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

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

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.

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 will defer to Will.

Thanks, -Takahiro AKASHI

-Kees

...

•   }
  

•    if (test_thread_flag(TIF_SYSCALL_TRACEPOINT))
             trace_sys_enter(regs, regs->syscallno);
  
  

-- 1.7.9.5