linux-kernel - Re: [libseccomp-discuss] ARM audit, seccomp, etc are broken wrt OABI syscalls

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Date:	Wed, 6 Nov 2013 13:49:43 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Kees Cook <keescook@...omium.org>
Cc:	Will Drewry <wad@...omium.org>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	Eric Paris <eparis@...hat.com>,
	Richard Weinberger <richard@....at>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	libseccomp-discuss@...ts.sourceforge.net,
	linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [libseccomp-discuss] ARM audit, seccomp, etc are broken wrt OABI syscalls

On Wed, Nov 6, 2013 at 1:26 PM, Kees Cook <keescook@...omium.org> wrote:
> On Wed, Nov 6, 2013 at 1:20 PM, Will Drewry <wad@...omium.org> wrote:
>> On Wed, Nov 6, 2013 at 9:51 AM, Russell King - ARM Linux
>> <linux@....linux.org.uk> wrote:
>>> On Wed, Nov 06, 2013 at 10:32:31AM -0500, Eric Paris wrote:
>>>> On Tue, 2013-11-05 at 14:36 -0800, Andy Lutomirski wrote:
>>>> > 1. Set a different audit arch for OABI syscalls (e.g.
>>>> > AUDIT_ARCH_ARMOABI).  That is, treat OABI syscall entries the same way
>>>> > that x86_64 treats int 80.
>>>>
>>>> As the audit maintainer, I like #1.  It might break ABI, but the ABI is
>>>> flat wrong now and not maintainable...
>>>
>>> If you read the whole thread, you will see that this corner case is just
>>> not worth the effort to support.  Audit may as well be disabled by
>>> kernel config if any OABI support is enabled.
>>
>> This might be the best move for seccomp too (as Kees suggested).  I'd
>> love to have audit arch visibility, but it's not clear that it's worth
>> any sort of larger changes ...
>>
>> ... like adding a task_thread_info.compat flag that bubbles up to
>> syscall_get_arch(), or if we assume consumers of syscall_get_nr() are
>> broken today (I haven't checked), then it would be possible to at
>> least re-add the 0x900000 bits, if compat, before handing back the
>> system call number but leave the audit arch pieces alone.

That would fix the main issue, but I bet that no one will ever do
anything on the userspace side other than treating those syscalls like
any other unknown syscall.

>
> How does this look, for the seccomp part?
>
> -Kees
>
> diff --git a/arch/Kconfig b/arch/Kconfig
> index af2cc6eabcc7..3610c2d9910f 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -331,7 +331,7 @@ config HAVE_ARCH_SECCOMP_FILTER
>
>  config SECCOMP_FILTER
>         def_bool y
> -       depends on HAVE_ARCH_SECCOMP_FILTER && SECCOMP && NET
> +       depends on HAVE_ARCH_SECCOMP_FILTER && SECCOMP && NET && !OABI_COMPAT
>         help
>           Enable tasks to build secure computing environments defined
>           in terms of Berkeley Packet Filter programs which implement
>

Works for me.  Of course, I don't maintain any of this stuff, so I
don't have to deal with it :)

It's probably work adding some text either in CONFIG_SECCOMP_FILTER or
CONFIG_OABI_COMPAT explaining what the problem is.


FWIW, does syscall restart work with OABI_COMPAT?  I've never
understood the syscall restart mechanism, but x86 had an issue awhile
back when with sysenter vs. syscall that was kind of similar.

--Andy
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