kvm/x86: Avoid clearing the C-bit in rsvd_bits()

The following commit:

  d0ec49d4de90 ("kvm/x86/svm: Support Secure Memory Encryption within KVM")

uses __sme_clr() to remove the C-bit in rsvd_bits(). rsvd_bits() is
just a simple function to return some 1 bits. Applying a mask based
on properties of the host MMU is incorrect. Additionally, the masks
computed by __reset_rsvds_bits_mask also apply to guest page tables,
where the C bit is reserved since we don't emulate SME.

The fix is to clear the C-bit from rsvd_bits_mask array after it has been
populated from __reset_rsvds_bits_mask()

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: kvm@vger.kernel.org
Cc: paolo.bonzini@gmail.com
Fixes: d0ec49d ("kvm/x86/svm: Support Secure Memory Encryption within KVM")
Link: http://lkml.kernel.org/r/20170825205540.123531-1-brijesh.singh@amd.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kvm/mmu.c

@@ -4109,16 +4109,28 @@ void
 reset_shadow_zero_bits_mask(struct kvm_vcpu *vcpu, struct kvm_mmu *context)
 {
 	bool uses_nx = context->nx || context->base_role.smep_andnot_wp;
+	struct rsvd_bits_validate *shadow_zero_check;
+	int i;
 
 	/*
 	 * Passing "true" to the last argument is okay; it adds a check
 	 * on bit 8 of the SPTEs which KVM doesn't use anyway.
 	 */
-	__reset_rsvds_bits_mask(vcpu, &context->shadow_zero_check,
+	shadow_zero_check = &context->shadow_zero_check;
+	__reset_rsvds_bits_mask(vcpu, shadow_zero_check,
 				boot_cpu_data.x86_phys_bits,
 				context->shadow_root_level, uses_nx,
 				guest_cpuid_has_gbpages(vcpu), is_pse(vcpu),
 				true);
+
+	if (!shadow_me_mask)
+		return;
+
+	for (i = context->shadow_root_level; --i >= 0;) {
+		shadow_zero_check->rsvd_bits_mask[0][i] &= ~shadow_me_mask;
+		shadow_zero_check->rsvd_bits_mask[1][i] &= ~shadow_me_mask;
+	}
+
 }
 EXPORT_SYMBOL_GPL(reset_shadow_zero_bits_mask);
 
@@ -4136,17 +4148,29 @@ static void
 reset_tdp_shadow_zero_bits_mask(struct kvm_vcpu *vcpu,
 				struct kvm_mmu *context)
 {
+	struct rsvd_bits_validate *shadow_zero_check;
+	int i;
+
+	shadow_zero_check = &context->shadow_zero_check;
+
 	if (boot_cpu_is_amd())
-		__reset_rsvds_bits_mask(vcpu, &context->shadow_zero_check,
+		__reset_rsvds_bits_mask(vcpu, shadow_zero_check,
 					boot_cpu_data.x86_phys_bits,
 					context->shadow_root_level, false,
 					boot_cpu_has(X86_FEATURE_GBPAGES),
 					true, true);
 	else
-		__reset_rsvds_bits_mask_ept(&context->shadow_zero_check,
+		__reset_rsvds_bits_mask_ept(shadow_zero_check,
 					    boot_cpu_data.x86_phys_bits,
 					    false);
 
+	if (!shadow_me_mask)
+		return;
+
+	for (i = context->shadow_root_level; --i >= 0;) {
+		shadow_zero_check->rsvd_bits_mask[0][i] &= ~shadow_me_mask;
+		shadow_zero_check->rsvd_bits_mask[1][i] &= ~shadow_me_mask;
+	}
 }
 
 /*

arch/x86/kvm/mmu.h

@@ -48,7 +48,7 @@
 
 static inline u64 rsvd_bits(int s, int e)
 {
-	return __sme_clr(((1ULL << (e - s + 1)) - 1) << s);
+	return ((1ULL << (e - s + 1)) - 1) << s;
 }
 
 void kvm_mmu_set_mmio_spte_mask(u64 mmio_mask, u64 mmio_value);