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

[AKASHI Takahiro]

On 08/22/2014 01:47 AM, Kees Cook wrote:
> On Thu, Aug 21, 2014 at 3:56 AM, AKASHI Takahiro
> <takahiro.akashi@xxxxxxxxxx> 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@xxxxxxxxxx>
>
> Thanks, I like having this on both arm and arm64.

Yeah, having this simplified the code of syscall_trace_enter() a bit, but
also imposes some restriction on arm64, too.

> I wonder if other archs should add this option too.

Do you think so? I assumed that SET_SYSCALL is to be avoided if possible.

I also think that SET_SYSCALL should take an extra argument for a return value
just in case of -1 (or we have SKIP_SYSCALL?).

-Takahiro AKASHI

> Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
>
> > ---
> >   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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/