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Date:   Wed, 29 Mar 2023 12:09:59 +0100
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Subject: Re: [PATCH v17 2/8] cpu/hotplug: Reset task stack state in _cpu_up()
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From:   Usama Arif <usama.arif@bytedance.com>
To:     dwmw2@infradead.org, tglx@linutronix.de, kim.phillips@amd.com,
        brgerst@gmail.com
Cc:     piotrgorski@cachyos.org, oleksandr@natalenko.name,
        arjan@linux.intel.com, mingo@redhat.com, bp@alien8.de,
        dave.hansen@linux.intel.com, hpa@zytor.com, x86@kernel.org,
        pbonzini@redhat.com, paulmck@kernel.org,
        linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        rcu@vger.kernel.org, mimoja@mimoja.de, hewenliang4@huawei.com,
        thomas.lendacky@amd.com, seanjc@google.com, pmenzel@molgen.mpg.de,
        fam.zheng@bytedance.com, punit.agrawal@bytedance.com,
        simon.evans@bytedance.com, liangma@liangbit.com,
        gpiccoli@igalia.com, David Woodhouse <dwmw@amazon.co.uk>,
        Mark Rutland <mark.rutland@arm.com>
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 <20230328195758.1049469-3-usama.arif@bytedance.com>
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On 28/03/2023 20:57, Usama Arif wrote:
> From: David Woodhouse <dwmw@amazon.co.uk>
> 
> Commit dce1ca0525bf ("sched/scs: Reset task stack state in bringup_cpu()")
> ensured that the shadow call stack was reset and KASAN poisoning removed
> from a CPU's stack each time that CPU is brought up, not just once.
> 
> This is not incorrect. However, with parallel bringup, an architecture
> may obtain the idle thread for a new CPU from a pre-bringup stage, by
> calling idle_thread_get() for itself. This would mean that the cleanup
> in bringup_cpu() would be too late.
> 
> Move the SCS/KASAN cleanup to the generic _cpu_up() function instead,
> which already ensures that the new CPU's stack is available, purely to
> allow for early failure. This occurs when the CPU to be brought up is
> in the CPUHP_OFFLINE state, which should correctly do the cleanup any
> time the CPU has been taken down to the point where such is needed.
> 
> Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
> Reviewed-by: Mark Rutland <mark.rutland@arm.com>
> Tested-by: Mark Rutland <mark.rutland@arm.com> [arm64]

Forgot to include my sign-off. Thanks David for pointing it out.

Signed-off-by: Usama Arif <usama.arif@bytedance.com>

> ---
>   kernel/cpu.c | 12 ++++++------
>   1 file changed, 6 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/cpu.c b/kernel/cpu.c
> index 6c0a92ca6bb5..43e0a77f21e8 100644
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -591,12 +591,6 @@ static int bringup_cpu(unsigned int cpu)
>   	struct task_struct *idle = idle_thread_get(cpu);
>   	int ret;
>   
> -	/*
> -	 * Reset stale stack state from the last time this CPU was online.
> -	 */
> -	scs_task_reset(idle);
> -	kasan_unpoison_task_stack(idle);
> -
>   	/*
>   	 * Some architectures have to walk the irq descriptors to
>   	 * setup the vector space for the cpu which comes online.
> @@ -1383,6 +1377,12 @@ static int _cpu_up(unsigned int cpu, int tasks_frozen, enum cpuhp_state target)
>   			ret = PTR_ERR(idle);
>   			goto out;
>   		}
> +
> +		/*
> +		 * Reset stale stack state from the last time this CPU was online.
> +		 */
> +		scs_task_reset(idle);
> +		kasan_unpoison_task_stack(idle);
>   	}
>   
>   	cpuhp_tasks_frozen = tasks_frozen;