On Wed, Jul 23, 2014 at 12:03 AM, AKASHI Takahiro
[off-list ref] wrote:

On 07/23/2014 05:15 AM, Kees Cook wrote:

quoted

On Tue, Jul 22, 2014 at 2:14 AM, AKASHI Takahiro
[off-list ref] wrote:

quoted

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 <redacted>
---
  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.

Yeah, I know this issue, but was not sure that setting a return value
is mandatory. (x86 seems to return -ENOSYS by default if not explicitly
specified.)
Is "fake a system call" a more appropriate word than "skip"?

I think this is just a matter of semantics and perspective. From the
kernel's perspective, it's always a "skip" since the syscall is never
actually executed. But from the perspective of userspace, it's really
up to the tracer to decide how it should be seen: the tracer could
return -ENOSYS, or a fake return value, etc. But generally, I think
"skip" is the most accurate term for this.

-Kees

-- 
Kees Cook
Chrome OS Security