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

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Date:   Mon, 14 Oct 2019 16:43:59 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Daniel Axtens <dja@...ens.net>
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

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.

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.

Thanks,
Mark.