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

[Christophe Leroy]

Hi Daniel,

Le 08/02/2019 à 17:18, Daniel Axtens a écrit :
> 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.

I mainly used ARM arch as an exemple when I implemented KASAN for ppc32.

> 
> At the moment, I don't do any early init, and like Aneesh's series for
> book3s, I end up needing a special flag to disable kasan until after
> kasan_init. Also, as with Balbir's seris for Radix, some tests didn't
> fire, although my missing tests are a superset of his. I suspect the
> early init has something to do with these...?

I think you should really focus on establishing a zero shadow area as 
early as possible instead of trying to ack the core parts of KASAN.

> 
> (I'm happy to collate answers into a patch to the docs, btw!)

We can also have the discussion going via 
https://github.com/linuxppc/issues/issues/106

> 
> In the long term I hope to revive Aneesh's and Balbir's series for hash
> and radix as well.

Great.

Christophe

> 
> Regards,
> Daniel
> 
>> +
>> +static void __init kasan_init_region(struct memblock_region *reg)
>> +{
>> +	void *start = __va(reg->base);
>> +	void *end = __va(reg->base + reg->size);
>> +	unsigned long k_start, k_end, k_cur, k_next;
>> +	pmd_t *pmd;
>> +
>> +	if (start >= end)
>> +		return;
>> +
>> +	k_start = (unsigned long)kasan_mem_to_shadow(start);
>> +	k_end = (unsigned long)kasan_mem_to_shadow(end);
>> +	pmd = pmd_offset(pud_offset(pgd_offset_k(k_start), k_start), k_start);
>> +
>> +	for (k_cur = k_start; k_cur != k_end; k_cur = k_next, pmd++) {
>> +		k_next = pgd_addr_end(k_cur, k_end);
>> +		if ((void *)pmd_page_vaddr(*pmd) == kasan_early_shadow_pte) {
>> +			pte_t *new = pte_alloc_one_kernel(&init_mm);
>> +
>> +			if (!new)
>> +				panic("kasan: pte_alloc_one_kernel() failed");
>> +			memcpy(new, kasan_early_shadow_pte, PTE_TABLE_SIZE);
>> +			pmd_populate_kernel(&init_mm, pmd, new);
>> +		}
>> +	};
>> +
>> +	for (k_cur = k_start; k_cur < k_end; k_cur += PAGE_SIZE) {
>> +		void *va = memblock_alloc(PAGE_SIZE, PAGE_SIZE);
>> +		pte_t pte = pfn_pte(PHYS_PFN(__pa(va)), PAGE_KERNEL);
>> +
>> +		if (!va)
>> +			panic("kasan: memblock_alloc() failed");
>> +		pmd = pmd_offset(pud_offset(pgd_offset_k(k_cur), k_cur), k_cur);
>> +		pte_update(pte_offset_kernel(pmd, k_cur), ~0, pte_val(pte));
>> +	}
>> +	flush_tlb_kernel_range(k_start, k_end);
>> +}
>> +
>> +void __init kasan_init(void)
>> +{
>> +	struct memblock_region *reg;
>> +
>> +	for_each_memblock(memory, reg)
>> +		kasan_init_region(reg);
>> +
>> +	kasan_init_tags();
>> +
>> +	/* At this point kasan is fully initialized. Enable error messages */
>> +	init_task.kasan_depth = 0;
>> +	pr_info("KASAN init done\n");
>> +}
>> diff --git a/arch/powerpc/mm/mem.c b/arch/powerpc/mm/mem.c
>> index 33cc6f676fa6..ae7db88b72d6 100644
>> --- a/arch/powerpc/mm/mem.c
>> +++ b/arch/powerpc/mm/mem.c
>> @@ -369,6 +369,10 @@ void __init mem_init(void)
>>   	pr_info("  * 0x%08lx..0x%08lx  : highmem PTEs\n",
>>   		PKMAP_BASE, PKMAP_ADDR(LAST_PKMAP));
>>   #endif /* CONFIG_HIGHMEM */
>> +#ifdef CONFIG_KASAN
>> +	pr_info("  * 0x%08lx..0x%08lx  : kasan shadow mem\n",
>> +		KASAN_SHADOW_START, KASAN_SHADOW_END);
>> +#endif
>>   #ifdef CONFIG_NOT_COHERENT_CACHE
>>   	pr_info("  * 0x%08lx..0x%08lx  : consistent mem\n",
>>   		IOREMAP_TOP, IOREMAP_TOP + CONFIG_CONSISTENT_SIZE);
>> -- 
>> 2.13.3