Date:	Thu, 15 Jan 2015 14:54:54 -0800
From:	Kees Cook <keescook@...omium.org>
To:	Roman Peniaev <r.peniaev@...il.com>
Cc:	Will Deacon <will.deacon@....com>,
	Russell King <linux@....linux.org.uk>,
	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	Marc Zyngier <Marc.Zyngier@....com>,
	Catalin Marinas <Catalin.Marinas@....com>,
	Sekhar Nori <nsekhar@...com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"stable@...r.kernel.org" <stable@...r.kernel.org>,
	Christoffer Dall <christoffer.dall@...aro.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 1/2] ARM: entry-common: fix forgotten set of thread_info->syscall

On Wed, Jan 14, 2015 at 5:54 PM, Roman Peniaev <r.peniaev@...il.com> wrote:
> On Thu, Jan 15, 2015 at 5:51 AM, Kees Cook <keescook@...omium.org> wrote:
>> On Tue, Jan 13, 2015 at 12:35 AM, Roman Peniaev <r.peniaev@...il.com> wrote:
>>> On Tue, Jan 13, 2015 at 3:39 AM, Will Deacon <will.deacon@....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@...il.com>
>>>>> Cc: Russell King <linux@....linux.org.uk>
>>>>> Cc: Marc Zyngier <marc.zyngier@....com>
>>>>> Cc: Catalin Marinas <catalin.marinas@....com>
>>>>> Cc: Christoffer Dall <christoffer.dall@...aro.org>
>>>>> Cc: Stefano Stabellini <stefano.stabellini@...citrix.com>
>>>>> Cc: Sekhar Nori <nsekhar@...com>
>>>>> Cc: linux-arm-kernel@...ts.infradead.org
>>>>> Cc: linux-kernel@...r.kernel.org
>>>>> Cc: stable@...r.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@...omium.org>

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?

-Kees

>
> --
> Roman
>
>
>>
>> -Kees
>>
>>>
>>>
>>> --
>>> Roman
>>>
>>> _______________________________________________
>>> linux-arm-kernel mailing list
>>> linux-arm-kernel@...ts.infradead.org
>>> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
>>
>>
>>
>> --
>> Kees Cook
>> Chrome OS Security



-- 
Kees Cook
Chrome OS Security
--
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