linux-kernel - Re: [PATCH v6 1/6] arm64: ptrace: add PTRACE_SET

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Message-ID: <CAGXu5jJrTEhdDZ8fipNeaVm1_wto7u2f=uq_KzyMcVHJMT57vw@mail.gmail.com>
Date:	Thu, 21 Aug 2014 11:47:32 -0500
From:	Kees Cook <keescook@...omium.org>
To:	AKASHI Takahiro <takahiro.akashi@...aro.org>
Cc:	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	Deepak Saxena <dsaxena@...aro.org>, arndb@...db.de,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	linaro-kernel@...ts.linaro.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v6 1/6] arm64: ptrace: add PTRACE_SET_SYSCALL

On Thu, Aug 21, 2014 at 3:56 AM, AKASHI Takahiro
<takahiro.akashi@...aro.org> wrote:
> To allow tracer to be able to change/skip a system call by re-writing
> a syscall number, there are several approaches:
>
> (1) modify x8 register with ptrace(PTRACE_SETREGSET), and handle this case
>     later on in syscall_trace_enter(), or
> (2) support ptrace(PTRACE_SET_SYSCALL) as on arm
>
> Thinking of the fact that user_pt_regs doesn't expose 'syscallno' to
> tracer as well as that secure_computing() expects a changed syscall number
> to be visible, especially case of -1, before this function returns in
> syscall_trace_enter(), we'd better take (2).
>
> Signed-off-by: AKASHI Takahiro <takahiro.akashi@...aro.org>

Thanks, I like having this on both arm and arm64. I wonder if other
archs should add this option too.

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

> ---
>  arch/arm64/include/uapi/asm/ptrace.h |    1 +
>  arch/arm64/kernel/ptrace.c           |   14 +++++++++++++-
>  2 files changed, 14 insertions(+), 1 deletion(-)
>
> diff --git a/arch/arm64/include/uapi/asm/ptrace.h b/arch/arm64/include/uapi/asm/ptrace.h
> index 6913643..49c6174 100644
> --- a/arch/arm64/include/uapi/asm/ptrace.h
> +++ b/arch/arm64/include/uapi/asm/ptrace.h
> @@ -23,6 +23,7 @@
>
>  #include <asm/hwcap.h>
>
> +#define PTRACE_SET_SYSCALL     23
>
>  /*
>   * PSR bits
> diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
> index 0310811..8876049 100644
> --- a/arch/arm64/kernel/ptrace.c
> +++ b/arch/arm64/kernel/ptrace.c
> @@ -1077,7 +1077,19 @@ const struct user_regset_view *task_user_regset_view(struct task_struct *task)
>  long arch_ptrace(struct task_struct *child, long request,
>                  unsigned long addr, unsigned long data)
>  {
> -       return ptrace_request(child, request, addr, data);
> +       int ret;
> +
> +       switch (request) {
> +               case PTRACE_SET_SYSCALL:
> +                       task_pt_regs(child)->syscallno = data;
> +                       ret = 0;
> +                       break;
> +               default:
> +                       ret = ptrace_request(child, request, addr, data);
> +                       break;
> +       }
> +
> +       return ret;
>  }
>
>  enum ptrace_syscall_dir {
> --
> 1.7.9.5
>



-- 
Kees Cook
Chrome OS Security
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