linux-kernel - Re: [PULL] seccomp update (next)

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Date:	Sun, 19 Jun 2016 15:45:27 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Kees Cook <keescook@...omium.org>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	James Morris <jmorris@...ei.org>,
	LSM List <linux-security-module@...r.kernel.org>
Subject: Re: [PULL] seccomp update (next)

On Sat, Jun 18, 2016 at 3:21 AM, Andy Lutomirski <luto@...capital.net> wrote:
> On Jun 18, 2016 12:02 AM, "Kees Cook" <keescook@...omium.org> wrote:
>>
>> On Fri, Jun 17, 2016 at 11:55 AM, Andy Lutomirski <luto@...capital.net> wrote:
>> > On Fri, Jun 17, 2016 at 12:15 AM, James Morris <jmorris@...ei.org> wrote:
>> >> On Tue, 14 Jun 2016, Kees Cook wrote:
>> >>
>> >>> Hi,
>> >>>
>> >>> Please pull these seccomp changes for next. These have been tested by
>> >>> myself and Andy, and close a long-standing issue with seccomp where tracers
>> >>> could change the syscall out from under seccomp.
>> >>
>> >> Pulled to security -next.
>> >
>> > As a heads up: I think this doesn't quite close the hole on x86.  Consider:
>> >
>> > 64-bit task arranges to be traced by a 32-bit task (or presumably a
>> > 64-bit task that calls ptrace via int80).
>> >
>> > Tracer does PTRACE_SYSCALL.
>> >
>> > Tracee does a normal syscall.
>> >
>> > Tracer writes tracee's orig_ax, thus invoking this thing in
>> > arch/x86/kernel/ptrace.c:
>> >
>> >         if (syscall_get_nr(child, regs) >= 0)
>> >             child->thread.status |= TS_COMPAT;
>> >
>> > Tracer resumes and gets confused.
>> >
>> > I think the right fix is to just delete:
>> >
>> >         if (syscall_get_nr(child, regs) >= 0)
>> >             child->thread.status |= TS_COMPAT;
>> >
>> > from ptrace.c.   The comment above it is garbage, too.
>>
>> I'm perfectly happy to see it removed. I can't make sense of the comment. :)
>>
>> That said, the only confusion I see is pretty minor. The arch is saved
>> before the tracer could force TS_COMPAT, so nothing confused is handed
>> to seccomp (the first time). And the syscall will continue to be
>> looked up on sys_call_table not ia32_sys_call_table.
>
> Hmm, right, but...
>
>>
>> The only thing I see is if the tracer has also added a
>> SECCOMP_RET_TRACE filter, after which the recheck will reload all the
>> seccomp info, including the arch. And at this point, a sensible filter
>> will reject a non-matching architecture.
>>
>
> Yes for some filters, but others might have different logic for
> different arches, at which point there's a minor bypass.
>
>> Maybe I'm missing something more?
>>
>
> You can also use this to do a 64-bit syscall with in_compat_syscall()
> returning true, which could cause issues for audit and maybe some
> ioctl handlers irrespective of this patch series.  I'll see about
> getting it fixed in x86/urgent.

Hi Kees and James-

On further consideration:

(a) that TS_COMPAT thing is highly, highly buggy -- much buggier and
more confusing than I thought, and not in the way I thought.

(b) it doesn't interfere with seccomp at all, so fixing it is not at
all a prerequisite for these changes.


-- 
Andy Lutomirski
AMA Capital Management, LLC