Re: [PATCH 1/4] KVM: arm64: Fix read-side race on updates to vcpu reset state

From: Marc Zyngier <maz@kernel.org>
To: Oliver Upton <oupton@google.com>
Cc: kvm@vger.kernel.org, kvmarm@lists.cs.columbia.edu,
	Peter Shier <pshier@google.com>,
	Ricardo Koller <ricarkol@google.com>,
	Jing Zhang <jingzhangos@google.com>,
	Raghavendra Rao Anata <rananta@google.com>,
	James Morse <james.morse@arm.com>,
	Alexandru Elisei <alexandru.elisei@arm.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>
Subject: Re: [PATCH 1/4] KVM: arm64: Fix read-side race on updates to vcpu reset state
Date: Wed, 18 Aug 2021 11:06:28 +0100	[thread overview]
Message-ID: <87sfz7rrrv.wl-maz@kernel.org> (raw)
In-Reply-To: <20210818085047.1005285-2-oupton@google.com>

On Wed, 18 Aug 2021 09:50:44 +0100,
Oliver Upton <oupton@google.com> wrote:
> 
> KVM correctly serializes writes to a vCPU's reset state, however since
> we do not take the KVM lock on the read side it is entirely possible to
> read state from two different reset requests.
> 
> Cure the race for now by taking the KVM lock when reading the
> reset_state structure.
> 
> Fixes: 358b28f09f0a ("arm/arm64: KVM: Allow a VCPU to fully reset itself")
> Signed-off-by: Oliver Upton <oupton@google.com>
> ---
>  arch/arm64/kvm/reset.c | 16 ++++++++++------
>  1 file changed, 10 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/arm64/kvm/reset.c b/arch/arm64/kvm/reset.c
> index 18ffc6ad67b8..3507e64ff8ad 100644
> --- a/arch/arm64/kvm/reset.c
> +++ b/arch/arm64/kvm/reset.c
> @@ -210,10 +210,16 @@ static bool vcpu_allowed_register_width(struct kvm_vcpu *vcpu)
>   */
>  int kvm_reset_vcpu(struct kvm_vcpu *vcpu)
>  {
> +	struct vcpu_reset_state reset_state;
>  	int ret;
>  	bool loaded;
>  	u32 pstate;
>  
> +	mutex_lock(&vcpu->kvm->lock);
> +	memcpy(&reset_state, &vcpu->arch.reset_state, sizeof(reset_state));

nit: "reset_state = vcpu->arch.reset_state;" should do the trick.

> +	vcpu->arch.reset_state.reset = false;

We should probably need to upgrade this to a WRITE_ONCE() to match the
PSCI side.

> +	mutex_unlock(&vcpu->kvm->lock);
> +
>  	/* Reset PMU outside of the non-preemptible section */
>  	kvm_pmu_vcpu_reset(vcpu);
>  
> @@ -276,8 +282,8 @@ int kvm_reset_vcpu(struct kvm_vcpu *vcpu)
>  	 * Additional reset state handling that PSCI may have imposed on us.
>  	 * Must be done after all the sys_reg reset.
>  	 */
> -	if (vcpu->arch.reset_state.reset) {
> -		unsigned long target_pc = vcpu->arch.reset_state.pc;
> +	if (reset_state.reset) {
> +		unsigned long target_pc = reset_state.pc;
>  
>  		/* Gracefully handle Thumb2 entry point */
>  		if (vcpu_mode_is_32bit(vcpu) && (target_pc & 1)) {
> @@ -286,13 +292,11 @@ int kvm_reset_vcpu(struct kvm_vcpu *vcpu)
>  		}
>  
>  		/* Propagate caller endianness */
> -		if (vcpu->arch.reset_state.be)
> +		if (reset_state.be)
>  			kvm_vcpu_set_be(vcpu);
>  
>  		*vcpu_pc(vcpu) = target_pc;
> -		vcpu_set_reg(vcpu, 0, vcpu->arch.reset_state.r0);
> -
> -		vcpu->arch.reset_state.reset = false;
> +		vcpu_set_reg(vcpu, 0, reset_state.r0);
>  	}
>  
>  	/* Reset timer */

Thanks,

	M.

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