linux-kernel - Re: [PATCH 1/2] KVM: arm64: Fix host stage-2 PGD refcount

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Date:   Mon, 04 Oct 2021 10:55:13 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Quentin Perret <qperret@...gle.com>
Cc:     James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, Fuad Tabba <tabba@...gle.com>,
        David Brazdil <dbrazdil@...gle.com>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-kernel@...r.kernel.org, kernel-team@...roid.com
Subject: Re: [PATCH 1/2] KVM: arm64: Fix host stage-2 PGD refcount

Hi Quentin,

On Mon, 04 Oct 2021 10:03:13 +0100,
Quentin Perret <qperret@...gle.com> wrote:
> 
> The KVM page-table library refcounts the pages of concatenated stage-2
> PGDs individually. However, the host's stage-2 PGD is currently managed
> by EL2 as a single high-order compound page, which can cause the
> refcount of the tail pages to reach 0 when they really shouldn't, hence
> corrupting the page-table.

nit: this comment only applies to the protected mode, right? As far as
I can tell, 'classic' KVM is just fine.

> 
> Fix this by introducing a new hyp_split_page() helper in the EL2 page
> allocator (matching EL1's split_page() function), and make use of it

uber nit: split_page() is not an EL1 function. more of a standard
kernel function.

> from host_s2_zalloc_page().
> 
> Fixes: 1025c8c0c6ac ("KVM: arm64: Wrap the host with a stage 2")
> Suggested-by: Will Deacon <will@...nel.org>
> Signed-off-by: Quentin Perret <qperret@...gle.com>
> ---
>  arch/arm64/kvm/hyp/include/nvhe/gfp.h |  1 +
>  arch/arm64/kvm/hyp/nvhe/mem_protect.c |  6 +++++-
>  arch/arm64/kvm/hyp/nvhe/page_alloc.c  | 14 ++++++++++++++
>  3 files changed, 20 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kvm/hyp/include/nvhe/gfp.h b/arch/arm64/kvm/hyp/include/nvhe/gfp.h
> index fb0f523d1492..0a048dc06a7d 100644
> --- a/arch/arm64/kvm/hyp/include/nvhe/gfp.h
> +++ b/arch/arm64/kvm/hyp/include/nvhe/gfp.h
> @@ -24,6 +24,7 @@ struct hyp_pool {
>  
>  /* Allocation */
>  void *hyp_alloc_pages(struct hyp_pool *pool, unsigned short order);
> +void hyp_split_page(struct hyp_page *page);
>  void hyp_get_page(struct hyp_pool *pool, void *addr);
>  void hyp_put_page(struct hyp_pool *pool, void *addr);
>  
> diff --git a/arch/arm64/kvm/hyp/nvhe/mem_protect.c b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> index bacd493a4eac..93a79736c283 100644
> --- a/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> +++ b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> @@ -35,7 +35,11 @@ const u8 pkvm_hyp_id = 1;
>  
>  static void *host_s2_zalloc_pages_exact(size_t size)
>  {
> -	return hyp_alloc_pages(&host_s2_pool, get_order(size));
> +	void *addr = hyp_alloc_pages(&host_s2_pool, get_order(size));
> +
> +	hyp_split_page(hyp_virt_to_page(addr));

The only reason this doesn't lead to a subsequent memory leak is that
concatenated page tables are always a power of two, right?

If so, that deserves a comment, because I don't think this works in
the general case unless you actively free the pages that are between
size and (1 << order).

> +
> +	return addr;
>  }
>  
>  static void *host_s2_zalloc_page(void *pool)
> diff --git a/arch/arm64/kvm/hyp/nvhe/page_alloc.c b/arch/arm64/kvm/hyp/nvhe/page_alloc.c
> index 41fc25bdfb34..a6e874e61a40 100644
> --- a/arch/arm64/kvm/hyp/nvhe/page_alloc.c
> +++ b/arch/arm64/kvm/hyp/nvhe/page_alloc.c
> @@ -193,6 +193,20 @@ void hyp_get_page(struct hyp_pool *pool, void *addr)
>  	hyp_spin_unlock(&pool->lock);
>  }
>  
> +void hyp_split_page(struct hyp_page *p)
> +{
> +	unsigned short order = p->order;
> +	unsigned int i;
> +
> +	p->order = 0;
> +	for (i = 1; i < (1 << order); i++) {
> +		struct hyp_page *tail = p + i;
> +
> +		tail->order = 0;
> +		hyp_set_page_refcounted(tail);
> +	}
> +}
> +
>  void *hyp_alloc_pages(struct hyp_pool *pool, unsigned short order)
>  {
>  	unsigned short i = order;

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.