Re: [PATCH v6 1/6] arm64: ptrace: add PTRACE_SET_SYSCALL

Kees Cook
2014-08-21T16:47:34Z

On Thu, Aug 21, 2014 at 3:56 AM, AKASHI Takahiro
<takahiro.akashi@linaro.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@linaro.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@chromium.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