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

To: Kees Cook <keescook@chromium.org>
Subject: Re: [PATCH v5 3/5] x86: Split syscall_trace_enter into two phases
From: Andy Lutomirski <luto@amacapital.net>
Date: Fri, 6 Feb 2015 12:20:20 -0800
Cc: "Dmitry V. Levin" <ldv@altlinux.org>, LKML <linux-kernel@vger.kernel.org>, Will Drewry <wad@chromium.org>, Oleg Nesterov <oleg@redhat.com>, "x86@kernel.org" <x86@kernel.org>, "linux-arm-kernel@lists.infradead.org" <linux-arm-kernel@lists.infradead.org>, Linux MIPS Mailing List <linux-mips@linux-mips.org>, linux-arch <linux-arch@vger.kernel.org>, linux-security-module <linux-security-module@vger.kernel.org>, Alexei Starovoitov <ast@plumgrid.com>, "H. Peter Anvin" <hpa@zytor.com>, Frederic Weisbecker <fweisbec@gmail.com>, Michael Kerrisk-manpages <mtk.manpages@gmail.com>
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References: <cover.1409954077.git.luto@amacapital.net> <2df320a600020fda055fccf2b668145729dd0c04.1409954077.git.luto@amacapital.net> <20150205211916.GA31367@altlinux.org> <CAGXu5j+aXxt55LsxxbNkfGGF719ubXBZ2JAFwUPNARwKMVFgng@mail.gmail.com> <20150205214027.GB31367@altlinux.org> <CALCETrXFzcXngHsX=_72hYZqms32Zf7oFYDBgC3XNw7zOGdDCA@mail.gmail.com> <CAGXu5jJtHT9o8WMoynifN13=uZoARt4G9iVcgZsc9xYOBEwWsg@mail.gmail.com> <20150205233945.GA31540@altlinux.org> <CAGXu5jLTH+mUF0JxeR2qA_r=ocWjPHPSK2OPgE0Fu_JKoQyQ9w@mail.gmail.com> <CALCETrXsCUje+_V=Ud+TB4A2jH2M7yqyoCFMLEyxOD6pd7Di5w@mail.gmail.com> <20150206023249.GB31540@altlinux.org> <CALCETrWTnqKDoatK+5FN=yYDOeENoW5=r5YMToYKhZ8Zfv5wWA@mail.gmail.com> <CAGXu5j+nopAMFukwMu=Cy0GOapziOLTb-ryJhA-aywk_uerg9A@mail.gmail.com> <CALCETrVaF+3ETn5nfcbvyWKUYb71jNXK-zo9V6uOK0cEW4TCNQ@mail.gmail.com> <CAGXu5jJXspS_34KBJ5VxvyKuj4bA+zg267dNiEkqR1LuvjoA1Q@mail.gmail.com> <CALCETrUmMA9sK4SJCSiF24iAiPLMBf=-JBw6TcLV+aLt_eN=Sw@mail.gmail.com> <CAGXu5jKuLUCwptoL=5Hcz7ME-SKdVcuYoRPw+JJ2nktz5273-w@mail.gmail.com>
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On Fri, Feb 6, 2015 at 12:16 PM, Kees Cook <keescook@chromium.org> wrote:
> On Fri, Feb 6, 2015 at 12:12 PM, Andy Lutomirski <luto@amacapital.net> wrote:
>> On Fri, Feb 6, 2015 at 12:07 PM, Kees Cook <keescook@chromium.org> wrote:
>>> On Fri, Feb 6, 2015 at 11:32 AM, Andy Lutomirski <luto@amacapital.net> 
>>> wrote:
>>>> On Fri, Feb 6, 2015 at 11:23 AM, Kees Cook <keescook@chromium.org> wrote:
>>>>> And especially since a ptracer
>>>>> can change syscalls during syscall-enter-stop to any syscall it wants,
>>>>> bypassing seccomp. This condition is already documented.
>>>>
>>>> If a ptracer (using PTRACE_SYSCALL) were to get the entry callback
>>>> before seccomp, then this oddity would go away, which might be a good
>>>> thing.  A ptracer could change the syscall, but seccomp would based on
>>>> what the ptracer changed the syscall to.
>>>
>>> I want kill events to trigger immediately. I don't want to leave the
>>> ptrace surface available on a SECCOMP_RET_KILL. So maybe it can be
>>> seccomp phase 1, then ptrace, then seccomp phase 2? And pass more
>>> information between phases to determine how things should behave
>>> beyond just "skip"?
>>
>> I thought so too, originally, but I'm far less convinced now, for two 
>> reasons:
>>
>> 1. I think that a lot of filters these days use RET_ERRNO heavily, so
>> this won't benefit them.
>>
>> 2. I'm not convinced it really reduces the attack surface for anyone.
>> Unless your filter is literally "return SECCOMP_RET_KILL", then the
>> seccomp-filtered task can always cause the ptracer to get a pair of
>> syscall notifications.  Also, the task can send itself signals (using
>> page faults, breakpoints, etc) and cause ptrace events via other
>> paths.
>
> What are you thinking for a solution?
>

I'm writing a patch now.  It's an ABI break, but this thread seems to
show that the ABI was somewhat useless before the split-phase changes,
and it's differently broken now, so I would be surprised if the change
broke anything that was currently working.  I'll send it later today,
hopefully.

> As for capping SECCOMP_RET_ERRNO to MAX_ERRNO, how about this (sorry
> if gmail butchers the paste):
>
> diff --git a/kernel/seccomp.c b/kernel/seccomp.c
> index 4ef9687ac115..c88148d20bd5 100644
> --- a/kernel/seccomp.c
> +++ b/kernel/seccomp.c
> @@ -629,7 +629,9 @@ static u32 __seccomp_phase1_filter(int this_syscall, 
> struct
>
>         switch (action) {
>         case SECCOMP_RET_ERRNO:
> -               /* Set the low-order 16-bits as a errno. */
> +               /* Set the low-order bits as a errno. */
> +               if (data > MAX_ERRNO)
> +                       data = MAX_ERRNO;
>                 syscall_set_return_value(current, task_pt_regs(current),
>                                          -data, 0);
>                 goto skip;
>

I'm fine with this, but I'm not entirely convinced it solves a
problem.  SECCOMP_RET_ERRNO | 5000 didn't work before, and it doesn't
work now.  Admittedly, the new failure mode is possibly better.

--Andy

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