Re: [PATCH v2 2/2] KVM: arm64: Pin the host vCPU before adjusting its PC under pKVM

[Vincent Donnefort <vdonnefort@google.com>]

On Tue, Sep 15, 2026 at 08:04:18AM +0100, Fuad Tabba wrote:
> Under pKVM, a page the host shares with EL2 is mapped at EL2 only while
> it's pinned, and the host vCPU is pinned at its first KVM_RUN.
> KVM_SET_VCPU_EVENTS with ext_dabt_pending reaches __kvm_adjust_pc() at
> EL2 before that, and dereferencing the unmapped host vCPU panics the
> hypervisor. Any process holding /dev/kvm on a pKVM host can trigger it.
> 
> Pin the host vCPU around the adjustment when no hyp vCPU is loaded for
> it. A loaded hyp vCPU implies it's pinned. A pin fails only for memory
> the host never shared, and the request is then dropped like any other
> bad host pointer.
> 
> Fixes: efa1368ba9f4b ("KVM: arm64: Commit exceptions from KVM_SET_VCPU_EVENTS immediately")
> Cc: stable@vger.kernel.org
> Signed-off-by: Fuad Tabba <fuad.tabba@linux.dev>
> ---
>  arch/arm64/kvm/hyp/nvhe/hyp-main.c | 19 ++++++++++++++++++-
>  1 file changed, 18 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kvm/hyp/nvhe/hyp-main.c b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
> index 9a3b92e626adb..4cb7347db2514 100644
> --- a/arch/arm64/kvm/hyp/nvhe/hyp-main.c
> +++ b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
> @@ -542,8 +542,25 @@ static void handle___pkvm_host_mkyoung_guest(struct kvm_cpu_context *host_ctxt)
>  static void handle___kvm_adjust_pc(struct kvm_cpu_context *host_ctxt)
>  {
>  	DECLARE_REG(struct kvm_vcpu *, vcpu, host_ctxt, 1);
> +	struct pkvm_hyp_vcpu *hyp_vcpu;
> +	struct kvm_vcpu *host_vcpu;
>  
> -	__kvm_adjust_pc(kern_hyp_va(vcpu));
> +	host_vcpu = __get_host_hyp_vcpus(vcpu, &hyp_vcpu);
> +	if (host_vcpu) {
> +		__kvm_adjust_pc(host_vcpu);
> +		return;
> +	}
> +
> +	/*
> +	 * With no hyp vCPU loaded for it, the host vCPU may be unpinned,
> +	 * and so unmapped at EL2: its first run pins it.
> +	 */
> +	host_vcpu = kern_hyp_va(vcpu);
> +	if (hyp_pin_shared_mem(host_vcpu, host_vcpu + 1))
> +		return;
> +
> +	__kvm_adjust_pc(host_vcpu);
> +	hyp_unpin_shared_mem(host_vcpu, host_vcpu + 1);
>  }
>  
>  static void handle___kvm_flush_vm_context(struct kvm_cpu_context *host_ctxt)
> -- 
> 2.39.5
>

Reviewed-by: Vincent Donnefort <vdonnefor@google.com>

-- 
Vincent