From: Eiichi Tsukata <eiichi.tsukata@nutanix.com>
Date: Sat, 6 Jun 2020 13:26:27 +0900
Subject: KVM: x86: Fix APIC page invalidation race
Git-commit: e649b3f0188f8fd34dd0dde8d43fd3312b902fb2
Patch-mainline: v5.8-rc1
References: bsc#1174122

Commit b1394e745b94 ("KVM: x86: fix APIC page invalidation") tried
to fix inappropriate APIC page invalidation by re-introducing arch
specific kvm_arch_mmu_notifier_invalidate_range() and calling it from
kvm_mmu_notifier_invalidate_range_start. However, the patch left a
possible race where the VMCS APIC address cache is updated *before*
it is unmapped:

  (Invalidator) kvm_mmu_notifier_invalidate_range_start()
  (Invalidator) kvm_make_all_cpus_request(kvm, KVM_REQ_APIC_PAGE_RELOAD)
  (KVM VCPU) vcpu_enter_guest()
  (KVM VCPU) kvm_vcpu_reload_apic_access_page()
  (Invalidator) actually unmap page

Because of the above race, there can be a mismatch between the
host physical address stored in the APIC_ACCESS_PAGE VMCS field and
the host physical address stored in the EPT entry for the APIC GPA
(0xfee0000).  When this happens, the processor will not trap APIC
accesses, and will instead show the raw contents of the APIC-access page.
Because Windows OS periodically checks for unexpected modifications to
the LAPIC register, this will show up as a BSOD crash with BugCheck
CRITICAL_STRUCTURE_CORRUPTION (109) we are currently seeing in
https://bugzilla.redhat.com/show_bug.cgi?id=1751017.

The root cause of the issue is that kvm_arch_mmu_notifier_invalidate_range()
cannot guarantee that no additional references are taken to the pages in
the range before kvm_mmu_notifier_invalidate_range_end().  Fortunately,
this case is supported by the MMU notifier API, as documented in
include/linux/mmu_notifier.h:

	 * If the subsystem
         * can't guarantee that no additional references are taken to
         * the pages in the range, it has to implement the
         * invalidate_range() notifier to remove any references taken
         * after invalidate_range_start().

The fix therefore is to reload the APIC-access page field in the VMCS
from kvm_mmu_notifier_invalidate_range() instead of ..._range_start().

Cc: stable@vger.kernel.org
Fixes: b1394e745b94 ("KVM: x86: fix APIC page invalidation")
Fixes: https://bugzilla.kernel.org/show_bug.cgi?id=197951
Signed-off-by: Eiichi Tsukata <eiichi.tsukata@nutanix.com>
Message-Id: <20200606042627.61070-1-eiichi.tsukata@nutanix.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: Joerg Roedel <jroedel@suse.de>
---
 arch/x86/kvm/x86.c       |  7 ++-----
 include/linux/kvm_host.h |  4 ++--
 virt/kvm/kvm_main.c      | 26 ++++++++++++++++----------
 3 files changed, 20 insertions(+), 17 deletions(-)

--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -389,6 +389,18 @@ static void kvm_mmu_notifier_invalidate_
 	srcu_read_unlock(&kvm->srcu, idx);
 }
 
+static void kvm_mmu_notifier_invalidate_range(struct mmu_notifier *mn,
+					      struct mm_struct *mm,
+					      unsigned long start, unsigned long end)
+{
+	struct kvm *kvm = mmu_notifier_to_kvm(mn);
+	int idx;
+
+	idx = srcu_read_lock(&kvm->srcu);
+	kvm_arch_mmu_notifier_invalidate_range(kvm, start, end);
+	srcu_read_unlock(&kvm->srcu, idx);
+}
+
 static void kvm_mmu_notifier_change_pte(struct mmu_notifier *mn,
 					struct mm_struct *mm,
 					unsigned long address,
@@ -429,8 +441,6 @@ static void kvm_mmu_notifier_invalidate_
 
 	spin_unlock(&kvm->mmu_lock);
 
-	kvm_arch_mmu_notifier_invalidate_range(kvm, start, end);
-
 	srcu_read_unlock(&kvm->srcu, idx);
 }
 
@@ -540,6 +550,7 @@ static void kvm_mmu_notifier_release(str
 
 static const struct mmu_notifier_ops kvm_mmu_notifier_ops = {
 	.invalidate_page	= kvm_mmu_notifier_invalidate_page,
+	.invalidate_range	= kvm_mmu_notifier_invalidate_range,
 	.invalidate_range_start	= kvm_mmu_notifier_invalidate_range_start,
 	.invalidate_range_end	= kvm_mmu_notifier_invalidate_range_end,
 	.clear_flush_young	= kvm_mmu_notifier_clear_flush_young,