From: Vitaly Kuznetsov <vkuznets@redhat.com>
To: kvm@vger.kernel.org, Paolo Bonzini <pbonzini@redhat.com>
Cc: Sean Christopherson <sean.j.christopherson@intel.com>,
Wanpeng Li <wanpengli@tencent.com>,
Jim Mattson <jmattson@google.com>,
Junaid Shahid <junaids@google.com>,
linux-kernel@vger.kernel.org
Subject: [PATCH] KVM: x86: drop erroneous mmu_check_root() from fast_pgd_switch()
Date: Tue, 30 Jun 2020 12:07:42 +0200 [thread overview]
Message-ID: <20200630100742.1167961-1-vkuznets@redhat.com> (raw)
Undesired triple fault gets injected to L1 guest on SVM when L2 is
launched with certain CR3 values. It seems the mmu_check_root()
check in fast_pgd_switch() is wrong: first of all we don't know
if 'new_pgd' is a GPA or a nested GPA and, in case it is a nested
GPA, we can't check it with kvm_is_visible_gfn().
The problematic code path is:
nested_svm_vmrun()
...
nested_prepare_vmcb_save()
kvm_set_cr3(..., nested_vmcb->save.cr3)
kvm_mmu_new_pgd()
...
mmu_check_root() -> TRIPLE FAULT
The mmu_check_root() check in fast_pgd_switch() seems to be
superfluous even for non-nested case: when GPA is outside of the
visible range cached_root_available() will fail for non-direct
roots (as we can't have a matching one on the list) and we don't
seem to care for direct ones.
Also, raising #TF immediately when a non-existent GFN is written to CR3
doesn't seem to mach architecture behavior.
Fixes: 7c390d350f8b ("kvm: x86: Add fast CR3 switch code path")
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
---
- The patch fixes the immediate issue and doesn't seem to break any
tests even with shadow PT but I'm not sure I properly understood
why the check was there in the first place. Please review!
---
arch/x86/kvm/mmu/mmu.c | 3 +--
1 file changed, 1 insertion(+), 2 deletions(-)
diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 76817d13c86e..286c74d2ae8d 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -4277,8 +4277,7 @@ static bool fast_pgd_switch(struct kvm_vcpu *vcpu, gpa_t new_pgd,
*/
if (mmu->shadow_root_level >= PT64_ROOT_4LEVEL &&
mmu->root_level >= PT64_ROOT_4LEVEL)
- return !mmu_check_root(vcpu, new_pgd >> PAGE_SHIFT) &&
- cached_root_available(vcpu, new_pgd, new_role);
+ return cached_root_available(vcpu, new_pgd, new_role);
return false;
}
--
2.25.4
next reply other threads:[~2020-06-30 10:07 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-06-30 10:07 Vitaly Kuznetsov [this message]
2020-07-01 3:24 ` [PATCH] KVM: x86: drop erroneous mmu_check_root() from fast_pgd_switch() Junaid Shahid
2020-07-01 7:14 ` Vitaly Kuznetsov
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