Date:	Wed, 2 Mar 2016 15:53:36 +0300
From:	Andrey Ryabinin <ryabinin.a.a@...il.com>
To:	Mark Rutland <mark.rutland@....com>,
	Alexander Potapenko <glider@...gle.com>
Cc:	Andrey Konovalov <adech.fo@...il.com>,
	Dmitriy Vyukov <dvyukov@...gle.com>, will.deacon@....com,
	catalin.marinas@....com, Andrew Morton <akpm@...ux-foundation.org>,
	kasan-dev@...glegroups.com, LKML <linux-kernel@...r.kernel.org>,
	linux-arm-kernel@...ts.infradead.org,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v1] kasan, arm64: Unpoison dirty stack frames when
 resuming from suspend.



On 03/01/2016 10:42 PM, Mark Rutland wrote:
> On Fri, Feb 26, 2016 at 06:28:27PM +0100, Alexander Potapenko wrote:
>> On Fri, Feb 26, 2016 at 2:53 PM, Mark Rutland <mark.rutland@....com> wrote:
>>> Hi,
>>>
>>> On Fri, Feb 26, 2016 at 01:38:37PM +0100, Alexander Potapenko wrote:
>>>> Before an ARM64 CPU is suspended, the kernel saves the context which will
>>>> be used to initialize the register state upon resume. After that and
>>>> before the actual execution of the SMC instruction the kernel creates
>>>> several stack frames which are never unpoisoned because arm_smccc_smc()
>>>> does not return. This may cause false positive stack buffer overflow
>>>> reports from KASAN.
>>>>
>>>> The solution is to record the stack pointer value just before the CPU is
>>>> suspended, and unpoison the part of stack between the saved value and
>>>> the stack pointer upon resume.
>>>
>>> Thanks for looking into this! That's much appreciated.
>>>
>>> I think the general approach (unposioning the stack upon cold return to
>>> the kernel) is fine, but I have concerns with the implementation, which
>>> I've noted below.
> 
> For the idle case I intend to respin my patch [1] which calls
> kasan_unpoison_shadow from assembly in the resume path, as I think
> that's the only reliable approach.
> 
>>> The problem also applies for hotplug, as leftover poison from the
>>> hot-unplug path isn't cleaned before a CPU is hotplugged back on. The
>>> first few functions are likely deterministic in their stack usage, so
>>> it's not seen with a defconfig, but I think it's possible to trigger,
>>> and it's also a cross-architecture problem shared with x86.
>> Agreed, but since I haven't yet seen problems with hotplug, it's hard
>> to test the fix for them.
> 
> For the common hotplug case, how about the below?
> 

Nah, looks a bit hacky IMO. I think it's better to use cpu hotplug notifier.
I'll send patch shortly.

> I've given it a spin locally on arm64 with the reproducer I posted
> earlier.
> 
> Thanks,
> Mark.
> 
> [1] http://lists.infradead.org/pipermail/linux-arm-kernel/2016-February/409466.html
> 
> ---->8----
> From 34839286826c88338cd91a142b1bcc3c077a87aa Mon Sep 17 00:00:00 2001
> From: Mark Rutland <mark.rutland@....com>
> Date: Tue, 1 Mar 2016 19:27:23 +0000
> Subject: [PATCH] sched/kasan: clear stale stack poison
> 
> CPUs get hotplugged out some levels deep in C code, and hence when KASAN
> is in use, the instrumented function preambles will have left the stack
> shadow area poisoned.
> 
> This poison is not cleared, so when a CPU re-enters the kernel, it is
> possible for accesses in instrumented functions to hit this stale
> poison, resulting in (spurious) KASAN splats.
> 
> This patch forcefully unpoisons an idle task's stack shadow when it is
> re-initialised prior to a hotplug, avoiding spurious hits against stale
> poison.
> 
> Signed-off-by: Mark Rutland <mark.rutland@....com>
> ---
>  include/linux/kasan.h |  4 ++++
>  kernel/sched/core.c   |  3 +++
>  mm/kasan/kasan.c      | 10 ++++++++++
>  3 files changed, 17 insertions(+)
> 
> diff --git a/include/linux/kasan.h b/include/linux/kasan.h
> index 4b9f85c..e00486f 100644
> --- a/include/linux/kasan.h
> +++ b/include/linux/kasan.h
> @@ -43,6 +43,8 @@ static inline void kasan_disable_current(void)
>  
>  void kasan_unpoison_shadow(const void *address, size_t size);
>  
> +void kasan_unpoison_task_stack(struct task_struct *idle);
> +
>  void kasan_alloc_pages(struct page *page, unsigned int order);
>  void kasan_free_pages(struct page *page, unsigned int order);
>  
> @@ -66,6 +68,8 @@ void kasan_free_shadow(const struct vm_struct *vm);
>  
>  static inline void kasan_unpoison_shadow(const void *address, size_t size) {}
>  
> +static inline void kasan_unpoison_task_stack(struct task_struct *idle) {}
> +
>  static inline void kasan_enable_current(void) {}
>  static inline void kasan_disable_current(void) {}
>  
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 9503d59..41f6b22 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -26,6 +26,7 @@
>   *              Thomas Gleixner, Mike Kravetz
>   */
>  
> +#include <linux/kasan.h>
>  #include <linux/mm.h>
>  #include <linux/module.h>
>  #include <linux/nmi.h>
> @@ -5096,6 +5097,8 @@ void init_idle(struct task_struct *idle, int cpu)
>  	idle->state = TASK_RUNNING;
>  	idle->se.exec_start = sched_clock();
>  
> +	kasan_unpoison_task_stack(idle);
> +
>  #ifdef CONFIG_SMP
>  	/*
>  	 * Its possible that init_idle() gets called multiple times on a task,
> diff --git a/mm/kasan/kasan.c b/mm/kasan/kasan.c
> index bc0a8d8..467f394 100644
> --- a/mm/kasan/kasan.c
> +++ b/mm/kasan/kasan.c
> @@ -60,6 +60,16 @@ void kasan_unpoison_shadow(const void *address, size_t size)
>  	}
>  }
>  
> +/*
> + * Remove any poison left on the stack from a prior hot-unplug.
> + */
> +void kasan_unpoison_task_stack(struct task_struct *idle)
> +{
> +	void *base = task_stack_page(idle) + sizeof(struct thread_info);
> +	size_t size = THREAD_SIZE - sizeof(struct thread_info);
> +
> +	kasan_unpoison_shadow(base, size);
> +}
>  
>  /*
>   * All functions below always inlined so compiler could
> 

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