On 08/22/2014 01:47 AM, Kees Cook wrote:

On Thu, Aug 21, 2014 at 3:56 AM, AKASHI Takahiro
[off-list ref] wrote:

quoted

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 <redacted>

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 <redacted>

quoted

---
  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