linux-kernel - Re: [PATCH v5 3/3] powerpc/32: Add KASAN support

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Message-ID: <0c8f357d-a0b9-2dcd-63cc-44edec153b6c@virtuozzo.com>
Date:   Fri, 15 Feb 2019 13:38:56 +0300
From:   Andrey Ryabinin <aryabinin@...tuozzo.com>
To:     Christophe Leroy <christophe.leroy@....fr>,
        Daniel Axtens <dja@...ens.net>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        kasan-dev@...glegroups.com, linux-mm@...ck.org
Subject: Re: [PATCH v5 3/3] powerpc/32: Add KASAN support



On 2/15/19 1:10 PM, Christophe Leroy wrote:
> 
> 
> Le 15/02/2019 à 11:01, Andrey Ryabinin a écrit :
>>
>>
>> On 2/15/19 11:41 AM, Christophe Leroy wrote:
>>>
>>>
>>> Le 14/02/2019 à 23:04, Daniel Axtens a écrit :
>>>> Hi Christophe,
>>>>
>>>>> --- a/arch/powerpc/include/asm/string.h
>>>>> +++ b/arch/powerpc/include/asm/string.h
>>>>> @@ -27,6 +27,20 @@ extern int memcmp(const void *,const void *,__kernel_size_t);
>>>>>    extern void * memchr(const void *,int,__kernel_size_t);
>>>>>    extern void * memcpy_flushcache(void *,const void *,__kernel_size_t);
>>>>>    +void *__memset(void *s, int c, __kernel_size_t count);
>>>>> +void *__memcpy(void *to, const void *from, __kernel_size_t n);
>>>>> +void *__memmove(void *to, const void *from, __kernel_size_t n);
>>>>> +
>>>>> +#if defined(CONFIG_KASAN) && !defined(__SANITIZE_ADDRESS__)
>>>>> +/*
>>>>> + * For files that are not instrumented (e.g. mm/slub.c) we
>>>>> + * should use not instrumented version of mem* functions.
>>>>> + */
>>>>> +#define memcpy(dst, src, len) __memcpy(dst, src, len)
>>>>> +#define memmove(dst, src, len) __memmove(dst, src, len)
>>>>> +#define memset(s, c, n) __memset(s, c, n)
>>>>> +#endif
>>>>> +
>>>>
>>>> I'm finding that I miss tests like 'kasan test: kasan_memcmp
>>>> out-of-bounds in memcmp' because the uninstrumented asm version is used
>>>> instead of an instrumented C version. I ended up guarding the relevant
>>>> __HAVE_ARCH_x symbols behind a #ifndef CONFIG_KASAN and only exporting
>>>> the arch versions if we're not compiled with KASAN.
>>>>
>>>> I find I need to guard and unexport strncpy, strncmp, memchr and
>>>> memcmp. Do you need to do this on 32bit as well, or are those tests
>>>> passing anyway for some reason?
>>>
>>> Indeed, I didn't try the KASAN test module recently, because my configs don't have CONFIG_MODULE by default.
>>>
>>> Trying to test it now, I am discovering that module loading oopses with latest version of my series, I need to figure out exactly why. Here below the oops by modprobing test_module (the one supposed to just say hello to the world).
>>>
>>> What we see is an access to the RO kasan zero area.
>>>
>>> The shadow mem is 0xf7c00000..0xffc00000
>>> Linear kernel memory is shadowed by 0xf7c00000-0xf8bfffff
>>> 0xf8c00000-0xffc00000 is shadowed read only by the kasan zero page.
>>>
>>> Why is kasan trying to access that ? Isn't kasan supposed to not check stuff in vmalloc area ?
>>
>> It tries to poison global variables in modules. If module is in vmalloc, than it will try to poison vmalloc.
>> Given that the vmalloc area is not so big on 32bits, the easiest solution is to cover all vmalloc with RW shadow.
>>
> 
> Euh ... Not so big ?
> 
> Memory: 96448K/131072K available (8016K kernel code, 1680K rwdata
> , 2720K rodata, 624K init, 4678K bss, 34624K reserved, 0K cma-reserved)
> Kernel virtual memory layout:
>   * 0xffefc000..0xffffc000  : fixmap
>   * 0xf7c00000..0xffc00000  : kasan shadow mem
>   * 0xf7a00000..0xf7c00000  : consistent mem
>   * 0xf7a00000..0xf7a00000  : early ioremap
>   * 0xc9000000..0xf7a00000  : vmalloc & ioremap
> 
> Here, vmalloc area size 0x2ea00000, that is 746Mbytes. Shadow for this would be 93Mbytes and we are already using 16Mbytes to shadow the linear memory area .... this poor board has 128Mbytes RAM in total.
> 
> So another solution is needed.
> 

Ok.
As a temporary workaround your can make __asan_register_globals() to skip globals in vmalloc(). 
Obviously it means that out-of-bounds accesses to in modules will be missed.

Non temporary solution would making kasan to fully support vmalloc, i.e. remove RO shadow and allocate/free shadow on vmalloc()/vfree().
But this feels like separate task, out of scope of this patch set.

It is also possible to follow some other arches - dedicate separate address range for modules, allocate/free shadow in module_alloc/free.
But it doesn't seem worthy to implement this only for the sake of kasan, since vmalloc support needs to be done anyway.