Re: [PATCH v8 1/5] kasan: support backing vmalloc space with real shadow memory

Daniel Axtens
2019-10-15T06:28:06Z

Mark Rutland <mark.rutland@arm.com> writes:

> On Tue, Oct 01, 2019 at 04:58:30PM +1000, Daniel Axtens wrote:
>> Hook into vmalloc and vmap, and dynamically allocate real shadow
>> memory to back the mappings.
>>
>> Most mappings in vmalloc space are small, requiring less than a full
>> page of shadow space. Allocating a full shadow page per mapping would
>> therefore be wasteful. Furthermore, to ensure that different mappings
>> use different shadow pages, mappings would have to be aligned to
>> KASAN_SHADOW_SCALE_SIZE * PAGE_SIZE.
>>
>> Instead, share backing space across multiple mappings. Allocate a
>> backing page when a mapping in vmalloc space uses a particular page of
>> the shadow region. This page can be shared by other vmalloc mappings
>> later on.
>>
>> We hook in to the vmap infrastructure to lazily clean up unused shadow
>> memory.
>>
>> To avoid the difficulties around swapping mappings around, this code
>> expects that the part of the shadow region that covers the vmalloc
>> space will not be covered by the early shadow page, but will be left
>> unmapped. This will require changes in arch-specific code.
>>
>> This allows KASAN with VMAP_STACK, and may be helpful for architectures
>> that do not have a separate module space (e.g. powerpc64, which I am
>> currently working on). It also allows relaxing the module alignment
>> back to PAGE_SIZE.
>>
>> Link: https://bugzilla.kernel.org/show_bug.cgi?id=202009
>> Acked-by: Vasily Gorbik <gor@linux.ibm.com>
>> Signed-off-by: Daniel Axtens <dja@axtens.net>
>> [Mark: rework shadow allocation]
>> Signed-off-by: Mark Rutland <mark.rutland@arm.com>
>
> Sorry to point this out so late, but your S-o-B should come last in the
> chain per Documentation/process/submitting-patches.rst. Judging by the
> rest of that, I think you want something like:
>
> Co-developed-by: Mark Rutland <mark.rutland@arm.com>
> Signed-off-by: Mark Rutland <mark.rutland@arm.com> [shadow rework]
> Signed-off-by: Daniel Axtens <dja@axtens.net>
>
> ... leaving yourself as the Author in the headers.

no worries, I wasn't really sure how best to arrange them, so thanks for
clarifying!

>
> Sorry to have made that more complicated!
>
> [...]
>
>> +static int kasan_depopulate_vmalloc_pte(pte_t *ptep, unsigned long addr,
>> + void *unused)
>> +{
>> + unsigned long page;
>> +
>> + page = (unsigned long)__va(pte_pfn(*ptep) << PAGE_SHIFT);
>> +
>> + spin_lock(&init_mm.page_table_lock);
>> +
>> + if (likely(!pte_none(*ptep))) {
>> + pte_clear(&init_mm, addr, ptep);
>> + free_page(page);
>> + }
>
> There should be TLB maintenance between clearing the PTE and freeing the
> page here.

Fixed for v9.

Regards,
Daniel

>
> Thanks,
> Mark.