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

From: [email-masked] (AKASHI Takahiro)
Date: Wed, 23 Jul 2014 16:03:47 +0900

On 07/23/2014 05:15 AM, Kees Cook wrote:
> On Tue, Jul 22, 2014 at 2:14 AM, AKASHI Takahiro
> <[email-masked]> 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 <[email-masked]>
>> ---
>>   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
>>
>
>
>