Re: [PATCH v11 07/12] seccomp: add SECCOMP_RET_ERRNO

[Will Drewry <wad@chromium.org>]

On Mon, Feb 27, 2012 at 1:14 PM, Kees Cook <keescook@chromium.org> wrote:
> On Mon, Feb 27, 2012 at 10:35 AM, Andrew Lutomirski <luto@mit.edu> wrote:
>> On Mon, Feb 27, 2012 at 10:14 AM, Oleg Nesterov <oleg@redhat.com> wrote:
>>> On 02/27, Kees Cook wrote:
>>>>
>>>> On Mon, Feb 27, 2012 at 9:11 AM, Oleg Nesterov <oleg@redhat.com> wrote:
>>>> > On 02/24, Will Drewry wrote:
>>>> >>
>>>> >>  static u32 seccomp_run_filters(int syscall)
>>>> >>  {
>>>> >>       struct seccomp_filter *f;
>>>> >> -     u32 ret = SECCOMP_RET_KILL;
>>>> >>       static const struct bpf_load_fn fns = {
>>>> >>               bpf_load,
>>>> >>               sizeof(struct seccomp_data),
>>>> >>       };
>>>> >> +     u32 ret = SECCOMP_RET_ALLOW;
>>>> >>       const void *sc_ptr = (const void *)(uintptr_t)syscall;
>>>> >>
>>>> >> +     /* Ensure unexpected behavior doesn't result in failing open. */
>>>> >> +     if (unlikely(current->seccomp.filter == NULL))
>>>> >> +             ret = SECCOMP_RET_KILL;
>>>> >
>>>> > Is "seccomp.filter == NULL" really possible?
>>>>
>>>> It should not be, but I'm much more comfortable with this failing
>>>> closed. I think it's important to be as defensive as possible with
>>>> this code given its intended use.
>>>
>>> Can't resists... Sorry, I know I am troll but personally I think
>>> in this case the most defensive code is BUG_ON(->filter == NULL)
>>> or at least WARN_ON().
>>
>> Linus will probably object because he objected (correctly) to a very
>> similar problem in my old vsyscall emulation series.  A userspace
>> security feature shouldn't have a failure mode in which it confuses
>> the kernel and results in an oops, unless the situation is really
>> unrecoverable.  So WARN_ON plus do_exit would be okay but BUG_ON would
>> not.
>
> Yeah, actually, add WARN_ON would be preferred here because it should
> be an impossible situation. It should still fail closed, though:
>
>      /* Ensure unexpected behavior doesn't result in failing open. */
>      if (WARN_ON(current->seccomp.filter == NULL))
>              return SECCOMP_RET_KILL;

I'll do that - thanks!