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Date: Mon, 5 Oct 2015 13:23:41 +0200
From: Ingo Molnar <mingo@kernel.org>
To: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org, Andy Lutomirski <luto@amacapital.net>,
        Andrey Konovalov <andreyknvl@google.com>,
        Kostya Serebryany <kcc@google.com>,
        Alexander Potapenko <glider@google.com>,
        kasan-dev <kasan-dev@googlegroups.com>, Borislav Petkov <bp@alien8.de>,
        Denys Vlasenko <dvlasenk@redhat.com>, Andi Kleen <ak@linux.intel.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        Sasha Levin <sasha.levin@oracle.com>,
        Wolfram Gloger <wmglo@dent.med.uni-muenchen.de>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH] x86/process: Silence KASAN warnings in get_wchan()
Message-ID: <20151005112341.GA1101@gmail.com>
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* Andrey Ryabinin <aryabinin@virtuozzo.com> wrote:

> get_wchan() is racy by design, it may access volatile stack
> of running task, thus it may access redzone in a stack frame
> and cause KASAN to warn about this.
> 
> Use kasan_disable_current()/kasan_enable_current() to silence
> these warnings.
> 
> Reported-by: Sasha Levin <sasha.levin@oracle.com>
> Signed-off-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
> ---
> 
>  Perhaps it would be better to add something like this:
> 	READ_ONCE_NOCHECK()
> 	{
> 		kasan_disable_current();
> 		READ_ONCE();
> 		kasan_enable_current();
> 	}
>   ?
> 
>  arch/x86/kernel/process.c | 18 +++++++++++++-----
>  1 file changed, 13 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
> index 39e585a..0488eb9 100644
> --- a/arch/x86/kernel/process.c
> +++ b/arch/x86/kernel/process.c
> @@ -13,6 +13,7 @@
>  #include <linux/random.h>
>  #include <linux/user-return-notifier.h>
>  #include <linux/dmi.h>
> +#include <linux/kasan.h>
>  #include <linux/utsname.h>
>  #include <linux/stackprotector.h>
>  #include <linux/tick.h>
> @@ -514,7 +515,7 @@ unsigned long arch_randomize_brk(struct mm_struct *mm)
>   */
>  unsigned long get_wchan(struct task_struct *p)
>  {
> -	unsigned long start, bottom, top, sp, fp, ip;
> +	unsigned long start, bottom, top, sp, fp, ip, ret = 0;
>  	int count = 0;
>  
>  	if (!p || p == current || p->state == TASK_RUNNING)
> @@ -550,14 +551,21 @@ unsigned long get_wchan(struct task_struct *p)
>  	if (sp < bottom || sp > top)
>  		return 0;
>  
> +	kasan_disable_current();
>  	fp = READ_ONCE(*(unsigned long *)sp);
>  	do {
>  		if (fp < bottom || fp > top)
> -			return 0;
> +			goto out;

a break would do just fine too.

> +
>  		ip = READ_ONCE(*(unsigned long *)(fp + sizeof(unsigned long)));
> -		if (!in_sched_functions(ip))
> -			return ip;
> +		if (!in_sched_functions(ip)) {
> +			ret = ip;
> +			goto out;

ditto.

> +		}
>  		fp = READ_ONCE(*(unsigned long *)fp);
>  	} while (count++ < 16 && p->state != TASK_RUNNING);
> -	return 0;
> +
> +out:

and then the label would not be needed.

> +	kasan_enable_current();
> +	return ret;

But that's all pretty disgusting really.

Cannot we do better, such as annotating the function and then KASAN sorting out 
its false positives, or something like that?

Thanks,

	Ingo