Date:   Tue, 16 Jan 2018 08:39:45 +0000
From:   "Liuwenliang (Abbott Liu)" <liuwenliang@...wei.com>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
CC:     Russell King - ARM Linux <linux@...linux.org.uk>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
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        Laura Abbott <labbott@...hat.com>,
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        Heshaoliang <heshaoliang@...wei.com>
Subject: Re: [PATCH 06/11] change memory_is_poisoned_16 for aligned error

On 6 December 2017 at 1:09  Ard Biesheuvel [ard.biesheuvel@...aro.org] wrote:
>On 5 December 2017 at 14:19, Liuwenliang (Abbott Liu)
><liuwenliang@...wei.com> wrote:
>> On Nov 23, 2017  20:30  Russell King - ARM Linux [mailto:linux@...linux.org.uk]  wrote:
>>>On Thu, Oct 12, 2017 at 11:27:40AM +0000, Liuwenliang (Lamb) wrote:
>>>> >> - 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.
>>
>>>What about using get_unaligned() ?
>>
>> Thanks for your review.
>>
>> I think it is good idea to use get_unaligned. But ARMv7 support CONFIG_ HAVE_EFFICIENT_UNALIGNED_ACCESS
>> (arch/arm/Kconfig : select HAVE_EFFICIENT_UNALIGNED_ACCESS if (CPU_V6 || CPU_V6K || CPU_V7) && MMU).
>> So on ARMv7, the code:
>> u16 *shadow_addr = get_unaligned((u16 *)kasan_mem_to_shadow((void *)addr));
>> equals the code:000
>> u16 *shadow_addr = (u16 *)kasan_mem_to_shadow((void *)addr);
>>
>
>No it does not. The compiler may merge adjacent accesses into ldm or
>ldrd instructions, which do not tolerate misalignment regardless of
>the SCTLR.A bit.
>
>This is actually something we may need to fix for ARM, i.e., drop
>HAVE_EFFICIENT_UNALIGNED_ACCESS altogether, or carefully review the
>way it is used currently.
>
>> On ARMv7, if SCRLR.A is 0, unaligned access is OK.  Here is the description comes from ARM(r) Architecture Reference
>> Manual ARMv7-A and ARMv7-R edition :
>>
><snip>
>
>Could you *please* stop quoting the ARM ARM at us? People who are
>seeking detailed information like that will know where to find it.
>
>--
>Ard.

Thanks for Ard Biesheuvel's review.
Using get_unaligned does not give us too much benefit, and get_unaligned may have some problem.
So it may be better to not use get_unaligned.

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