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SubjectRe: [PATCH v5 1/3] arm64: ptrace: reload a syscall number after ptrace operations
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On Wed, Jul 23, 2014 at 12:03 AM, AKASHI Takahiro
<takahiro.akashi@linaro.org> wrote:
> 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 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


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