linux-kernel - Re: [PATCH v2] seccomp: not compatible with ARM OABI

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Date:	Thu, 7 Nov 2013 10:39:00 -0800
From:	Kees Cook <keescook@...omium.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Russell King <linux@....linux.org.uk>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Eric Paris <eparis@...hat.com>,
	libseccomp-discuss@...ts.sourceforge.net,
	Will Drewry <wad@...omium.org>,
	James Hogan <james.hogan@...tec.com>
Subject: Re: [PATCH v2] seccomp: not compatible with ARM OABI

On Thu, Nov 7, 2013 at 10:16 AM, Andy Lutomirski <luto@...capital.net> wrote:
> On Thu, Nov 7, 2013 at 9:47 AM, Kees Cook <keescook@...omium.org> wrote:
>> Make sure that seccomp filter won't be built when ARM OABI is in use,
>> since there is work needed to distinguish calling conventions. Until
>> that is done (which is likely never since OABI is deprecated), make
>> sure seccomp filter is unavailable in the OABI world.
>>
>> Signed-off-by: Kees Cook <keescook@...omium.org>
>> ---
>> v2:
>>  - toggle availability via HAVE_ARCH_SECCOMP_FILTER; James Hogan.
>> ---
>>  arch/arm/Kconfig |    7 ++++++-
>>  1 file changed, 6 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
>> index 0a1dc697333c..a0a8590f3609 100644
>> --- a/arch/arm/Kconfig
>> +++ b/arch/arm/Kconfig
>> @@ -23,7 +23,7 @@ config ARM
>>         select HARDIRQS_SW_RESEND
>>         select HAVE_ARCH_JUMP_LABEL if !XIP_KERNEL
>>         select HAVE_ARCH_KGDB
>> -       select HAVE_ARCH_SECCOMP_FILTER
>> +       select HAVE_ARCH_SECCOMP_FILTER if (AEABI && !OABI_COMPAT)
>>         select HAVE_ARCH_TRACEHOOK
>>         select HAVE_BPF_JIT
>>         select HAVE_CONTEXT_TRACKING
>> @@ -1735,6 +1735,11 @@ config OABI_COMPAT
>>           in memory differs between the legacy ABI and the new ARM EABI
>>           (only for non "thumb" binaries). This option adds a tiny
>>           overhead to all syscalls and produces a slightly larger kernel.
>> +
>> +         The seccomp filter system will not be available when this is
>> +         selected, since there is no way yet to sensibly distinguish
>> +         between calling conventions during filtering.
>> +
>>           If you know you'll be using only pure EABI user space then you
>>           can say N here. If this option is not selected and you attempt
>>           to execute a legacy ABI binary then the result will be
>> --
>> 1.7.9.5
>>
>>
>
> FWIW, OABI-only (i.e. !AEABI, as opposed to OABI_COMPAT) is, in
> principle, supportable -- userspace would just have to know that, if
> build for OABI, the calling convention is different.

Right -- I opted for enforcing seccomp-on-ARM-means-EABI.

> I doubt this is worth supporting, though, and, if no one complains
> about your patch for a couple releases, then that would mean we could
> get away with adding AUDIT_ARCH_ARM_OABI or something (maybe for
> seccomp only) if needed.

Agreed.

Thanks again for looking at all this!

-Kees

-- 
Kees Cook
Chrome OS Security
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