linux-kernel - Re: [PATCH v2 1/3] kasan: support backing vmalloc space with real shadow memory

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Date:   Tue, 30 Jul 2019 18:38:47 +1000
From:   Daniel Axtens <dja@...ens.net>
To:     Mark Rutland <mark.rutland@....com>
Cc:     kasan-dev@...glegroups.com, linux-mm@...ck.org, x86@...nel.org,
        aryabinin@...tuozzo.com, glider@...gle.com, luto@...nel.org,
        linux-kernel@...r.kernel.org, dvyukov@...gle.com
Subject: Re: [PATCH v2 1/3] kasan: support backing vmalloc space with real shadow memory

Hi Mark,

Thanks for your email - I'm very new to mm stuff and the feedback is
very helpful.

>> +#ifndef CONFIG_KASAN_VMALLOC
>>  int kasan_module_alloc(void *addr, size_t size)
>>  {
>>  	void *ret;
>> @@ -603,6 +604,7 @@ void kasan_free_shadow(const struct vm_struct *vm)
>>  	if (vm->flags & VM_KASAN)
>>  		vfree(kasan_mem_to_shadow(vm->addr));
>>  }
>> +#endif
>
> IIUC we can drop MODULE_ALIGN back to PAGE_SIZE in this case, too.

Yes, done.

>>  core_initcall(kasan_memhotplug_init);
>>  #endif
>> +
>> +#ifdef CONFIG_KASAN_VMALLOC
>> +void kasan_cover_vmalloc(unsigned long requested_size, struct vm_struct *area)
>
> Nit: I think it would be more consistent to call this
> kasan_populate_vmalloc().
>

Absolutely. I didn't love the name but just didn't 'click' that populate
would be a better verb.

>> +{
>> +	unsigned long shadow_alloc_start, shadow_alloc_end;
>> +	unsigned long addr;
>> +	unsigned long backing;
>> +	pgd_t *pgdp;
>> +	p4d_t *p4dp;
>> +	pud_t *pudp;
>> +	pmd_t *pmdp;
>> +	pte_t *ptep;
>> +	pte_t backing_pte;
>
> Nit: I think it would be preferable to use 'page' rather than 'backing',
> and 'pte' rather than 'backing_pte', since there's no otehr namespace to
> collide with here. Otherwise, using 'shadow' rather than 'backing' would
> be consistent with the existing kasan code.

Not a problem, done.

>> +	addr = shadow_alloc_start;
>> +	do {
>> +		pgdp = pgd_offset_k(addr);
>> +		p4dp = p4d_alloc(&init_mm, pgdp, addr);
>> +		pudp = pud_alloc(&init_mm, p4dp, addr);
>> +		pmdp = pmd_alloc(&init_mm, pudp, addr);
>> +		ptep = pte_alloc_kernel(pmdp, addr);
>> +
>> +		/*
>> +		 * we can validly get here if pte is not none: it means we
>> +		 * allocated this page earlier to use part of it for another
>> +		 * allocation
>> +		 */
>> +		if (pte_none(*ptep)) {
>> +			backing = __get_free_page(GFP_KERNEL);
>> +			backing_pte = pfn_pte(PFN_DOWN(__pa(backing)),
>> +					      PAGE_KERNEL);
>> +			set_pte_at(&init_mm, addr, ptep, backing_pte);
>> +		}
>
> Does anything prevent two threads from racing to allocate the same
> shadow page?
>
> AFAICT it's possible for two threads to get down to the ptep, then both
> see pte_none(*ptep)), then both try to allocate the same page.
>
> I suspect we have to take init_mm::page_table_lock when plumbing this
> in, similarly to __pte_alloc().

Good catch. I think you're right, I'll add the lock.

>> +	} while (addr += PAGE_SIZE, addr != shadow_alloc_end);
>> +
>> +	kasan_unpoison_shadow(area->addr, requested_size);
>> +	requested_size = round_up(requested_size, KASAN_SHADOW_SCALE_SIZE);
>> +	kasan_poison_shadow(area->addr + requested_size,
>> +			    area->size - requested_size,
>> +			    KASAN_VMALLOC_INVALID);
>
> IIUC, this could leave the final portion of an allocated page
> unpoisoned.
>
> I think it might make more sense to poison each page when it's
> allocated, then plumb it into the page tables, then unpoison the object.
>
> That way, we can rely on any shadow allocated by another thread having
> been initialized to KASAN_VMALLOC_INVALID, and only need mutual
> exclusion when allocating the shadow, rather than when poisoning
> objects.

Yes, that makes sense, will do.

Thanks again,
Daniel