Re: [PATCH v1] KVM: arm64: Fix benign bug with incorrect use of VA_BITS.

From: Marc Zyngier <maz@kernel.org>
To: Ryan Roberts <ryan.roberts@arm.com>
Cc: James Morse <james.morse@arm.com>,
	Alexandru Elisei <alexandru.elisei@arm.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Oliver Upton <oliver.upton@linux.dev>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev,
	kvmarm@lists.cs.columbia.edu
Subject: Re: [PATCH v1] KVM: arm64: Fix benign bug with incorrect use of VA_BITS.
Date: Mon, 05 Dec 2022 13:49:22 +0000	[thread overview]
Message-ID: <86y1rmkljx.wl-maz@kernel.org> (raw)
In-Reply-To: <20221205114031.3972780-1-ryan.roberts@arm.com>

Hi Ryan,

Thanks for that.

On Mon, 05 Dec 2022 11:40:31 +0000,
Ryan Roberts <ryan.roberts@arm.com> wrote:
> 
> get_user_mapping_size() uses kvm's pgtable library to walk a user space
> page table created by the kernel, and in doing so, fakes up the metadata
> that the library needs, including ia_bits, which defines the size of the
> input address.

It isn't supposed to "fake" anything. It simply provides the
information that the walker needs to correctly parse the page tables.

> 
> For the case where the kernel is compiled for 52 VA bits but runs on HW
> that does not support LVA, it will fall back to 48 VA bits at runtime.
> Therefore we must use vabits_actual rather than VA_BITS to get the true
> address size.
> 
> This is benign in the current code base because the pgtable library only
> uses it for error checking.
> 
> Fixes: 6011cf68c885 ("KVM: arm64: Walk userspace page tables to compute
> the THP mapping size")

nit: this should appear on a single line, without a line-break in the
middle [1]...

>

... without a blank line between Fixes: and the rest of the tags.

And while I'm on the "trivial remarks" train, drop the full stop at
the end of the subject line.

> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
> ---
>  arch/arm64/kvm/mmu.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index 4efb983cff43..1ef0704420d9 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -641,7 +641,7 @@ static int get_user_mapping_size(struct kvm *kvm, u64 addr)
>  {
>  	struct kvm_pgtable pgt = {
>  		.pgd		= (kvm_pte_t *)kvm->mm->pgd,
> -		.ia_bits	= VA_BITS,
> +		.ia_bits	= vabits_actual,
>  		.start_level	= (KVM_PGTABLE_MAX_LEVELS -
>  				   CONFIG_PGTABLE_LEVELS),
>  		.mm_ops		= &kvm_user_mm_ops,
> --
> 2.25.1
> 
> 

Other than the above nits, this is well spotted. I need to regenerate
the kvmarm/next branch after the sysreg attack from James, so I'll try
and fold that in.

Thanks,

	M.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/process/submitting-patches.rst#n139

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

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