KVM: x86: SYSENTER emulation is broken

commit f3747379accba8e95d70cec0eae0582c8c182050 upstream.

SYSENTER emulation is broken in several ways:
1. It misses the case of 16-bit code segments completely (CVE-2015-0239).
2. MSR_IA32_SYSENTER_CS is checked in 64-bit mode incorrectly (bits 0 and 1 can
   still be set without causing #GP).
3. MSR_IA32_SYSENTER_EIP and MSR_IA32_SYSENTER_ESP are not masked in
   legacy-mode.
4. There is some unneeded code.

Fix it.

Signed-off-by: Nadav Amit <namit@cs.technion.ac.il>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
[zhangzhiqiang: backport to 3.10:
 - adjust context
 - in 3.10 context "ctxt->eflags &= ~(EFLG_VM | EFLG_IF | EFLG_RF)" is replaced by
   "ctxt->eflags &= ~(EFLG_VM | EFLG_IF)" in upstream, which was changed by another commit.
 - After the above adjustments, becomes same to the original patch:
       https://github.com/torvalds/linux/commit/f3747379accba8e95d70cec0eae0582c8c182050


]
Signed-off-by: Zhiqiang Zhang <zhangzhiqiang.zhang@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>