KVM: nVMX: Apply addr size mask to effective address for VMX instructions

The address size of an instruction affects the effective address, not
the virtual/linear address.  The final address may still be truncated,
e.g. to 32-bits outside of long mode, but that happens irrespective of
the address size, e.g. a 32-bit address size can yield a 64-bit virtual
address when using FS/GS with a non-zero base.

Fixes: 064aea77 ("KVM: nVMX: Decoding memory operands of VMX instructions")
Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx/nested.c

@@ -4029,20 +4029,41 @@ int get_vmx_mem_address(struct kvm_vcpu *vcpu, unsigned long exit_qualification,
 	if (index_is_valid)
 		off += kvm_register_read(vcpu, index_reg)<<scaling;
 	vmx_get_segment(vcpu, &s, seg_reg);
-	*ret = s.base + off;
 
+	/*
+	 * The effective address, i.e. @off, of a memory operand is truncated
+	 * based on the address size of the instruction.  Note that this is
+	 * the *effective address*, i.e. the address prior to accounting for
+	 * the segment's base.
+	 */
 	if (addr_size == 1) /* 32 bit */
-		*ret &= 0xffffffff;
+		off &= 0xffffffff;
+	else if (addr_size == 0) /* 16 bit */
+		off &= 0xffff;
 
 	/* Checks for #GP/#SS exceptions. */
 	exn = false;
 	if (is_long_mode(vcpu)) {
+		/*
+		 * The virtual/linear address is never truncated in 64-bit
+		 * 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;
+
 		/* Long mode: #GP(0)/#SS(0) if the memory address is in a
 		 * non-canonical form. This is the only check on the memory
 		 * destination for long mode!
 		 */
 		exn = is_noncanonical_address(*ret, vcpu);
 	} else if (is_protmode(vcpu)) {
+		/*
+		 * When not in long mode, the virtual/linear address is
+		 * unconditionally truncated to 32 bits regardless of the
+		 * address size.
+		 */
+		*ret = (s.base + off) & 0xffffffff;
+
 		/* Protected mode: apply checks for segment validity in the
 		 * following order:
 		 * - segment type check (#GP(0) may be thrown)