Re: [PATCH v5 1/3] arm64: ptrace: reload a syscall number after ptrace operations

[Kees Cook <keescook@chromium.org>]

On Tue, Jul 22, 2014 at 2:14 AM, AKASHI Takahiro
<takahiro.akashi@linaro.org> wrote:
> Arm64 holds a syscall number in w8(x8) register. Ptrace tracer may change
> its value either to:
>   * any valid syscall number to alter a system call, or
>   * -1 to skip a system call
>
> This patch implements this behavior by reloading that value into syscallno
> in struct pt_regs after tracehook_report_syscall_entry() or
> secure_computing(). In case of '-1', a return value of system call can also
> be changed by the tracer setting the value to x0 register, and so
> sys_ni_nosyscall() should not be called.
>
> See also:
>     42309ab4, ARM: 8087/1: ptrace: reload syscall number after
>               secure_computing() check
>
> Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
> ---
>  arch/arm64/kernel/entry.S  |    2 ++
>  arch/arm64/kernel/ptrace.c |   13 +++++++++++++
>  2 files changed, 15 insertions(+)
>
> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
> index 5141e79..de8bdbc 100644
> --- a/arch/arm64/kernel/entry.S
> +++ b/arch/arm64/kernel/entry.S
> @@ -628,6 +628,8 @@ ENDPROC(el0_svc)
>  __sys_trace:
>         mov     x0, sp
>         bl      syscall_trace_enter
> +       cmp     w0, #-1                         // skip syscall?
> +       b.eq    ret_to_user
>         adr     lr, __sys_trace_return          // return address
>         uxtw    scno, w0                        // syscall number (possibly new)
>         mov     x1, sp                          // pointer to regs
> diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
> index 70526cf..100d7d1 100644
> --- a/arch/arm64/kernel/ptrace.c
> +++ b/arch/arm64/kernel/ptrace.c
> @@ -21,6 +21,7 @@
>
>  #include <linux/audit.h>
>  #include <linux/compat.h>
> +#include <linux/errno.h>
>  #include <linux/kernel.h>
>  #include <linux/sched.h>
>  #include <linux/mm.h>
> @@ -1109,9 +1110,21 @@ static void tracehook_report_syscall(struct pt_regs *regs,
>
>  asmlinkage int syscall_trace_enter(struct pt_regs *regs)
>  {
> +       unsigned long saved_x0, saved_x8;
> +
> +       saved_x0 = regs->regs[0];
> +       saved_x8 = regs->regs[8];
> +
>         if (test_thread_flag(TIF_SYSCALL_TRACE))
>                 tracehook_report_syscall(regs, PTRACE_SYSCALL_ENTER);
>
> +       regs->syscallno = regs->regs[8];
> +       if ((long)regs->syscallno == ~0UL) { /* skip this syscall */
> +               regs->regs[8] = saved_x8;
> +               if (regs->regs[0] == saved_x0) /* not changed by user */
> +                       regs->regs[0] = -ENOSYS;

I'm not sure this is right compared to other architectures. Generally
when a tracer performs a syscall skip, it's up to them to also adjust
the return value. They may want to be faking a syscall, and what if
the value they want to return happens to be what x0 was going into the
tracer? It would have no way to avoid this -ENOSYS case. I think
things are fine without this test.

-Kees

> +       }
> +
>         if (test_thread_flag(TIF_SYSCALL_TRACEPOINT))
>                 trace_sys_enter(regs, regs->syscallno);
>
> --
> 1.7.9.5
>



-- 
Kees Cook
Chrome OS Security