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

From: Andy Lutomirski
Date: Thu Nov 07 2013 - 13:16:46 EST


On Thu, Nov 7, 2013 at 9:47 AM, Kees Cook <keescook@xxxxxxxxxxxx> 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@xxxxxxxxxxxx>
> ---
> 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.

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.

--Andy

> --
> Kees Cook
> Chrome OS Security



--
Andy Lutomirski
AMA Capital Management, LLC
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