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

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Date:   Tue, 15 Oct 2019 17:27:57 +1100
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,
        christophe.leroy@....fr, linuxppc-dev@...ts.ozlabs.org,
        gor@...ux.ibm.com
Subject: Re: [PATCH v8 1/5] kasan: support backing vmalloc space with real shadow memory

Mark Rutland <mark.rutland@....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@...ux.ibm.com>
>> Signed-off-by: Daniel Axtens <dja@...ens.net>
>> [Mark: rework shadow allocation]
>> Signed-off-by: Mark Rutland <mark.rutland@....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@....com>
> Signed-off-by: Mark Rutland <mark.rutland@....com> [shadow rework]
> Signed-off-by: Daniel Axtens <dja@...ens.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.