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Date: Thu, 26 Jan 2017 17:40:47 +0100
From: Yann Droneaud <ydroneaud@...eya.com>
To: Will Deacon <will.deacon@....com>, kpark3469@...il.com
Cc: kernel-hardening@...ts.openwall.com, catalin.marinas@....com, 
	keescook@...omium.org, mark.rutland@....com, james.morse@....com, 
	panand@...hat.com, keun-o.park@...kmatter.ae, takahiro.akashi@...aro.org
Subject: Re: Re: [PATCH] arm64: usercopy: Implement stack
 frame object validation

Hi,

Le mercredi 25 janvier 2017 à 13:54 +0000, Will Deacon a écrit :
> diff --git a/arch/arm64/include/asm/thread_info.h
> > b/arch/arm64/include/asm/thread_info.h
> > index 46c3b93..f610c44 100644
> > --- a/arch/arm64/include/asm/thread_info.h
> > +++ b/arch/arm64/include/asm/thread_info.h
> > @@ -68,7 +68,62 @@ struct thread_info {
> > +	const void *oldframe;
> > +	const void *callee_fp = NULL;
> > +	const void *caller_fp = NULL;
> > +
> > +	oldframe = __builtin_frame_address(1);
> > +	if (oldframe) {
> > +		callee_fp = __builtin_frame_address(2);
> > +		if (callee_fp)
> > +			caller_fp = __builtin_frame_address(3);
> > +	}
> > 
> Which compilers have you tested this with? The GCC folks don't
> guarantee a frame layout, and they have changed it in the past, so I
> suspect this is pretty fragile. In particularly, if
> __builtin_frame_address just points at the frame record, then I don't
> think you can make assumptions about the placement of local variables
> and arguments with respect to that.
> 

https://gcc.gnu.org/onlinedocs/gcc-6.3.0/gcc/Return-Address.html#index-
g_t_005f_005fbuiltin_005fframe_005faddress-3701

"Calling this function with a nonzero argument can have unpredictable 
 effects, including crashing the calling program. As a result, calls 
 that are considered unsafe are diagnosed when the -Wframe-address 
 option is in effect. Such calls should only be made in debugging 
 situations."

-- 
Yann Droneaud
OPTEYA

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