KVM: x86: Add more protection against undefined behavior in rsvd_bits()

Add compile-time asserts in rsvd_bits() to guard against KVM passing in
garbage hardcoded values, and cap the upper bound at '63' for dynamic
values to prevent generating a mask that would overflow a u64.

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210113204515.3473079-1-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu.h

@@ -44,8 +44,15 @@
 #define PT32_ROOT_LEVEL 2
 #define PT32E_ROOT_LEVEL 3
 
-static inline u64 rsvd_bits(int s, int e)
+static __always_inline u64 rsvd_bits(int s, int e)
 {
+	BUILD_BUG_ON(__builtin_constant_p(e) && __builtin_constant_p(s) && e < s);
+
+	if (__builtin_constant_p(e))
+		BUILD_BUG_ON(e > 63);
+	else
+		e &= 63;
+
 	if (e < s)
 		return 0;