linux-kernel - Re: [PATCH 3/4] arch/arm: allow a scno of -1 to not cause a SIGILL

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Date:	Thu, 1 Nov 2012 14:51:28 -0700
From:	Kees Cook <keescook@...omium.org>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	linux-kernel@...r.kernel.org, Will Deacon <will.deacon@....com>,
	Will Drewry <wad@...omium.org>,
	Geremy Condra <gcondra@...gle.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Catalin Marinas <catalin.marinas@....com>
Subject: Re: [PATCH 3/4] arch/arm: allow a scno of -1 to not cause a SIGILL

On Thu, Nov 1, 2012 at 1:25 PM, Russell King - ARM Linux
<linux@....linux.org.uk> wrote:
> On Thu, Nov 01, 2012 at 12:46:38PM -0700, Kees Cook wrote:
>> From: Will Drewry <wad@...omium.org>
>>
>> On tracehook-friendly platforms, a system call number of -1 falls
>> through without running much code or taking much action.
>>
>> ARM is different.  This adds a lightweight check to arm_syscall()
>> to make sure that ARM behaves the same way.
>>
>> Signed-off-by: Will Drewry <wad@...omium.org>
>> Signed-off-by: Kees Cook <keescook@...omium.org>
>> ---
>>  arch/arm/kernel/traps.c |    4 ++++
>>  1 file changed, 4 insertions(+)
>>
>> diff --git a/arch/arm/kernel/traps.c b/arch/arm/kernel/traps.c
>> index b0179b8..f303ea6 100644
>> --- a/arch/arm/kernel/traps.c
>> +++ b/arch/arm/kernel/traps.c
>> @@ -540,6 +540,10 @@ asmlinkage int arm_syscall(int no, struct pt_regs *regs)
>>       struct thread_info *thread = current_thread_info();
>>       siginfo_t info;
>>
>> +     /* Emulate/fallthrough. */
>> +     if (no == -1)
>> +             return regs->ARM_r0;
>> +
>
> This won't work properly with OABI.  The problem is that OABI has an
> offset on its syscall numbers which is removed/added at appropriate
> times, and this is one of the places where it's put back.  So you end
> up with -1 XOR 0x900000 here, not -1.
>
> It'd probably be better to do this check in the asm code here, which
> prevents that yuckyness from affecting this.
>
> __sys_trace:
>         mov     r1, scno
>         add     r0, sp, #S_OFF
>         bl      syscall_trace_enter
>
>         adr     lr, BSYM(__sys_trace_return)    @ return address
>         mov     scno, r0                        @ syscall number (possibly new)
>         add     r1, sp, #S_R0 + S_OFF           @ pointer to regs
>         cmp     scno, #NR_syscalls              @ check upper syscall limit
>         ldmccia r1, {r0 - r6}                   @ have to reload r0 - r6
>         stmccia sp, {r4, r5}                    @ and update the stack args
>         ldrcc   pc, [tbl, scno, lsl #2]         @ call sys_* routine
> +       cmp     scno, #-1
>         bne     2b
> +       b       ret_slow_syscall
>

Ah! Good call, yes. I'll use this and include it in a v3 posting. Thanks!

-Kees

-- 
Kees Cook
Chrome OS Security
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