linux-kernel - Re: [PATCH 09/11] powerpc/kernel: Enable seccomp filter

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Date:	Mon, 27 Jul 2015 11:56:22 -0700
From:	Kees Cook <keescook@...omium.org>
To:	Michael Ellerman <mpe@...erman.id.au>
Cc:	"linuxppc-dev@...abs.org" <linuxppc-dev@...abs.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Andy Lutomirski <luto@...capital.net>,
	Will Drewry <wad@...omium.org>, strosake@...ux.vnet.ibm.com,
	bogdan.purcareata@...escale.com
Subject: Re: [PATCH 09/11] powerpc/kernel: Enable seccomp filter

On Thu, Jul 23, 2015 at 3:21 AM, Michael Ellerman <mpe@...erman.id.au> wrote:
> This commit enables seccomp filter on powerpc, now that we have all the
> necessary pieces in place.
>
> To support seccomp's desire to modify the syscall return value under
> some circumstances, we use a different ABI to the ptrace ABI. That is we
> use r3 as the syscall return value, and orig_gpr3 is the first syscall
> parameter.
>
> This means the seccomp code, or a ptracer via SECCOMP_RET_TRACE, will
> see -ENOSYS preloaded in r3. This is identical to the behaviour on x86,
> and allows seccomp or the ptracer to either leave the -ENOSYS or change
> it to something else, as well as rejecting or not the syscall by
> modifying r0.
>
> If seccomp does not reject the syscall, we restore the register state to
> match what ptrace and audit expect, ie. r3 is the first syscall
> parameter again. We do this restore using orig_gpr3, which may have been
> modified by seccomp, which allows seccomp to modify the first syscall
> paramater and allow the syscall to proceed.
>
> Signed-off-by: Michael Ellerman <mpe@...erman.id.au>

Reviewed-by: Kees Cook <keescook@...omium.org>

-Kees

> ---
>  arch/powerpc/Kconfig         |  1 +
>  arch/powerpc/kernel/ptrace.c | 28 +++++++++++++++++++++++++++-
>  2 files changed, 28 insertions(+), 1 deletion(-)
>
> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
> index 5ef27113b898..b6cb6a87b7a2 100644
> --- a/arch/powerpc/Kconfig
> +++ b/arch/powerpc/Kconfig
> @@ -155,6 +155,7 @@ config PPC
>         select HAVE_PERF_EVENTS_NMI if PPC64
>         select EDAC_SUPPORT
>         select EDAC_ATOMIC_SCRUB
> +       select HAVE_ARCH_SECCOMP_FILTER
>
>  config GENERIC_CSUM
>         def_bool CPU_LITTLE_ENDIAN
> diff --git a/arch/powerpc/kernel/ptrace.c b/arch/powerpc/kernel/ptrace.c
> index 7484221bb3f8..de79eb5218c6 100644
> --- a/arch/powerpc/kernel/ptrace.c
> +++ b/arch/powerpc/kernel/ptrace.c
> @@ -1787,7 +1787,33 @@ long do_syscall_trace_enter(struct pt_regs *regs)
>
>         user_exit();
>
> -       secure_computing_strict(regs->gpr[0]);
> +       if (test_thread_flag(TIF_SECCOMP)) {
> +               /*
> +                * The ABI we present to seccomp tracers is that r3 contains
> +                * the syscall return value and orig_gpr3 contains the first
> +                * syscall parameter. This is different to the ptrace ABI where
> +                * both r3 and orig_gpr3 contain the first syscall parameter.
> +                */
> +               regs->gpr[3] = -ENOSYS;
> +
> +               /*
> +                * We use the __ version here because we have already checked
> +                * TIF_SECCOMP. If this fails, there is nothing left to do, we
> +                * have already loaded -ENOSYS into r3, or seccomp has put
> +                * something else in r3 (via SECCOMP_RET_ERRNO/TRACE).
> +                */
> +               if (__secure_computing())
> +                       return -1;
> +
> +               /*
> +                * The syscall was allowed by seccomp, restore the register
> +                * state to what ptrace and audit expect.
> +                * Note that we use orig_gpr3, which means a seccomp tracer can
> +                * modify the first syscall parameter (in orig_gpr3) and also
> +                * allow the syscall to proceed.
> +                */
> +               regs->gpr[3] = regs->orig_gpr3;
> +       }
>
>         if (test_thread_flag(TIF_SYSCALL_TRACE)) {
>                 /*
> --
> 2.1.0
>



-- 
Kees Cook
Chrome OS Security
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