[PATCH] KVM: x86/mmu: Guard against collision with KVM-defined PFERR_IMPLICIT_ACCESS

[Sean Christopherson <seanjc@google.com>]

Add an assertion in kvm_mmu_page_fault() to ensure the error code provided
by hardware doesn't conflict with KVM's software-defined IMPLICIT_ACCESS
flag.  In the unlikely scenario that future hardware starts using bit 48
for a hardware-defined flag, preserving the bit could result in KVM
incorrectly interpreting the unknown flag as KVM's IMPLICIT_ACCESS flag.

WARN so that any such conflict can be surfaced to KVM developers and
resolved, but otherwise ignore the bit as KVM can't possibly rely on a
flag it knows nothing about.

Fixes: 4f4aa80e3b88 ("KVM: X86: Handle implicit supervisor access with SMAP")
Signed-off-by: Sean Christopherson <seanjc@google.com>
---

Note, Isaku already posted an RFC version of this, but that doesn't have
Isaku's SoB, so I'm taking the easy (for me) route so that this can land
sooner than later.

https://lore.kernel.org/all/0d71b1cdd5d901478cbfd421b4b0071cce44e16a.1689893403.git.isaku.yamahata@intel.com

 arch/x86/kvm/mmu/mmu.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index ec169f5c7dce..ef554fe9f477 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -5742,6 +5742,17 @@ int noinline kvm_mmu_page_fault(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa, u64 err
 	int r, emulation_type = EMULTYPE_PF;
 	bool direct = vcpu->arch.mmu->root_role.direct;
 
+	/*
+	 * IMPLICIT_ACCESS is a KVM-defined flag used to correctly perform SMAP
+	 * checks when emulating instructions that triggers implicit access.
+	 * WARN if hardware generates a fault with an error code that collides
+	 * with the KVM-defined value.  Clear the flag and continue on, i.e.
+	 * don't terminate the VM, as KVM can't possibly be relying on a flag
+	 * that KVM doesn't know about.
+	 */
+	if (WARN_ON_ONCE(error_code & PFERR_IMPLICIT_ACCESS))
+		error_code &= ~PFERR_IMPLICIT_ACCESS;
+
 	if (WARN_ON(!VALID_PAGE(vcpu->arch.mmu->root.hpa)))
 		return RET_PF_RETRY;
 

base-commit: fdf0eaf11452d72945af31804e2a1048ee1b574c
-- 
2.41.0.487.g6d72f3e995-goog