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

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Message-ID: <CALCETrXRGUfkzbOcijsjWTh10TsCT_y8=zM3eQHrhMdVL=ow_g@mail.gmail.com>
Date:	Sat, 18 Jun 2016 03:21:04 -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 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.