > __u32 pid;
> > > __u32 flags;
> > > struct seccomp_data data;
> > > __u32 data_size;
> > > };
> >
> > I guess you need to put data_size before data, otherwise old userspace
> > with a smaller struct seccomp_data will look in the wrong place.
> >
> > But yes, that'll work if you put two sizes in, which is probably
> > reasonable since we're talking about two structs.
>
> Well, no, it doesn't either. Suppose we add a new field first to
> struct seccomp_notif2:
>
> struct seccomp_notif2 {
> __u32 notif_size;
> __u64 id;
> __u32 pid;
> __u32 flags;
> struct seccomp_data data;
> __u32 data_size;
> __u32 new_field;
> };
>
> And next we add a new field to struct secccomp_data. When a userspace
> compiled with just the new seccomp_notif2 field does:
>
> seccomp_notif2.new_field = ...;
>
> the compiler will put it in the wrong place for the kernel with the
> new seccomp_data field too.
>
> Sort of feels like we should do:
>
> struct seccomp_notif2 {
> struct seccomp_notif *notif;
> struct seccomp_data *data;
> };

I'm going read this thread more carefully tomorrow, but I just wanted to
mention that I'd *like* to extend seccomp_data for doing deep argument
inspection of the new syscalls. I think it's the least bad of many
designs, and I'll write that up in more detail. (I would *really* like
to avoid extending seccomp's BPF language, and instead allow probing
into the struct copied from userspace, etc.)

Anyway, it's very related to this, so, yeah, probably we need a v2 of the
notif API, but I'll try to get all the ideas here collected in one place.

--
Kees Cook