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

[Christophe LEROY]

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@xxxxxx> 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