linux-kernel - Re: [PATCH] KVM: arm64: Fix unaligned addr case in mmu walking

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Date:   Wed, 3 Mar 2021 11:29:34 +0000
From:   Will Deacon <will@...nel.org>
To:     Jia He <justin.he@....com>
Cc:     Marc Zyngier <maz@...nel.org>, kvmarm@...ts.cs.columbia.edu,
        James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Gavin Shan <gshan@...hat.com>,
        Yanan Wang <wangyanan55@...wei.com>,
        Quentin Perret <qperret@...gle.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: arm64: Fix unaligned addr case in mmu walking

On Wed, Mar 03, 2021 at 10:42:25AM +0800, Jia He wrote:
> If the start addr is not aligned with the granule size of that level.
> loop step size should be adjusted to boundary instead of simple
> kvm_granual_size(level) increment. Otherwise, some mmu entries might miss
> the chance to be walked through.
> E.g. Assume the unmap range [data->addr, data->end] is
> [0xff00ab2000,0xff00cb2000] in level 2 walking and NOT block mapping.
> And the 1st part of that pmd entry is [0xff00ab2000,0xff00c00000]. The
> pmd value is 0x83fbd2c1002 (not valid entry). In this case, data->addr
> should be adjusted to 0xff00c00000 instead of 0xff00cb2000.
> 
> Without this fix, userspace "segment fault" error can be easily
> triggered by running simple gVisor runsc cases on an Ampere Altra
> server:
>     docker run --runtime=runsc -it --rm  ubuntu /bin/bash
> 
> In container:
>     for i in `seq 1 100`;do ls;done
> 
> Reported-by: Howard Zhang <Howard.Zhang@....com>
> Signed-off-by: Jia He <justin.he@....com>
> ---
>  arch/arm64/kvm/hyp/pgtable.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
> index bdf8e55ed308..4d99d07c610c 100644
> --- a/arch/arm64/kvm/hyp/pgtable.c
> +++ b/arch/arm64/kvm/hyp/pgtable.c
> @@ -225,6 +225,7 @@ static inline int __kvm_pgtable_visit(struct kvm_pgtable_walk_data *data,
>  		goto out;
>  
>  	if (!table) {
> +		data->addr = ALIGN_DOWN(data->addr, kvm_granule_size(level));
>  		data->addr += kvm_granule_size(level);

Can you replace both of these lines with:

	data->addr = ALIGN(data->addr, kvm_granule_size(level));

instead?

Will