linux-kernel - Re: [PATCH 5/9] KVM: arm: move has_run_once after the map

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Date:   Tue, 12 Jan 2021 14:55:49 +0000
From:   Alexandru Elisei <alexandru.elisei@....com>
To:     Eric Auger <eric.auger@...hat.com>, eric.auger.pro@...il.com,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        kvmarm@...ts.cs.columbia.edu, maz@...nel.org, drjones@...hat.com
Cc:     james.morse@....com, julien.thierry.kdev@...il.com,
        suzuki.poulose@....com, shuah@...nel.org, pbonzini@...hat.com
Subject: Re: [PATCH 5/9] KVM: arm: move has_run_once after the map_resources

Hi Eric,

On 12/12/20 6:50 PM, Eric Auger wrote:
> has_run_once is set to true at the beginning of
> kvm_vcpu_first_run_init(). This generally is not an issue
> except when exercising the code with KVM selftests. Indeed,
> if kvm_vgic_map_resources() fails due to erroneous user settings,
> has_run_once is set and this prevents from continuing
> executing the test. This patch moves the assignment after the
> kvm_vgic_map_resources().
>
> Signed-off-by: Eric Auger <eric.auger@...hat.com>
> ---
>  arch/arm64/kvm/arm.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> index c0ffb019ca8b..331fae6bff31 100644
> --- a/arch/arm64/kvm/arm.c
> +++ b/arch/arm64/kvm/arm.c
> @@ -540,8 +540,6 @@ static int kvm_vcpu_first_run_init(struct kvm_vcpu *vcpu)
>  	if (!kvm_arm_vcpu_is_finalized(vcpu))
>  		return -EPERM;
>  
> -	vcpu->arch.has_run_once = true;
> -
>  	if (likely(irqchip_in_kernel(kvm))) {
>  		/*
>  		 * Map the VGIC hardware resources before running a vcpu the
> @@ -560,6 +558,8 @@ static int kvm_vcpu_first_run_init(struct kvm_vcpu *vcpu)
>  		static_branch_inc(&userspace_irqchip_in_use);
>  	}
>  
> +	vcpu->arch.has_run_once = true;

I have a few concerns regarding this:

1. Moving has_run_once = true here seems very arbitrary to me - kvm_timer_enable()
and kvm_arm_pmu_v3_enable(), below it, can both fail because of erroneous user
values. If there's a reason why the assignment cannot be moved at the end of the
function, I think it should be clearly stated in a comment for the people who
might be tempted to write similar tests for the timer or pmu.

2. There are many ways that kvm_vgic_map_resources() can fail, other than
incorrect user settings. I started digging into how
kvm_vgic_map_resources()->vgic_v2_map_resources() can fail for a VGIC V2 and this
is what I managed to find before I gave up:

* vgic_init() can fail in:
    - kvm_vgic_dist_init()
    - vgic_v3_init()
    - kvm_vgic_setup_default_irq_routing()
* vgic_register_dist_iodev() can fail in:
    - vgic_v3_init_dist_iodev()
    - kvm_io_bus_register_dev()(*)
* kvm_phys_addr_ioremap() can fail in:
    - kvm_mmu_topup_memory_cache()
    - kvm_pgtable_stage2_map()

So if any of the functions below fail, are we 100% sure it is safe to allow the
user to execute kvm_vgic_map_resources() again?

(*) It looks to me like kvm_io_bus_register_dev() doesn't take into account a
caller that tries to register the same device address range and it will create
another identical range. Is this intentional? Is it a bug that should be fixed? Or
am I misunderstanding the function?

Thanks,
Alex
> +
>  	ret = kvm_timer_enable(vcpu);
>  	if (ret)
>  		return ret;