Re: [PATCH] seccomp: Check flags on seccomp_notif is unset

[Sargun Dhillon <sargun@sargun.me>]

On Fri, Dec 27, 2019 at 6:47 AM Christian Brauner
<christian.brauner@ubuntu.com> wrote:
>
> On Fri, Dec 27, 2019 at 01:31:31PM +1100, Aleksa Sarai wrote:
> > On 2019-12-27, Aleksa Sarai <cyphar@cyphar.com> wrote:
> >
> > Scratch that -- as Tycho just mentioned, there is un-named padding in
> > the struct so check_zeroed_user() is the wrong thing to do. But this
>
> Hm, I don't think so.
> I understood Tycho's point as _if_ there ever is padding then this would
> not be zeroed.
> Right now, there is no padding since the struct is correctly padded:
>
> struct seccomp_data {
>         int nr;
>         __u32 arch;
>         __u64 instruction_pointer;
>         __u64 args[6];
> };
>
> struct seccomp_notif {
>         __u64 id;
>         __u32 pid;
>         __u32 flags;
>         struct seccomp_data data;
> };
>
> which would be - using pahole:
>
> struct seccomp_data {
>         int                        nr;                   /*     0     4 */
>         __u32                      arch;                 /*     4     4 */
>         __u64                      instruction_pointer;  /*     8     8 */
>         __u64                      args[6];              /*    16    48 */
>
>         /* size: 64, cachelines: 1, members: 4 */
> };
> struct seccomp_notif {
>         __u64                      id;                   /*     0     8 */
>         __u32                      pid;                  /*     8     4 */
>         __u32                      flags;                /*    12     4 */
>         struct seccomp_data data;                        /*    16    64 */
>
>         /* size: 80, cachelines: 2, members: 4 */
>         /* last cacheline: 16 bytes */
> };
>
> The only worry would be a 2byte int type but there's no architecture
> we support which does this right now afaict.
>
> > also will make extensions harder to deal with because (presumably) they
> > will also have un-named padding, making copy_struct_from_user() the
>
> This all will be a non-issue if we just use __u64 for extensions.
>
> My point about using copy_struct_from_user() was that we should verify
> that _all_ fields are uninitialized and not just the flags argument
> since we might introduce a flags argument that requires another already
> existing member in seccomp_notif to be set to a value. We should do this
> change now so we don't have to risk breaking someone in the future.
>
> I'm trying to get at least Mozilla/Firefox off of their crazy
> SECCOMP_RET_TRAP way of implementing their broker onto the user notifier
> and they will likely need some extensions. That includes the pidfd stuff
> for seccomp that Sargun will likely be doing and the new pidfd_getfd()
> syscall. So it's not unlikely that we might need other already existing
> fields in that struct to be set to some value.
>
> I don't particulary care how we do it:
> - We can do a simple copy_from_user() and check each field individually.
Just doing a simple copy_from_user, and for now, calling memchr_inv
on the whole thing. We can drop the memset, and just leave a note to
indicate that if unpadded fields are introduced in the future, this structure
must be manually zeroed out. Although, this might be laying a trap for
ourselves.

This leaves us in a good position for introducing a flag field in the future.
All we have to do is change the memchr_inv from checking on an
entire struct basis to checking on a per-field basis.

> - Use copy_struct_from_user().
>   That is safe to do right now since there is no padding afaict and
>   it'll automatically verify new fields as well.
>   If I understand the worry correctly then the argument against
>   copy_struct_from_user() here is that there might be padding introduced
>   and userspace will not do an explicit memset() but rather rely on an
>   empty inializer {} and will _accidently_ pass down a struct which has
>   __all fields cleared__ but __uninitialized padding__ and we tell them
>   EINVAL? That can only happen if we introduce padding in the struct
>   which I'd argue we just don't do. That'll be in line with what we
>   require from our ABIs already anyway.
>
> Christian