Re: [PATCH] seccomp: plug syscall-dodging ptrace hole

[Kees Cook]

On Thu, May 26, 2016 at 9:45 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> On Thu, May 26, 2016 at 7:41 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>> On Thu, May 26, 2016 at 7:10 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>>> On Thu, May 26, 2016 at 2:04 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>>>> One problem with seccomp was that ptrace could be used to change a
>>>> syscall after seccomp filtering had completed. This was a well documented
>>>> limitation, and it was recommended to block ptrace when defining a filter
>>>> to avoid this problem. This can be quite a limitation for containers or
>>>> other places where ptrace is desired even under seccomp filters.
>>>>
>>>> Since seccomp filtering has been split into pre-trace and trace phases
>>>> (phase1 and phase2 respectively), it's possible to re-run phase1 seccomp
>>>> after ptrace. This makes that change, and updates the test suite for
>>>> both SECCOMP_RET_TRACE and PTRACE_SYSCALL manipulation.
>>>
>>> I like fixing the hole, but I don't like this fix.
>>>
>>> The two-phase seccomp mechanism is messy.  I wrote it because it was a
>>> huge speedup.  Since then, I've made a ton of changes to the way that
>>> x86 syscalls work, and there are two relevant effects: the slow path
>>> is quite fast, and the phase-1-only path isn't really a win any more.
>>>
>>> I suggest that we fix the by simplifying the code instead of making it
>>> even more complicated.  Let's back out the two-phase mechanism (but
>>> keep the ability for arch code to supply seccomp_data) and then just
>>> reorder it so that seccomp happens after ptrace.  The result should be
>>> considerably simpler.  (We'll still have to answer the question of
>>> what happens when a SECCOMP_RET_TRACE event changes the syscall, but
>>> maybe the answer is to just let it through -- after all,
>>> SECCOMP_RET_TRACE might be a request by a tracer to do its own
>>> internal filtering.)
>>
>> I'm really against this. I think seccomp needs to stay first,
>
> Why?  What use case is improved with it going first?

I feel that the critical purpose of seccomp is to minimize attack
surface. To that end, I am strongly against anything coming before it
in the syscall path. I really do not want ptrace going first, I think
it's just asking for bugs.

>> and I
>> like the two-phase split because it gives us a lot of flexibility on
>> other architectures.
>
> I thought so too when I wrote it, and I even tried a bit to evangelize
> it to other arch maintainers.  So far, it's used *only* in x86, and it
> would IMO be a cleanup to stop using it in x86 now.  Given my
> experience cleanup up the x86 syscall path, my current advice to other
> arch maintainers would be to try hard to avoid having a context in
> which syscall args are known but ptrace can't be invoked (as x86 had
> before Linux 4.5).

Well, I've got most of the ARM 2-phase port done, but haven't gotten
it all the way finished. But I could be talked into removing the
2-phase just from the perspective of reducing complexity.

>> And we can't just let through RET_TRACE because
>> we'll have exactly the same problem: a process can add a RET_TRACE
>> filter for some syscall and then change it arbitrarily to escape the
>> filtering. The non-trace returns of seccomp need to be check first and
>> after ptrace manipulations. The patch seems like the best approach and
>> it covers all the corners.
>
> But RET_TRACE really is special.
>
> Suppose you have a tracer and you use SECCOMP_RET_TRACE.  If the
> tracer sees a syscall, approves, and calls PTRACE_CONT, then the
> syscall will be allowed, whereas the effect of SECCOMP_RET_TRACE run
> anew would be to either force -ENOSYS or to trap back to the tracer,
> depending on whether there is a tracer.  Your patch has a
> SECCOMP_RET_TRACE special case, whereas my approach wouldn't need a
> special case.

But after the seccomp re-trap, we'd still have to re-check the
filters. So it's not cleaner, and we gain attack surface. Don't get me
wrong, I totally see why you're suggesting doing ptrace first, but I
still think that attack surface reduction must remain the primary
principle of seccomp.

> I think your patch also has a minor hole: if you have
> SECCOMP_RET_TRACE *and* a tracer that's catching syscalls directly
> (PTRACE_SYSCALL), then the PTRACE_SYSCALL action can modify a syscall
> after TRACE does its thing but before recheck, and can then redirect
> to another RET_TRACE action that would otherwise be denied.  This is
> minor because it could only happen if the tracer actively fights with
> itself.

So, a few thoughts went into the patch design, and here's why I don't
think this hole is a hole (which you already talk about a bit):
- no filtered syscall could be suddenly made to execute (this is core
goal, obviously)
- worst case, the tracer doesn't notice a syscall marked for
RET_TRACE, but that would be it's own fault because it chose to put
itself in that state.
- RET_TRACE is just under RET_ALLOW in priority, so even if there were
some really weird unintended side-effects, we still wouldn't be
allowing a syscall.

> Finally, I think that the your approach would break an existing valid
> use case.  Suppose I have a tracer that wants to intercept some
> syscall sys_foo (using SECCOMP_RET_TRACE) and, when it sees a sys_foo
> attempt, it will implement it by redirecting it so some other syscall
> that wouldn't be allowed if called directly (i.e. it would return
> SECCOMP_RET_KILL or similar).  Currently, it'll work.  With your
> patch, it will kill the tracee.  I think the former behavior is
> better.  On the flip side, if you write a program that uses
> SECCOMP_RET_TRACE, you more or less have to trust the tracer to begin
> with.

Well, you say "valid" here, but I actually think this is a
mis-feature. Luckily, nothing actually uses this behavior, and
multiple seccomp users want to see this fixed. As for tracer trust,
yes. RET_TRACE was designed for a tracer to do things on behalf of the
process, not to redirect it through an otherwise blocked syscall. The
use cases for that behavior is few and far between (and is currently
unused), where-as the use case for actually blocking the syscalls the
filters claim to block is much more desired.

> One more reason to prefer my approach: currently, if you strace a
> process that gets killed by SECCOMP_RET_KILL, you can't tell what
> killed it.  For example:
>
> prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, {len = 1, filter =
> 0x7ffe7b2b7d30}) = 0
> +++ killed by SIGSYS +++
> Bad system call (core dumped)
>
> With my approach, strace will have the IMO much more sensible behavior
> of showing the fatal syscall entry before showing the "killed by
> SIGSYS".

Right, I know, it's aesthetically much nicer that way, but I really
want to stay totally paranoid and keep seccomp absolutely first on the
path.

How about this: we'll use this patch as-is for now, since I'd like to
be able to start getting feedback from the container-using folks ASAP,
and then we can redesign the 2-phase system going forward from there.

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security