KVM: x86: fix conversion of addresses to linear in 32-bit protected mode

Commit e8dd2d2d641c ("Silence compiler warning in arch/x86/kvm/emulate.c",
2015-09-06) broke boot of the Hurd.  The bug is that the "default:"
case actually could modify "la", but after the patch this change is
not reflected in *linear.

The bug is visible whenever a non-zero segment base causes the linear
address to wrap around the 4GB mark.

Fixes: e8dd2d2d641cb2724ee10e76c0ad02e04289c017
Cc: stable@vger.kernel.org
Reported-by: Aurelien Jarno <aurelien@aurel32.net>
Tested-by: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/emulate.c

@@ -650,10 +650,10 @@ static __always_inline int __linearize(struct x86_emulate_ctxt *ctxt,
 	u16 sel;
 
 	la = seg_base(ctxt, addr.seg) + addr.ea;
-	*linear = la;
 	*max_size = 0;
 	switch (mode) {
 	case X86EMUL_MODE_PROT64:
+		*linear = la;
 		if (is_noncanonical_address(la))
 			goto bad;
 
@@ -662,6 +662,7 @@ static __always_inline int __linearize(struct x86_emulate_ctxt *ctxt,
 			goto bad;
 		break;
 	default:
+		*linear = la = (u32)la;
 		usable = ctxt->ops->get_segment(ctxt, &sel, &desc, NULL,
 						addr.seg);
 		if (!usable)
@@ -689,7 +690,6 @@ static __always_inline int __linearize(struct x86_emulate_ctxt *ctxt,
 			if (size > *max_size)
 				goto bad;
 		}
-		la &= (u32)-1;
 		break;
 	}
 	if (insn_aligned(ctxt, size) && ((la & (size - 1)) != 0))