linux-kernel - Re: [PATCH v2 2/2] powerpc/32: add stack protector support

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Date:   Wed, 19 Sep 2018 09:32:43 -0500
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Christophe LEROY <christophe.leroy@....fr>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v2 2/2] powerpc/32: add stack protector support

On Wed, Sep 19, 2018 at 04:22:52PM +0200, Christophe LEROY wrote:
> >It looks like it will be easy to enable on 64 bit as well.
> 
> Will it ? It seems that PPC64 doesn't have r2 pointing to current task 
> struct, but instead it has r13 pointing to the paca struct. Which means 
> we should add a canary in the paca struct, and populate it at task 
> switch from current->stack_canary. Or am I missing something ?

No, I am just forgetting things :-)

> >>+	/* Try to get a semi random initial value. */
> >>+	get_random_bytes(&canary, sizeof(canary));
> >>+	canary ^= mftb();
> >>+	canary ^= LINUX_VERSION_CODE;
> >
> >These last two lines are useless (or worse, they may give people the idea
> >that they are not!)
> 
> Well, the last line is in all arches except x86
> The mftb() was suggested by Michael to add some entropy.
> x86 does the same sort of thing with their rdtsc()
> 
> >
> >You should use wait_for_random_bytes I think.
> 
> On the 8xx, it takes several minutes before crnd_is_ready(), while 
> boot_init_stack_canary() is called quite early in start_kernel()

If you do not provide real entropy to the canary, the canary doesn't help
providing protection as much as you may hope.


Segher