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 <[e-mail]>
Signed-off-by: Brijesh Singh <[e-mail]>
Acked-by: Paolo Bonzini <[e-mail]>
Cc: Linus Torvalds <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Radim Krčmář <[e-mail]>
Cc: Stephen Rothwell <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Cc: Tom Lendacky <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Fixes: d0ec49d ("kvm/x86/svm: Support Secure Memory Encryption within KVM")
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Ingo Molnar <[e-mail]>

arch/x86/kvm/mmu.c

···
 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);
 
···
 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

···
 
 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);