linux-kernel - Re: [PATCH 1/2] powerpc: initial stack protector (-fstack-protector) support

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Message-ID: <823f6257-0071-a8af-1789-56749da71dc9@c-s.fr>
Date:   Tue, 22 Nov 2016 11:51:33 +0100
From:   Christophe LEROY <christophe.leroy@....fr>
To:     Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Scott Wood <oss@...error.net>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        Nicolas Pitre <nicolas.pitre@...aro.org>
Subject: Re: [PATCH 1/2] powerpc: initial stack protector (-fstack-protector)
 support



Le 17/11/2016 à 12:05, Michael Ellerman a écrit :

Hi Michael,

I took your comments into account in v2. Shame on me, I forgot to add 
the list of changes from v1 to v2 in the commit log.

Christophe

> Christophe Leroy <christophe.leroy@....fr> writes:
>
>> diff --git a/arch/powerpc/include/asm/stackprotector.h b/arch/powerpc/include/asm/stackprotector.h
>> new file mode 100644
>> index 0000000..de00332
>> --- /dev/null
>> +++ b/arch/powerpc/include/asm/stackprotector.h
>> @@ -0,0 +1,38 @@
>> +/*
>> + * GCC stack protector support.
>> + *
>> + * Stack protector works by putting predefined pattern at the start of
>> + * the stack frame and verifying that it hasn't been overwritten when
>> + * returning from the function.  The pattern is called stack canary
>> + * and gcc expects it to be defined by a global variable called
>> + * "__stack_chk_guard" on ARM.  This unfortunately means that on SMP
>                              ^
>                              PPC
>
>> + * we cannot have a different canary value per task.
>> + */
>> +
>> +#ifndef _ASM_STACKPROTECTOR_H
>> +#define _ASM_STACKPROTECTOR_H 1
>
> We usually just define it, not define it to 1.
>
>> +
>> +#include <linux/random.h>
>> +#include <linux/version.h>
>> +
>> +extern unsigned long __stack_chk_guard;
>> +
>> +/*
>> + * Initialize the stackprotector canary value.
>> + *
>> + * NOTE: this must only be called from functions that never return,
>> + * and it must always be inlined.
>> + */
>> +static __always_inline void boot_init_stack_canary(void)
>> +{
>> +	unsigned long canary;
>> +
>> +	/* Try to get a semi random initial value. */
>> +	get_random_bytes(&canary, sizeof(canary));
>> +	canary ^= LINUX_VERSION_CODE;
>
> What about mixing in an mftb() as well ?
>
>> diff --git a/arch/powerpc/kernel/Makefile b/arch/powerpc/kernel/Makefile
>> index e59ed6a..4a62179 100644
>> --- a/arch/powerpc/kernel/Makefile
>> +++ b/arch/powerpc/kernel/Makefile
>> @@ -19,6 +19,11 @@ CFLAGS_init.o += $(DISABLE_LATENT_ENTROPY_PLUGIN)
>>  CFLAGS_btext.o += $(DISABLE_LATENT_ENTROPY_PLUGIN)
>>  CFLAGS_prom.o += $(DISABLE_LATENT_ENTROPY_PLUGIN)
>>
>> +# -fstack-protector triggers protection checks in this code,
>> +# but it is being used too early to link to meaningful stack_chk logic.
>> +nossp_flags := $(call cc-option, -fno-stack-protector)
>> +CFLAGS_prom_init.o := $(nossp_flags)
>
> We've already assigned to CFLAGS_prom_init.o so I think you should be
> using += not := shouldn't you?
>
> Also it could just be a single line:
>
> CFLAGS_prom_init.o += $(call cc-option, -fno-stack-protector)
>
>
> cheers
>