Re: [PATCH RFT] arm64: kasan: Make KASAN work with 16K pages + 48 bit VA

[Andrey Ryabinin]

On 11/26/2015 05:48 PM, Mark Rutland wrote:
> Hi,
> 
> On Thu, Nov 26, 2015 at 04:14:46PM +0300, Andrey Ryabinin wrote:
>> Currently kasan assumes that shadow memory covers one or more entire PGDs.
>> That's not true for 16K pages + 48bit VA space, where PGDIR_SIZE is bigger
>> than the whole shadow memory.
>>
>> This patch tries to fix that case.
>> clear_page_tables() is a new replacement of clear_pgs(). Instead of always
>> clearing pgds it clears top level page table entries that entirely belongs
>> to shadow memory.
>> In addition to 'tmp_pg_dir' we now have 'tmp_pud' which is used to store
>> puds that now might be cleared by clear_page_tables.
>>
>> Reported-by: Suzuki K. Poulose <Suzuki.Poulose@xxxxxxx>
>> Signed-off-by: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
>> ---
>>
>>  *** THIS is not tested with 16k pages ***
>>
>>  arch/arm64/mm/kasan_init.c | 87 ++++++++++++++++++++++++++++++++++++++++------
>>  1 file changed, 76 insertions(+), 11 deletions(-)
>>
>> diff --git a/arch/arm64/mm/kasan_init.c b/arch/arm64/mm/kasan_init.c
>> index cf038c7..ea9f92a 100644
>> --- a/arch/arm64/mm/kasan_init.c
>> +++ b/arch/arm64/mm/kasan_init.c
>> @@ -22,6 +22,7 @@
>>  #include <asm/tlbflush.h>
>>  
>>  static pgd_t tmp_pg_dir[PTRS_PER_PGD] __initdata __aligned(PGD_SIZE);
>> +static pud_t tmp_pud[PAGE_SIZE/sizeof(pud_t)] __initdata __aligned(PAGE_SIZE);
>>  
>>  static void __init kasan_early_pte_populate(pmd_t *pmd, unsigned long addr,
>>  					unsigned long end)
>> @@ -92,20 +93,84 @@ asmlinkage void __init kasan_early_init(void)
>>  {
>>  	BUILD_BUG_ON(KASAN_SHADOW_OFFSET != KASAN_SHADOW_END - (1UL << 61));
>>  	BUILD_BUG_ON(!IS_ALIGNED(KASAN_SHADOW_START, PGDIR_SIZE));
>> -	BUILD_BUG_ON(!IS_ALIGNED(KASAN_SHADOW_END, PGDIR_SIZE));
>> +	BUILD_BUG_ON(!IS_ALIGNED(KASAN_SHADOW_END, PUD_SIZE));
> 
> We also assume that even in the shared PUD case, the shadow region falls
> within the same PGD entry, or we would need more than a single tmp_pud.
> 
> It would be good to test for that.
> 

Something like this:

	#define KASAN_SHADOW_SIZE (KASAN_SHADOW_END - KASAN_SHADOW_START)

	BUILD_BUG_ON(!IS_ALIGNED(KASAN_SHADOW_END, PGD_SIZE)
			 && !((PGDIR_SIZE > KASAN_SHADOW_SIZE)
				 && IS_ALIGNED(KASAN_SHADOW_END, PUD_SIZE)));



>> +static void __init clear_puds(pgd_t *pgd, unsigned long addr, unsigned long end)
>> +{
>> +	pud_t *pud;
>> +	unsigned long next;
>> +
>> +	pud = pud_offset(pgd, addr);
>> +
>> +	do {
>> +		next = pud_addr_end(addr, end);
>> +		if (IS_ALIGNED(addr, PUD_SIZE) && end - addr >= PUD_SIZE)
>> +			pud_clear(pud);
> 
> I think this can just be:
> 
> 		if (next - addr == PUD_SIZE)
> 			pud_clear(pud);
> 
> Given that next can at most be PUD_SIZE from addr, and if so we knwo
> addr is aligned.
> 

Right.

