Subject: Re: [PATCH v10 4/5] x86/kasan: support KASAN_VMALLOC

From: Daniel Axtens <[email-masked]>
Date: Thu, 31 Oct 2019 01:21:13 +1100

Andrey Ryabinin <[email-masked]> writes:

> On 10/30/19 4:50 PM, Daniel Axtens wrote:
>> Andrey Ryabinin <[email-masked]> writes:
>> 
>>> On 10/29/19 7:20 AM, Daniel Axtens wrote:
>>>> In the case where KASAN directly allocates memory to back vmalloc
>>>> space, don't map the early shadow page over it.
>>>>
>>>> We prepopulate pgds/p4ds for the range that would otherwise be empty.
>>>> This is required to get it synced to hardware on boot, allowing the
>>>> lower levels of the page tables to be filled dynamically.
>>>>
>>>> Acked-by: Dmitry Vyukov <[email-masked]>
>>>> Signed-off-by: Daniel Axtens <[email-masked]>
>>>>
>>>> ---
>>>
>>>> +static void __init kasan_shallow_populate_pgds(void *start, void *end)
>>>> +{
>>>> +	unsigned long addr, next;
>>>> +	pgd_t *pgd;
>>>> +	void *p;
>>>> +	int nid = early_pfn_to_nid((unsigned long)start);
>>>
>>> This doesn't make sense. start is not even a pfn. With linear mapping 
>>> we try to identify nid to have the shadow on the same node as memory. But 
>>> in this case we don't have memory or the corresponding shadow (yet),
>>> we only install pgd/p4d.
>>> I guess we could just use NUMA_NO_NODE.
>> 
>> Ah wow, that's quite the clanger on my part.
>> 
>> There are a couple of other invocations of early_pfn_to_nid in that file
>> that use an address directly, but at least they reference actual memory.
>> I'll send a separate patch to fix those up.
>
> I see only one incorrect, in kasan_init(): early_pfn_to_nid(__pa(_stext))
> It should be wrapped with PFN_DOWN().
> Other usages in map_range() seems to be correct, range->start,end is pfns.
>

Oh, right, I didn't realise map_range was already using pfns.

>
>> 
>>> The rest looks ok, so with that fixed:
>>>
>>> Reviewed-by: Andrey Ryabinin <[email-masked]>
>> 
>> Thanks heaps! I've fixed up the nit you identifed in the first patch,
>> and I agree that the last patch probably isn't needed. I'll respin the
>> series shortly.
>> 
>
> Hold on a sec, just spotted another thing to fix.
>
>> @@ -352,9 +397,24 @@ void __init kasan_init(void)
>>  	shadow_cpu_entry_end = (void *)round_up(
>>  			(unsigned long)shadow_cpu_entry_end, PAGE_SIZE);
>>  
>> +	/*
>> +	 * If we're in full vmalloc mode, don't back vmalloc space with early
>> +	 * shadow pages. Instead, prepopulate pgds/p4ds so they are synced to
>> +	 * the global table and we can populate the lower levels on demand.
>> +	 */
>> +#ifdef CONFIG_KASAN_VMALLOC
>> +	kasan_shallow_populate_pgds(
>> +		kasan_mem_to_shadow((void *)PAGE_OFFSET + MAXMEM),
>
> This should be VMALLOC_START, there is no point to allocate pgds for the hole between linear mapping
> and vmalloc, just waste of memory. It make sense to map early shadow for that hole, because if code
> dereferences address in that hole we will see the page fault on that address instead of fault on the shadow.
>
> So something like this might work:
>
> 	kasan_populate_early_shadow(
> 		kasan_mem_to_shadow((void *)PAGE_OFFSET + MAXMEM),
> 		kasan_mem_to_shadow((void *)VMALLOC_START));
>
> 	if (IS_ENABLED(CONFIG_KASAN_VMALLOC)
> 		kasan_shallow_populate_pgds(kasan_mem_to_shadow(VMALLOC_START), kasan_mem_to_shadow((void *)VMALLOC_END))
> 	else
> 		kasan_populate_early_shadow(kasan_mem_to_shadow(VMALLOC_START), kasan_mem_to_shadow((void *)VMALLOC_END));
>
> 	kasan_populate_early_shadow(
> 		kasan_mem_to_shadow((void *)VMALLOC_END + 1),
> 		shadow_cpu_entry_begin);

Sounds good. It's getting late for me so I'll change and test that and
send a respin tomorrow my time.

Regards,
Daniel