KVM: x86: Add WARNs to detect out-of-bounds register indices

Add WARN_ON_ONCE() checks in kvm_register_{read,write}() to detect reg
values that would cause KVM to overflow vcpu->arch.regs.  Change the reg
param to an 'int' to make it clear that the reg index is unverified.

Regarding the overhead of WARN_ON_ONCE(), now that all fixed GPR reads
and writes use dedicated accessors, e.g. kvm_rax_read(), the overhead
is limited to flows where the reg index is generated at runtime.  And
there is at least one historical bug where KVM has generated an out-of-
bounds access to arch.regs (see commit b68f3cc7, "KVM: x86: Always
use 32-bit SMRAM save state for 32-bit kernels").

Adding the WARN_ON_ONCE() protection paves the way for additional
cleanup related to kvm_reg and kvm_reg_ex.
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/kvm_cache_regs.h

@@ -37,19 +37,23 @@ BUILD_KVM_GPR_ACCESSORS(r14, R14)
 BUILD_KVM_GPR_ACCESSORS(r15, R15)
 #endif
 
-static inline unsigned long kvm_register_read(struct kvm_vcpu *vcpu,
-					      enum kvm_reg reg)
+static inline unsigned long kvm_register_read(struct kvm_vcpu *vcpu, int reg)
 {
+	if (WARN_ON_ONCE((unsigned int)reg >= NR_VCPU_REGS))
+		return 0;
+
 	if (!test_bit(reg, (unsigned long *)&vcpu->arch.regs_avail))
 		kvm_x86_ops->cache_reg(vcpu, reg);
 
 	return vcpu->arch.regs[reg];
 }
 
-static inline void kvm_register_write(struct kvm_vcpu *vcpu,
-				      enum kvm_reg reg,
+static inline void kvm_register_write(struct kvm_vcpu *vcpu, int reg,
 				      unsigned long val)
 {
+	if (WARN_ON_ONCE((unsigned int)reg >= NR_VCPU_REGS))
+		return;
+
 	vcpu->arch.regs[reg] = val;
 	__set_bit(reg, (unsigned long *)&vcpu->arch.regs_dirty);
 	__set_bit(reg, (unsigned long *)&vcpu->arch.regs_avail);

arch/x86/kvm/x86.h

@@ -238,8 +238,7 @@ static inline bool vcpu_match_mmio_gpa(struct kvm_vcpu *vcpu, gpa_t gpa)
 	return false;
 }
 
-static inline unsigned long kvm_register_readl(struct kvm_vcpu *vcpu,
-					       enum kvm_reg reg)
+static inline unsigned long kvm_register_readl(struct kvm_vcpu *vcpu, int reg)
 {
 	unsigned long val = kvm_register_read(vcpu, reg);
 
@@ -247,8 +246,7 @@ static inline unsigned long kvm_register_readl(struct kvm_vcpu *vcpu,
 }
 
 static inline void kvm_register_writel(struct kvm_vcpu *vcpu,
-				       enum kvm_reg reg,
-				       unsigned long val)
+				       int reg, unsigned long val)
 {
 	if (!is_64_bit_mode(vcpu))
 		val = (u32)val;