>> +
>> +		if (!pud_none(*pud))
>> +			clear_pmds(pud, addr, next);
> 
> I don't understand this. The KASAN shadow region is PUD_SIZE aligned at
> either end, so KASAN should never own a partial pud entry like this.
> 
> Regardless, were this case to occur, surely we'd be clearing pmd entries
> in the active page tables? We didn't copy anything at the pmd level.
> 
> That doesn't seem right.
> 

Just take a look at p?d_clear() macroses, under CONFIG_PGTABLE_LEVELS=2 for example.
pgd_clear() and pud_clear() is nops, and pmd_clear() is actually clears pgd.

I could replace p?d_clear() with set_p?d(p?d, __p?d(0)).
In that case going down to pmds is not needed, set_p?d() macro will do it for us.


...

>> +static void copy_pagetables(void)
>> +{
>> +	pgd_t *pgd = tmp_pg_dir + pgd_index(KASAN_SHADOW_START);
>> +
>> +	memcpy(tmp_pg_dir, swapper_pg_dir, sizeof(tmp_pg_dir));
>> +
>>  	/*
>> -	 * Remove references to kasan page tables from
>> -	 * swapper_pg_dir. pgd_clear() can't be used
>> -	 * here because it's nop on 2,3-level pagetable setups
>> +	 * If kasan shadow shares PGD with other mappings,
>> +	 * clear_page_tables() will clear puds instead of pgd,
>> +	 * so we need temporary pud table to keep early shadow mapped.
>>  	 */
>> -	for (; start < end; start += PGDIR_SIZE)
>> -		set_pgd(pgd_offset_k(start), __pgd(0));
>> +	if (PGDIR_SIZE > KASAN_SHADOW_END - KASAN_SHADOW_START) {
>> +		pud_t *pud;
>> +		pmd_t *pmd;
>> +		pte_t *pte;
>> +
>> +		memcpy(tmp_pud, pgd_page_vaddr(*pgd), sizeof(tmp_pud));
>> +
>> +		pgd_populate(&init_mm, pgd, tmp_pud);
>> +		pud = pud_offset(pgd, KASAN_SHADOW_START);
>> +		pmd = pmd_offset(pud, KASAN_SHADOW_START);
>> +		pud_populate(&init_mm, pud, pmd);
>> +		pte = pte_offset_kernel(pmd, KASAN_SHADOW_START);
>> +		pmd_populate_kernel(&init_mm, pmd, pte);
> 
> I don't understand why we need to do anything below the pud level here.
> We only copy down to the pud level, and we already initialised the
> shared ptes and pmds earlier.
> 
> Regardless of this patch, we currently initialise the shared tables
> repeatedly, which is redundant after the first time we initialise them.
> We could improve that.
> 

Sure, just pgd_populate() will work here, because this code is only for 16K+48-bit,
which has 4-level pagetables.
But it wouldn't work if 16k+48-bit would have > 4-level.
Because pgd_populate() in nop in such case, so we need to go down to actually set 'tmp_pud'

I just tried to avoid assumptions about number of pagetables levels in that code.




>> +	}
>>  }
>>  
>>  static void __init cpu_set_ttbr1(unsigned long ttbr1)
>> @@ -123,16 +188,16 @@ void __init kasan_init(void)
>>  
>>  	/*
>>  	 * We are going to perform proper setup of shadow memory.
>> -	 * At first we should unmap early shadow (clear_pgds() call bellow).
>> +	 * At first we should unmap early shadow (clear_page_tables()).
>>  	 * However, instrumented code couldn't execute without shadow memory.
>>  	 * tmp_pg_dir used to keep early shadow mapped until full shadow
>>  	 * setup will be finished.
>>  	 */
>> -	memcpy(tmp_pg_dir, swapper_pg_dir, sizeof(tmp_pg_dir));
>> +	copy_pagetables();
>>  	cpu_set_ttbr1(__pa(tmp_pg_dir));
>>  	flush_tlb_all();
> 
> As a heads-up, I will shortly have patches altering the swap of TTBR1,
> as it risks conflicting TLB entries and misses barriers.
> 
> Otherwise, we need a dsb(ishst) between the memcpy and writing to the
> TTBR to ensure that the tables are visible to the MMU.
> 

Thanks.

> Thanks,
> Mark.
> 
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