[PATCH v5 3/5] x86: Split syscall_trace_enter into two phases

[Kees Cook]

On Thu, Feb 5, 2015 at 3:39 PM, Dmitry V. Levin <ldv at altlinux.org> wrote:
> On Thu, Feb 05, 2015 at 03:12:39PM -0800, Kees Cook wrote:
>> On Thu, Feb 5, 2015 at 1:52 PM, Andy Lutomirski <luto at amacapital.net> wrote:
>> > On Thu, Feb 5, 2015 at 1:40 PM, Dmitry V. Levin <ldv at altlinux.org> wrote:
>> >> On Thu, Feb 05, 2015 at 01:27:16PM -0800, Kees Cook wrote:
>> >>> On Thu, Feb 5, 2015 at 1:19 PM, Dmitry V. Levin <ldv at altlinux.org> wrote:
>> >>> > Hi,
>> >>> >
>> >>> > On Fri, Sep 05, 2014 at 03:13:54PM -0700, Andy Lutomirski wrote:
>> >>> >> This splits syscall_trace_enter into syscall_trace_enter_phase1 and
>> >>> >> syscall_trace_enter_phase2.  Only phase 2 has full pt_regs, and only
>> >>> >> phase 2 is permitted to modify any of pt_regs except for orig_ax.
>> >>> >
>> >>> > This breaks ptrace, see below.
>> >>> >
> [...]
>> >>> >> +             ret = seccomp_phase1(&sd);
>> >>> >> +             if (ret == SECCOMP_PHASE1_SKIP) {
>> >>> >> +                     regs->orig_ax = -1;
>> >>> >
>> >>> > How the tracer is expected to get the correct syscall number after that?
>> >>>
>> >>> There shouldn't be a tracer if a skip is encountered. (A seccomp skip
>> >>> would skip ptrace.) This behavior hasn't changed, but maybe I don't
>> >>> see what you mean? (I haven't encountered any problems with syscall
>> >>> tracing as a result of these changes.)
>> >>
>> >> SECCOMP_RET_ERRNO leads to SECCOMP_PHASE1_SKIP, and if there is a tracer,
>> >> it will get -1 as a syscall number.
>> >>
>> >> I've found this while testing a strace parser for
>> >> SECCOMP_MODE_FILTER/SECCOMP_SET_MODE_FILTER, so the problem is quite real.
>> >
>> > Hasn't it always been this way?
>>
>> As far as I know, yes, it's always been this way. The point is to the
>> skip the syscall, which is what the -1 signals. Userspace then reads
>> back the errno.
>
> There is a clear difference: before these changes, SECCOMP_RET_ERRNO used
> to keep the syscall number unchanged and suppress syscall-exit-stop event,
> which was awful because userspace cannot distinguish syscall-enter-stop
> from syscall-exit-stop and therefore relies on the kernel that
> syscall-enter-stop is followed by syscall-exit-stop (or tracee's death, etc.).
>
> After these changes, SECCOMP_RET_ERRNO no longer causes syscall-exit-stop
> events to be suppressed, but now the syscall number is lost.

Ah-ha! Okay, thanks, I understand now. I think this means seccomp
phase1 should not treat RET_ERRNO as a "skip" event. Andy, what do you
think here?

-Kees

-- 
Kees Cook
Chrome OS Security