linux-kernel - Re: [PATCH v6 4/8] KVM: arm64: Add guard pages for pKVM (protected nVHE) hypervisor stack

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Date:   Tue, 29 Mar 2022 09:51:00 +0100
From:   Fuad Tabba <tabba@...gle.com>
To:     Kalesh Singh <kaleshsingh@...gle.com>
Cc:     will@...nel.org, maz@...nel.org, qperret@...gle.com,
        surenb@...gle.com, kernel-team@...roid.com,
        James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Mark Rutland <mark.rutland@....com>,
        Mark Brown <broonie@...nel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Peter Collingbourne <pcc@...gle.com>,
        "Madhavan T. Venkataraman" <madvenka@...ux.microsoft.com>,
        Andrew Scull <ascull@...gle.com>,
        Ard Biesheuvel <ardb@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 4/8] KVM: arm64: Add guard pages for pKVM (protected
 nVHE) hypervisor stack

Hi Kalesh,

On Mon, Mar 14, 2022 at 8:04 PM Kalesh Singh <kaleshsingh@...gle.com> wrote:
>
> Map the stack pages in the flexible private VA range and allocate
> guard pages below the stack as unbacked VA space. The stack is aligned
> so that any valid stack address has PAGE_SHIFT bit as 1 - this is used
> for overflow detection (implemented in a subsequent patch in the series)
>
> Signed-off-by: Kalesh Singh <kaleshsingh@...gle.com>

Tested-by: Fuad Tabba <tabba@...gle.com>
Reviewed-by: Fuad Tabba <tabba@...gle.com>

Thanks,
/fuad


> ---
>
> Changes in v6:
>   - Update call to pkvm_alloc_private_va_range() (return val and params)
>
> Changes in v5:
>   - Use a single allocation for stack and guard pages to ensure they
>     are contiguous, per Marc
>
> Changes in v4:
>   - Replace IS_ERR_OR_NULL check with IS_ERR check now that
>     pkvm_alloc_private_va_range() returns an error for null
>     pointer, per Fuad
>
> Changes in v3:
>   - Handle null ptr in IS_ERR_OR_NULL checks, per Mark
>
>
>  arch/arm64/kvm/hyp/nvhe/setup.c | 31 ++++++++++++++++++++++++++++---
>  1 file changed, 28 insertions(+), 3 deletions(-)
>
> diff --git a/arch/arm64/kvm/hyp/nvhe/setup.c b/arch/arm64/kvm/hyp/nvhe/setup.c
> index 27af337f9fea..e8d4ea2fcfa0 100644
> --- a/arch/arm64/kvm/hyp/nvhe/setup.c
> +++ b/arch/arm64/kvm/hyp/nvhe/setup.c
> @@ -99,17 +99,42 @@ static int recreate_hyp_mappings(phys_addr_t phys, unsigned long size,
>                 return ret;
>
>         for (i = 0; i < hyp_nr_cpus; i++) {
> +               struct kvm_nvhe_init_params *params = per_cpu_ptr(&kvm_init_params, i);
> +               unsigned long hyp_addr;
> +
>                 start = (void *)kern_hyp_va(per_cpu_base[i]);
>                 end = start + PAGE_ALIGN(hyp_percpu_size);
>                 ret = pkvm_create_mappings(start, end, PAGE_HYP);
>                 if (ret)
>                         return ret;
>
> -               end = (void *)per_cpu_ptr(&kvm_init_params, i)->stack_hyp_va;
> -               start = end - PAGE_SIZE;
> -               ret = pkvm_create_mappings(start, end, PAGE_HYP);
> +               /*
> +                * Allocate a contiguous HYP private VA range for the stack
> +                * and guard page. The allocation is also aligned based on
> +                * the order of its size.
> +                */
> +               ret = pkvm_alloc_private_va_range(PAGE_SIZE * 2, &hyp_addr);
> +               if (ret)
> +                       return ret;
> +
> +               /*
> +                * Since the stack grows downwards, map the stack to the page
> +                * at the higher address and leave the lower guard page
> +                * unbacked.
> +                *
> +                * Any valid stack address now has the PAGE_SHIFT bit as 1
> +                * and addresses corresponding to the guard page have the
> +                * PAGE_SHIFT bit as 0 - this is used for overflow detection.
> +                */
> +               hyp_spin_lock(&pkvm_pgd_lock);
> +               ret = kvm_pgtable_hyp_map(&pkvm_pgtable, hyp_addr + PAGE_SIZE,
> +                                       PAGE_SIZE, params->stack_pa, PAGE_HYP);
> +               hyp_spin_unlock(&pkvm_pgd_lock);
>                 if (ret)
>                         return ret;
> +
> +               /* Update stack_hyp_va to end of the stack's private VA range */
> +               params->stack_hyp_va = hyp_addr + (2 * PAGE_SIZE);
>         }
>
>         /*
> --
> 2.35.1.723.g4982287a31-goog
>