Re: [PATCH 1/2] ARM: entry-common: fix forgotten set of thread_info->syscall

[Roman Peniaev <r.peniaev@gmail.com>]

On Fri, Jan 16, 2015 at 7:54 AM, Kees Cook <keescook@chromium.org> wrote:
> On Wed, Jan 14, 2015 at 5:54 PM, Roman Peniaev <r.peniaev@gmail.com> wrote:
>> On Thu, Jan 15, 2015 at 5:51 AM, Kees Cook <keescook@chromium.org> wrote:
>>> On Tue, Jan 13, 2015 at 12:35 AM, Roman Peniaev <r.peniaev@gmail.com> wrote:
>>>> On Tue, Jan 13, 2015 at 3:39 AM, Will Deacon <will.deacon@arm.com> wrote:
>>>>> On Sun, Jan 11, 2015 at 02:32:30PM +0000, Roman Pen wrote:
>>>>>> thread_info->syscall is used only for ptrace, but syscall number
>>>>>> is also used by syscall_get_nr and returned to userspace by the
>>>>>> following proc file access:
>>>>>>
>>>>>>  $ cat /proc/self/syscall
>>>>>>  0 0x3 0xbe928bd8 0x1000 0x0 0xac9e0 0x3 0xbe928bb4 0xb6f5dfbc
>>>>>>  ^
>>>>>> The first number is the syscall number, currently it is zero.
>>>>>> Patch fixes this:
>>>>>>
>>>>>>  $ cat /proc/self/syscall
>>>>>>  3 0x3 0xbefc7bd8 0x1000 0x0 0xac9e0 0x3 0xbefc7bb4 0xb6e82fbc
>>>>>>  ^
>>>>>> Right, read syscall
>>>>>
>>>>> Yes, it seems that despite requiring CONFIG_HAVE_ARCH_TRACEHOOK,
>>>>> the /proc code requires syscall_get_nr to work regardless of
>>>>> TIF_SYSCALL_TRACE.
>>>>>
>>>>>> Signed-off-by: Roman Pen <r.peniaev@gmail.com>
>>>>>> Cc: Russell King <linux@arm.linux.org.uk>
>>>>>> Cc: Marc Zyngier <marc.zyngier@arm.com>
>>>>>> Cc: Catalin Marinas <catalin.marinas@arm.com>
>>>>>> Cc: Christoffer Dall <christoffer.dall@linaro.org>
>>>>>> Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
>>>>>> Cc: Sekhar Nori <nsekhar@ti.com>
>>>>>> Cc: linux-arm-kernel@lists.infradead.org
>>>>>> Cc: linux-kernel@vger.kernel.org
>>>>>> Cc: stable@vger.kernel.org
>>>>>> ---
>>>>>>  arch/arm/kernel/asm-offsets.c  | 1 +
>>>>>>  arch/arm/kernel/entry-common.S | 1 +
>>>>>>  2 files changed, 2 insertions(+)
>>>>>>
>>>>>> diff --git a/arch/arm/kernel/asm-offsets.c b/arch/arm/kernel/asm-offsets.c
>>>>>> index 2d2d608..6911bad 100644
>>>>>> --- a/arch/arm/kernel/asm-offsets.c
>>>>>> +++ b/arch/arm/kernel/asm-offsets.c
>>>>>> @@ -70,6 +70,7 @@ int main(void)
>>>>>>    DEFINE(TI_CPU,             offsetof(struct thread_info, cpu));
>>>>>>    DEFINE(TI_CPU_DOMAIN,              offsetof(struct thread_info, cpu_domain));
>>>>>>    DEFINE(TI_CPU_SAVE,                offsetof(struct thread_info, cpu_context));
>>>>>> +  DEFINE(TI_SYSCALL,         offsetof(struct thread_info, syscall));
>>>>>>    DEFINE(TI_USED_CP,         offsetof(struct thread_info, used_cp));
>>>>>>    DEFINE(TI_TP_VALUE,                offsetof(struct thread_info, tp_value));
>>>>>>    DEFINE(TI_FPSTATE,         offsetof(struct thread_info, fpstate));
>>>>>> diff --git a/arch/arm/kernel/entry-common.S b/arch/arm/kernel/entry-common.S
>>>>>> index f8ccc21..89452ff 100644
>>>>>> --- a/arch/arm/kernel/entry-common.S
>>>>>> +++ b/arch/arm/kernel/entry-common.S
>>>>>> @@ -189,6 +189,7 @@ ENTRY(vector_swi)
>>>>>>  #endif
>>>>>>
>>>>>>  local_restart:
>>>>>> +     str scno, [tsk, #TI_SYSCALL]            @ set syscall number
>>>>>>       ldr     r10, [tsk, #TI_FLAGS]           @ check for syscall tracing
>>>>>>       stmdb   sp!, {r4, r5}                   @ push fifth and sixth args
>>>>>
>>>>> Do we definitely want to update scno on syscall restarting?
>>>>
>>>>
>>>> Good question.
>>>>
>>>> First thing to mention is __sys_trace will trace 'restart_syscall',
>>>> not the real syscall we are going to restart.
>>>>
>>>> E.g. in test application we do infinite poll and then send STOP and
>>>> CONT to this app:
>>>>
>>>>     test-243   [002] ...1  1792.067726: sys_enter: NR 168 (0, 0,
>>>> ffffffff, 0, 0, 0)
>>>>     test-243   [002] ...1  1802.299073: sys_exit: NR 168 = -516
>>>>     test-243   [004] ...1  1814.716264: sys_enter: NR 0 (0, 0,
>>>> ffffffff, 0, 0, 0)
>>>>     test-243   [004] ...1  2183.687225: sys_exit: NR 0 = -516
>>>>
>>>> the poll was restarted and trace shows that we are in restart_syscall.
>>>>
>>>> Is that expected?
>>>>
>>>> And the second thing is that my next patch did some tweaks in
>>>> 'syscall_trace_enter', where we take scno not from param we passed,
>>>> but from thread_info->syscall we previously set.
>>>>
>>>> So, regarding your question, if I set scno only once - I will break
>>>> previous behavior, and __sys_trace will trace the syscall we restarted.
>>>>
>>>> And I think this is what we need, because according to the
>>>> 'syscall_trace_enter' code we do 'secure_computing' and
>>>> 'audit_syscall_entry', which definitely expect original syscall, not
>>>> the 'restart_syscall'.
>>>
>>> Seccomp expects to see the __NR_restart_syscall syscall, since it
>>> interposes the syscall entry points.
>>
>>
>> Aha, thanks. So I should not break anything.
>
> I've tested on ARM now, seccomp doesn't see any change in behavior.
> Please consider both patches:
>
> Tested-by: Kees Cook <keescook@chromium.org>
>

Thanks.

> One interesting thing I noticed (which is unchanged by this series),
> but pulling ARM_r7 during the seccomp ptrace event shows __NR_poll,
> not __NR_restart_syscall, even though it was a __NR_restart_syscall
> trap from seccomp. Is there a better place to see the actual syscall?

As I understand we do not push new r7 to the stack, and ptrace uses the
old value.

I checked x86, x86-64 - ptrace returns __NR_restart_syscall.
So, probably, this should be fixed for ARM.

--
Roman