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

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Date:   Thu, 13 Oct 2022 21:12:21 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Michal Luczaj <mhal@...x.co>,
        David Woodhouse <dwmw2@...radead.org>
Subject: [PATCH v2 03/16] KVM: x86: Always use non-compat vcpu_runstate_info
 size for gfn=>pfn cache

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.

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,
-- 
2.38.0.413.g74048e4d9e-goog