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Date:   Tue, 25 Apr 2023 13:53:13 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Joel Fernandes <joel@joelfernandes.org>
Cc:     Boqun Feng <boqun.feng@gmail.com>,
        Segher Boessenkool <segher@kernel.crashing.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Zhouyi Zhou <zhouzhouyi@gmail.com>,
        linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
        rcu <rcu@vger.kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>, lance@osuosl.org,
        "Paul E. McKenney" <paulmck@kernel.org>
Subject: Re: BUG : PowerPC RCU: torture test failed with __stack_chk_fail
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On Tue, Apr 25, 2023 at 06:59:29AM -0400, Joel Fernandes wrote:
> > I'm a little confused; the way I understand the whole stack protector
> > thing to work is that we push a canary on the stack at call and on
> > return check it is still valid. Since in general tasks randomly migrate,
> > the per-cpu validation canary should be the same on all CPUs.

> AFAICS, the canary is randomly chosen both in the kernel [1]. This

Yes, at boot, once. But thereafter it should be the same for all CPUs.

> also appears to be the case in glibc. That makes sense because you
> don't want the canary to be something that the attacker can easily
> predict and store on the stack to bypass buffer overflow attacks:
> 
> [1] kernel :
> /*
>  * 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 = get_random_canary();
> 
>         current->stack_canary = canary;
> #ifdef CONFIG_PPC64
>         get_paca()->canary = canary;
> #endif
> }
> 
> thanks,
> 
>  - Joel