KVM: X86: Fix warning caused by stale emulation context

Reported by syzkaller:

  WARNING: CPU: 7 PID: 10526 at linux/arch/x86/kvm//x86.c:7621 x86_emulate_instruction+0x41b/0x510 [kvm]
  RIP: 0010:x86_emulate_instruction+0x41b/0x510 [kvm]
  Call Trace:
   kvm_mmu_page_fault+0x126/0x8f0 [kvm]
   vmx_handle_exit+0x11e/0x680 [kvm_intel]
   vcpu_enter_guest+0xd95/0x1b40 [kvm]
   kvm_arch_vcpu_ioctl_run+0x377/0x6a0 [kvm]
   kvm_vcpu_ioctl+0x389/0x630 [kvm]
   __x64_sys_ioctl+0x8e/0xd0
   do_syscall_64+0x3c/0xb0
   entry_SYSCALL_64_after_hwframe+0x44/0xae

Commit 4a1e10d5 ("KVM: x86: handle hardware breakpoints during emulation())
adds hardware breakpoints check before emulation the instruction and parts of
emulation context initialization, actually we don't have the EMULTYPE_NO_DECODE flag
here and the emulation context will not be reused. Commit c8848cee ("KVM: x86:
set ctxt->have_exception in x86_decode_insn()) triggers the warning because it
catches the stale emulation context has #UD, however, it is not during instruction
decoding which should result in EMULATION_FAILED. This patch fixes it by moving
the second part emulation context initialization into init_emulate_ctxt() and
before hardware breakpoints check. The ctxt->ud will be dropped by a follow-up
patch.

syzkaller source: https://syzkaller.appspot.com/x/repro.c?x=134683fdd00000



Reported-by:  <syzbot+71271244f206d17f6441@syzkaller.appspotmail.com>
Fixes: 4a1e10d5

 (KVM: x86: handle hardware breakpoints during emulation)
Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
Reviewed-by: Sean Christopherson <seanjc@google.com>
Message-Id: <1622160097-37633-1-git-send-email-wanpengli@tencent.com>