linux-kernel - Re: [PATCH v5 3/3] arm64: Add seccomp support

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Date:	Tue, 12 Aug 2014 15:57:25 +0900
From:	AKASHI Takahiro <takahiro.akashi@...aro.org>
To:	Will Deacon <will.deacon@....com>
CC:	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>,
	Catalin Marinas <Catalin.Marinas@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Andy Lutomirski <luto@...capital.net>,
	Deepak Saxena <dsaxena@...aro.org>,
	Lee Campbell <leecam@...gle.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v5 3/3] arm64: Add seccomp support

Will,

On 08/11/2014 06:24 PM, Will Deacon wrote:
> On Fri, Aug 08, 2014 at 08:35:42AM +0100, AKASHI Takahiro wrote:
>> On 08/06/2014 12:08 AM, Kees Cook wrote:
>>> On Fri, Jul 25, 2014 at 2:37 AM, AKASHI Takahiro
>>> <takahiro.akashi@...aro.org> wrote:
>>>> I found a bug in my current patch (v5). When 32-bit tracer skips a system call,
>>>> we should not update syscallno from x8 since syscallno is re-written directly
>>>> via ptrace(PTRACE_SET_SYSCALL).
>>>
>>> Ah, yes. Will aarch64 have a PTRACE_SET_SYSCALL option, or is this
>>> strictly a 32-bit vs 64-bit issue?
>>
>> As discussed in a few weeks ago, aarch64 won't support PTRACE_SET_SYSCALL.
>
> Well, I don't think anything was set in stone. If you have a compelling
> reason why adding the new request gives you something over setting w8
> directly, then we can extend ptrace.

Yeah, I think I may have to change my mind. Looking into __secure_computing(),
I found the code below:

 >     case SECCOMP_MODE_FILTER:
 >         case SECCOMP_RET_TRACE:
 >             ...
 >             if (syscall_get_nr(current, regs) < 0)
 >                 goto skip;

This implies that we should modify syscallno *before* __secure_computing() returns.

I assumed, in my next version, we could skip a system call by overwriting syscallno
with x8 in syscall_trace_enter() after __secure_computing() returns 0, and it actually
works.
But we'd better implement PTRACE_SET_SYSCALL to comply with what __secure_computing()
expects.

-Takahiro AKASHI

> Will
>
> _______________________________________________
> linux-arm-kernel mailing list
> linux-arm-kernel@...ts.infradead.org
> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
>
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