Re: [PATCH v4 3/3] powerpc/32: Add KASAN support

[Daniel Axtens]

Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx> writes:

> On 2/11/19 3:25 PM, Andrey Konovalov wrote:
>> On Sat, Feb 9, 2019 at 12:55 PM christophe leroy
>> <christophe.leroy@xxxxxx> wrote:
>>>
>>> Hi Andrey,
>>>
>>> Le 08/02/2019 Ã 18:40, Andrey Konovalov a Ãcrit :
>>>> On Fri, Feb 8, 2019 at 6:17 PM Christophe Leroy <christophe.leroy@xxxxxx> wrote:
>>>>>
>>>>> Hi Daniel,
>>>>>
>>>>> Le 08/02/2019 Ã 17:18, Daniel Axtens a Ãcrit :
>>>>>> Hi Christophe,
>>>>>>
>>>>>> I've been attempting to port this to 64-bit Book3e nohash (e6500),
>>>>>> although I think I've ended up with an approach more similar to Aneesh's
>>>>>> much earlier (2015) series for book3s.
>>>>>>
>>>>>> Part of this is just due to the changes between 32 and 64 bits - we need
>>>>>> to hack around the discontiguous mappings - but one thing that I'm
>>>>>> particularly puzzled by is what the kasan_early_init is supposed to do.
>>>>>
>>>>> It should be a problem as my patch uses a 'for_each_memblock(memory,
>>>>> reg)' loop.
>>>>>
>>>>>>
>>>>>>> +void __init kasan_early_init(void)
>>>>>>> +{
>>>>>>> +    unsigned long addr = KASAN_SHADOW_START;
>>>>>>> +    unsigned long end = KASAN_SHADOW_END;
>>>>>>> +    unsigned long next;
>>>>>>> +    pmd_t *pmd = pmd_offset(pud_offset(pgd_offset_k(addr), addr), addr);
>>>>>>> +    int i;
>>>>>>> +    phys_addr_t pa = __pa(kasan_early_shadow_page);
>>>>>>> +
>>>>>>> +    BUILD_BUG_ON(KASAN_SHADOW_START & ~PGDIR_MASK);
>>>>>>> +
>>>>>>> +    if (early_mmu_has_feature(MMU_FTR_HPTE_TABLE))
>>>>>>> +            panic("KASAN not supported with Hash MMU\n");
>>>>>>> +
>>>>>>> +    for (i = 0; i < PTRS_PER_PTE; i++)
>>>>>>> +            __set_pte_at(&init_mm, (unsigned long)kasan_early_shadow_page,
>>>>>>> +                         kasan_early_shadow_pte + i,
>>>>>>> +                         pfn_pte(PHYS_PFN(pa), PAGE_KERNEL_RO), 0);
>>>>>>> +
>>>>>>> +    do {
>>>>>>> +            next = pgd_addr_end(addr, end);
>>>>>>> +            pmd_populate_kernel(&init_mm, pmd, kasan_early_shadow_pte);
>>>>>>> +    } while (pmd++, addr = next, addr != end);
>>>>>>> +}
>>>>>>
>>>>>> As far as I can tell it's mapping the early shadow page, read-only, over
>>>>>> the KASAN_SHADOW_START->KASAN_SHADOW_END range, and it's using the early
>>>>>> shadow PTE array from the generic code.
>>>>>>
>>>>>> I haven't been able to find an answer to why this is in the docs, so I
>>>>>> was wondering if you or anyone else could explain the early part of
>>>>>> kasan init a bit better.
>>>>>
>>>>> See https://www.kernel.org/doc/html/latest/dev-tools/kasan.html for an
>>>>> explanation of the shadow.
>>>>>
>>>>> When shadow is 0, it means the memory area is entirely accessible.
>>>>>
>>>>> It is necessary to setup a shadow area as soon as possible because all
>>>>> data accesses check the shadow area, from the begining (except for a few
>>>>> files where sanitizing has been disabled in Makefiles).
>>>>>
>>>>> Until the real shadow area is set, all access are granted thanks to the
>>>>> zero shadow area beeing for of zeros.
>>>>
>>>> Not entirely correct. kasan_early_init() indeed maps the whole shadow
>>>> memory range to the same kasan_early_shadow_page. However as kernel
>>>> loads and memory gets allocated this shadow page gets rewritten with
>>>> non-zero values by different KASAN allocator hooks. Since these values
>>>> come from completely different parts of the kernel, but all land on
>>>> the same page, kasan_early_shadow_page's content can be considered
>>>> garbage. When KASAN checks memory accesses for validity it detects
>>>> these garbage shadow values, but doesn't print any reports, as the
>>>> reporting routine bails out on the current->kasan_depth check (which
>>>> has the value of 1 initially). Only after kasan_init() completes, when
>>>> the proper shadow memory is mapped, current->kasan_depth gets set to 0
>>>> and we start reporting bad accesses.
>>>
>>> That's surprising, because in the early phase I map the shadow area
>>> read-only, so I do not expect it to get modified unless RO protection is
>>> failing for some reason.
>> 
>> Actually it might be that the allocator hooks don't modify shadow at
>> this point, as the allocator is not yet initialized. However stack
>> should be getting poisoned and unpoisoned from the very start. But the
>> generic statement that early shadow gets dirtied should be correct.
>> Might it be that you don't use stack instrumentation?
>> 
>
> Yes, stack instrumentation is not used here, because shadow offset which we pass to
> the -fasan-shadow-offset= cflag is not specified here. So the logic in scrpits/Makefile.kasan
> just fallbacks to CFLAGS_KASAN_MINIMAL, which is outline and without stack instrumentation.
>
> Christophe, you can specify KASAN_SHADOW_OFFSET either in Kconfig (e.g. x86_64) or
> in Makefile (e.g. arm64). And make early mapping writable, because compiler generated code will write
> to shadow memory in function prologue/epilogue.

Hmm. Is this limitation just that compilers have not implemented
out-of-line support for stack instrumentation, or is there a deeper
reason that stack/global instrumentation relies upon inline
instrumentation?

I ask because it's very common on ppc64 to have the virtual address
space split up into discontiguous blocks. I know this means we lose
inline instrumentation, but I didn't realise we'd also lose stack and
global instrumentation...

I wonder if it would be worth, in the distant future, trying to
implement a smarter scheme in compilers where we could insert more
complex inline mapping schemes.

Regards,
Daniel