KVM: x86: Stack size is overridden by __linearize

When performing segmented-read/write in the emulator for stack operations, it
ignores the stack size, and uses the ad_bytes as indication for the pointer
size. As a result, a wrong address may be accessed.

To fix this behavior, we can remove the masking of address in __linearize and
perform it beforehand.  It is already done for the operands (so currently it is
inefficiently done twice). It is missing in two cases:
1. When using rip_relative
2. On fetch_bit_operand that changes the address.

This patch masks the address on these two occassions, and removes the masking
from __linearize.

Note that it does not mask EIP during fetch. In protected/legacy mode code
fetch when RIP >= 2^32 should result in #GP and not wrap-around. Since we make
limit checks within __linearize, this is the expected behavior.

Partial revert of commit 518547b3 (KVM: x86: Emulator does not
calculate address correctly, 2014-09-30).
Signed-off-by: Nadav Amit <namit@cs.technion.ac.il>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/emulate.c

@@ -665,8 +665,7 @@ static __always_inline int __linearize(struct x86_emulate_ctxt *ctxt,
 	u16 sel;
 	unsigned cpl;
 
-	la = seg_base(ctxt, addr.seg) +
-	    (fetch || ctxt->ad_bytes == 8 ? addr.ea : (u32)addr.ea);
+	la = seg_base(ctxt, addr.seg) + addr.ea;
 	*max_size = 0;
 	switch (ctxt->mode) {
 	case X86EMUL_MODE_PROT64:
@@ -1289,7 +1288,8 @@ static void fetch_bit_operand(struct x86_emulate_ctxt *ctxt)
 		else
 			sv = (s64)ctxt->src.val & (s64)mask;
 
-		ctxt->dst.addr.mem.ea += (sv >> 3);
+		ctxt->dst.addr.mem.ea = address_mask(ctxt,
+					   ctxt->dst.addr.mem.ea + (sv >> 3));
 	}
 
 	/* only subword offset */
@@ -4638,7 +4638,8 @@ int x86_decode_insn(struct x86_emulate_ctxt *ctxt, void *insn, int insn_len)
 	rc = decode_operand(ctxt, &ctxt->dst, (ctxt->d >> DstShift) & OpMask);
 
 	if (ctxt->rip_relative)
-		ctxt->memopp->addr.mem.ea += ctxt->_eip;
+		ctxt->memopp->addr.mem.ea = address_mask(ctxt,
+					ctxt->memopp->addr.mem.ea + ctxt->_eip);
 
 done:
 	return (rc != X86EMUL_CONTINUE) ? EMULATION_FAILED : EMULATION_OK;