[PATCH v7 1/6] arm64: ptrace: add PTRACE_SET_SYSCALL

On Wed, Oct 8, 2014 at 9:13 AM, Will Deacon [off-list ref] wrote:

Hi Akashi,

On Thu, Oct 02, 2014 at 10:46:11AM +0100, AKASHI Takahiro 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).

Reviewed-by: Kees Cook <redacted>
Signed-off-by: AKASHI Takahiro <redacted>
---
 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 fe63ac5..2842f9f 100644
--- a/arch/arm64/kernel/ptrace.c
+++ b/arch/arm64/kernel/ptrace.c

@@ -1082,7 +1082,19 @@ 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;
 }

I still don't understand why this needs to be in arch-specific code. Can't
we implement this in generic code and get architectures to implement
something like syscall_set_nr if they want the generic interface?

Personally, I'd rather see this land as-is in the arm64 tree, and then
later optimize PTRACE_SET_SYSCALL out of arm/ and arm64/, especially
since only these architectures implement this at the moment.

This is my plan for the asm-generic seccomp.h too -- I'd rather avoid
touching other architectures in this series, as it's easier to review
this way. Then we can optimize the code in a separate series, which
will have those changes isolated, etc.

-Kees

-- 
Kees Cook
Chrome OS Security