linux-kernel - Re: [PATCH 06/11] change memory_is_poisoned

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Date:   Thu, 12 Oct 2017 11:27:40 +0000
From:   "Liuwenliang (Lamb)" <liuwenliang@...wei.com>
To:     Dmitry Vyukov <dvyukov@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>
CC:     Russell King - ARM Linux <linux@...linux.org.uk>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        "afzal.mohd.ma@...il.com" <afzal.mohd.ma@...il.com>,
        "f.fainelli@...il.com" <f.fainelli@...il.com>,
        Laura Abbott <labbott@...hat.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Michal Hocko <mhocko@...e.com>,
        "cdall@...aro.org" <cdall@...aro.org>,
        "marc.zyngier@....com" <marc.zyngier@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        "Matthew Wilcox" <mawilcox@...rosoft.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Thomas Garnier" <thgarnie@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        "Arnd Bergmann" <arnd@...db.de>,
        Vladimir Murzin <vladimir.murzin@....com>,
        "tixy@...aro.org" <tixy@...aro.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        "robin.murphy@....com" <robin.murphy@....com>,
        Ingo Molnar <mingo@...nel.org>,
        "grygorii.strashko@...aro.org" <grygorii.strashko@...aro.org>,
        Alexander Potapenko <glider@...gle.com>,
        "opendmb@...il.com" <opendmb@...il.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        kasan-dev <kasan-dev@...glegroups.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Jiazhenghua <jiazhenghua@...wei.com>,
        Dailei <dylix.dailei@...wei.com>,
        Zengweilin <zengweilin@...wei.com>,
        Heshaoliang <heshaoliang@...wei.com>,
        "Liuwenliang (Lamb)" <liuwenliang@...wei.com>
Subject: Re: [PATCH 06/11] change memory_is_poisoned_16 for aligned error

>> - I don't understand why this is necessary.  memory_is_poisoned_16()
>>   already handles unaligned addresses?
>>
>> - If it's needed on ARM then presumably it will be needed on other
>>   architectures, so CONFIG_ARM is insufficiently general.
>>
>> - If the present memory_is_poisoned_16() indeed doesn't work on ARM,
>>   it would be better to generalize/fix it in some fashion rather than
>>   creating a new variant of the function.


>Yes, I think it will be better to fix the current function rather then
>have 2 slightly different copies with ifdef's.
>Will something along these lines work for arm? 16-byte accesses are
>not too common, so it should not be a performance problem. And
>probably modern compilers can turn 2 1-byte checks into a 2-byte check
>where safe (x86).

>static __always_inline bool memory_is_poisoned_16(unsigned long addr)
>{
>        u8 *shadow_addr = (u8 *)kasan_mem_to_shadow((void *)addr);
>
>        if (shadow_addr[0] || shadow_addr[1])
>                return true;
>        /* Unaligned 16-bytes access maps into 3 shadow bytes. */
>        if (unlikely(!IS_ALIGNED(addr, KASAN_SHADOW_SCALE_SIZE)))
>                return memory_is_poisoned_1(addr + 15);
>        return false;
>}

Thanks for Andrew Morton and Dmitry Vyukov's review. 
If the parameter addr=0xc0000008, now in function:
static __always_inline bool memory_is_poisoned_16(unsigned long addr)
{
 ---     //shadow_addr = (u16 *)(KASAN_OFFSET+0x18000001(=0xc0000008>>3)) is not 
 ---     // unsigned by 2 bytes.
        u16 *shadow_addr = (u16 *)kasan_mem_to_shadow((void *)addr); 

        /* Unaligned 16-bytes access maps into 3 shadow bytes. */
        if (unlikely(!IS_ALIGNED(addr, KASAN_SHADOW_SCALE_SIZE)))
                return *shadow_addr || memory_is_poisoned_1(addr + 15);
----      //here is going to be error on arm, specially when kernel has not finished yet.
----      //Because the unsigned accessing cause DataAbort Exception which is not
----      //initialized when kernel is starting. 
        return *shadow_addr;
}

I also think it is better to fix this problem.