KVM: nVMX: Ignore segment base for VMX memory operand when segment not FS or GS

As reported by Maxime at
https://bugzilla.kernel.org/show_bug.cgi?id=204175:

In vmx/nested.c::get_vmx_mem_address(), when the guest runs in long mode,
the base address of the memory operand is computed with a simple:
    *ret = s.base + off;

This is incorrect, the base applies only to FS and GS, not to the others.
Because of that, if the guest uses a VMX instruction based on DS and has
a DS.base that is non-zero, KVM wrongfully adds the base to the
resulting address.
Reported-by: NMaxime Villard <max@m00nbsd.net>
Reviewed-by: NJoao Martins <joao.m.martins@oracle.com>
Signed-off-by: NLiran Alon <liran.alon@oracle.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx/nested.c

@@ -4194,7 +4194,10 @@ int get_vmx_mem_address(struct kvm_vcpu *vcpu, unsigned long exit_qualification,
 		 * mode, e.g. a 32-bit address size can yield a 64-bit virtual
 		 * address when using FS/GS with a non-zero base.
 		 */
-		*ret = s.base + off;
+		if (seg_reg == VCPU_SREG_FS || seg_reg == VCPU_SREG_GS)
+			*ret = s.base + off;
+		else
+			*ret = off;
 
 		/* Long mode: #GP(0)/#SS(0) if the memory address is in a
 		 * non-canonical form. This is the only check on the memory