[PATCH v3 01/37] KVM: VMX: Flush all EPTP/VPID contexts on remote TLB flush

[Sean Christopherson]

Flush all EPTP/VPID contexts if a TLB flush _may_ have been triggered by
a remote or deferred TLB flush, i.e. by KVM_REQ_TLB_FLUSH.  Remote TLB
flushes require all contexts to be invalidated, not just the active
contexts, e.g. all mappings in all contexts for a given HVA need to be
invalidated on a mmu_notifier invalidation.  Similarly, the instigator
of the deferred TLB flush may be expecting all contexts to be flushed,
e.g. vmx_vcpu_load_vmcs().

Without nested VMX, flushing only the current EPTP/VPID context isn't
problematic because KVM uses a constant VPID for each vCPU, and
mmu_alloc_direct_roots() all but guarantees KVM will use a single EPTP
for L1.  In the rare case where a different EPTP is created or reused,
KVM (currently) unconditionally flushes the new EPTP context prior to
entering the guest.

With nested VMX, KVM conditionally uses a different VPID for L2, and
unconditionally uses a different EPTP for L2.  Because KVM doesn't
_intentionally_ guarantee L2's EPTP/VPID context is flushed on nested
VM-Enter, it'd be possible for a malicious L1 to attack the host and/or
different VMs by exploiting the lack of flushing for L2.

  1) Launch nested guest from malicious L1.

  2) Nested VM-Enter to L2.

  3) Access target GPA 'g'.  CPU inserts TLB entry tagged with L2's ASID
     mapping 'g' to host PFN 'x'.

  2) Nested VM-Exit to L1.

  3) L1 triggers kernel same-page merging (ksm) by duplicating/zeroing
     the page for PFN 'x'.

  4) Host kernel merges PFN 'x' with PFN 'y', i.e. unmaps PFN 'x' and
     remaps the page to PFN 'y'.  mmu_notifier sends invalidate command,
     KVM flushes TLB only for L1's ASID.

  4) Host kernel reallocates PFN 'x' to some other task/guest.

  5) Nested VM-Enter to L2.  KVM does not invalidate L2's EPTP or VPID.

  6) L2 accesses GPA 'g' and gains read/write access to PFN 'x' via its
     stale TLB entry.

However, current KVM unconditionally flushes L1's EPTP/VPID context on
nested VM-Exit.  But, that behavior is mostly unintentional, KVM doesn't
go out of its way to flush EPTP/VPID on nested VM-Enter/VM-Exit, rather
a TLB flush is guaranteed to occur prior to re-entering L1 due to
__kvm_mmu_new_cr3() always being called with skip_tlb_flush=false.  On
nested VM-Enter, this happens via kvm_init_shadow_ept_mmu() (nested EPT
enabled) or in nested_vmx_load_cr3() (nested EPT disabled).  On nested
VM-Exit it occurs via nested_vmx_load_cr3().

This also fixes a bug where a deferred TLB flush in the context of L2,
with EPT disabled, would flush L1's VPID instead of L2's VPID, as
vmx_flush_tlb() flushes L1's VPID regardless of is_guest_mode().

Cc: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
Cc: Ben Gardon <bgardon@xxxxxxxxxx>
Cc: Jim Mattson <jmattson@xxxxxxxxxx>
Cc: Junaid Shahid <junaids@xxxxxxxxxx>
Cc: Liran Alon <liran.alon@xxxxxxxxxx>
Cc: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
Cc: John Haxby <john.haxby@xxxxxxxxxx>
Reviewed-by: Liran Alon <liran.alon@xxxxxxxxxx>
Fixes: efebf0aaec3d ("KVM: nVMX: Do not flush TLB on L1<->L2 transitions if L1 uses VPID and EPT")
Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
---
 arch/x86/kvm/vmx/vmx.h | 28 +++++++++++++++++++++++++++-
 1 file changed, 27 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
index be93d597306c..d6d67b816ebe 100644
--- a/arch/x86/kvm/vmx/vmx.h
+++ b/arch/x86/kvm/vmx/vmx.h
@@ -518,7 +518,33 @@ static inline void __vmx_flush_tlb(struct kvm_vcpu *vcpu, int vpid,
 
 static inline void vmx_flush_tlb(struct kvm_vcpu *vcpu, bool invalidate_gpa)
 {
-	__vmx_flush_tlb(vcpu, to_vmx(vcpu)->vpid, invalidate_gpa);
+	struct vcpu_vmx *vmx = to_vmx(vcpu);
+
+	/*
+	 * Flush all EPTP/VPID contexts if the TLB flush _may_ have been
+	 * invoked via kvm_flush_remote_tlbs(), which always passes %true for
+	 * @invalidate_gpa.  Flushing remote TLBs requires all contexts to be
+	 * flushed, not just the active context.
+	 *
+	 * Note, this also ensures a deferred TLB flush with VPID enabled and
+	 * EPT disabled invalidates the "correct" VPID, by nuking both L1 and
+	 * L2's VPIDs.
+	 */
+	if (invalidate_gpa) {
+		if (enable_ept) {
+			ept_sync_global();
+		} else if (enable_vpid) {
+			if (cpu_has_vmx_invvpid_global()) {
+				vpid_sync_vcpu_global();
+			} else {
+				WARN_ON_ONCE(!cpu_has_vmx_invvpid_single());
+				vpid_sync_vcpu_single(vmx->vpid);
+				vpid_sync_vcpu_single(vmx->nested.vpid02);
+			}
+		}
+	} else {
+		__vmx_flush_tlb(vcpu, vmx->vpid, false);
+	}
 }
 
 static inline void decache_tsc_multiplier(struct vcpu_vmx *vmx)
-- 
2.24.1