linux-kernel - Re: [PATCH v2 03/16] KVM: x86: Always use non-compat vcpu_runstate

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Date:   Thu, 27 Oct 2022 13:11:01 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Michal Luczaj <mhal@...x.co>,
        David Woodhouse <dwmw2@...radead.org>
Subject: Re: [PATCH v2 03/16] KVM: x86: Always use non-compat
 vcpu_runstate_info size for gfn=>pfn cache

On 10/13/22 23:12, Sean Christopherson wrote:
> Always use the size of Xen's non-compat vcpu_runstate_info struct when
> checking that the GPA+size doesn't cross a page boundary.  Conceptually,
> using the current mode is more correct, but KVM isn't consistent with
> itself as kvm_xen_vcpu_set_attr() unconditionally uses the "full" size
> when activating the cache.  More importantly, prior to the introduction
> of the gfn_to_pfn_cache, KVM _always_ used the full size, i.e. allowing
> the guest (userspace?) to use a poorly aligned GPA in 32-bit mode but not
> 64-bit mode is more of a bug than a feature, and fixing the bug doesn't
> break KVM's historical ABI.

I'd rather not introduce additional restrictions in KVM, mostly because 
it's actually easy to avoid this patch by instead enforcing that 
attributes are set in a sensible order:

- long mode cannot be changed after the shared info page is enabled 
(which makes sense because the shared info page also has a compat version)

- the caches must be activated after the shared info page (which 
enforces that the vCPU attributes are set after the VM attributes)

This is technically a userspace API break, but nobody is really using 
this API outside Amazon so...  Patches coming after I finish testing.

Paolo


> Always using the non-compat size will allow for future gfn_to_pfn_cache
> clenups as this is (was) the only case where KVM uses a different size at
> check()+refresh() than at activate().  E.g. the length/size of the cache
> can be made immutable and dropped from check()+refresh(), which yields a
> cleaner set of APIs and avoids potential bugs that could occur if check()
> where invoked with a different size than refresh().
> 
> Fixes: a795cd43c5b5 ("KVM: x86/xen: Use gfn_to_pfn_cache for runstate area")
> Cc: David Woodhouse <dwmw2@...radead.org>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>   arch/x86/kvm/xen.c | 5 +----
>   1 file changed, 1 insertion(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
> index b2be60c6efa4..9e79ef2cca99 100644
> --- a/arch/x86/kvm/xen.c
> +++ b/arch/x86/kvm/xen.c
> @@ -212,10 +212,7 @@ void kvm_xen_update_runstate_guest(struct kvm_vcpu *v, int state)
>   	if (!vx->runstate_cache.active)
>   		return;
>   
> -	if (IS_ENABLED(CONFIG_64BIT) && v->kvm->arch.xen.long_mode)
> -		user_len = sizeof(struct vcpu_runstate_info);
> -	else
> -		user_len = sizeof(struct compat_vcpu_runstate_info);
> +	user_len = sizeof(struct vcpu_runstate_info);
>   
>   	read_lock_irqsave(&gpc->lock, flags);
>   	while (!kvm_gfn_to_pfn_cache_check(v->kvm, gpc, gpc->gpa,