[PATCH v4 3/9] KVM: SVM: Properly check RAX on #GP intercept of SVM instructions

[Yosry Ahmed <yosry@kernel.org>]

When KVM intercepts #GP on an SVM instruction, it re-injects the #GP if
the instruction was executed with a mis-algined RAX. However, a #GP
should also be reinjected if RAX contains an illegal GPA, according to
the APM, one of #GP conditions is:

  rAX referenced a physical address above the maximum
  supported physical address.

Replace the PAGE_MASK check with page_address_valid(), which checks both
page-alignment as well as the legality of the GPA based on the vCPU's
MAXPHYADDR. Use kvm_register_read() to read RAX to avoid
page_address_valid() failing on 32-bit due to garbage in the higher
bits.

Note that this is currently only a problem if KVM is running an L2 guest
and ends up synthesizing a #VMEXIT to L1, as the RAX check takes
precedence over the intercept. Otherwise, if KVM emulates the
instruction, kvm_vcpu_map() should fail on illegal GPAs and inject a #GP
anyway. However, following patches will change the failure behavior of
kvm_vcpu_map(), so make sure the #GP interception handler does this
appropriately.

Opportunistically drop a teaser FIXME about the SVM instructions
handling on #GP belonging in the emulator.

Fixes: 82a11e9c6fa2 ("KVM: SVM: Add emulation support for #GP triggered by SVM instructions")
Fixes: d1cba6c92237 ("KVM: x86: nSVM: test eax for 4K alignment for GP errata workaround")
Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Yosry Ahmed <yosry@kernel.org>
---
 arch/x86/kvm/svm/svm.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 392a5088f20bf..3122a98745ab7 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -2277,10 +2277,12 @@ static int gp_interception(struct kvm_vcpu *vcpu)
 	if (x86_decode_emulated_instruction(vcpu, 0, NULL, 0) != EMULATION_OK)
 		goto reinject;
 
+	/* FIXME: Handle SVM instructions through the emulator */
 	svm_exit_code = svm_instr_exit_code(vcpu);
 	if (svm_exit_code) {
-		/* All SVM instructions expect page aligned RAX */
-		if (svm->vmcb->save.rax & ~PAGE_MASK)
+		unsigned long rax = kvm_register_read(vcpu, VCPU_REGS_RAX);
+
+		if (!page_address_valid(vcpu, rax))
 			goto reinject;
 
 		if (is_guest_mode(vcpu)) {
-- 
2.53.0.851.ga537e3e6e9-goog