linux-kernel - Re: [PATCH] x86/kasan: map shadow for percpu pages on demand

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Message-ID: <7afa3099-cc62-b439-24ea-4d04f03ab402@intel.com>
Date:   Mon, 31 Oct 2022 08:15:40 +0800
From:   "Yin, Fengwei" <fengwei.yin@...el.com>
To:     Andrey Ryabinin <ryabinin.a.a@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "kernel test robot" <yujie.liu@...el.com>
CC:     Seth Jenkins <sethjenkins@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        <linux-kernel@...r.kernel.org>, <x86@...nel.org>,
        Alexander Potapenko <glider@...gle.com>,
        Andrey Konovalov <andreyknvl@...il.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        <kasan-dev@...glegroups.com>, Thomas Gleixner <tglx@...utronix.de>,
        "Ingo Molnar" <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH] x86/kasan: map shadow for percpu pages on demand



On 10/28/2022 10:20 PM, Andrey Ryabinin wrote:
> 
> 
> On 10/28/22 05:51, Yin, Fengwei wrote:
>> Hi Andrey,
>>
> 
>>>  void __init kasan_init(void)
>>>  {
>>>  	int i;
>>> @@ -393,9 +405,6 @@ void __init kasan_init(void)
>>>  		kasan_mem_to_shadow((void *)VMALLOC_END + 1),
>>>  		shadow_cpu_entry_begin);
>>>  
>>> -	kasan_populate_shadow((unsigned long)shadow_cpu_entry_begin,
>>> -			      (unsigned long)shadow_cpu_entry_end, 0);
>>> -
>> There will be address in the range (shadow_cpu_entry_begin, shadow_cpu_entry_end)
>> which has no KASAN shadow mapping populated after the patch. Not sure whether
>> it could be a problem. Thanks.
>>
> 
> 
> This shouldn't be a problem. It's vital to have shadow *only* for addresses with mapped memory.
> Shadow address accessed only if the address itself accessed. So the difference between not having shadow
> for address with no mapping vs having it, is whether we crash on access to KASAN shadow or crash few
> instructions later on access to the address itself.

Thanks for clarification.

Regards
Yin, Fengwei