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>

arch/x86/kvm/x86.c

@@ -7228,6 +7228,11 @@ static void init_emulate_ctxt(struct kvm_vcpu *vcpu)
 	BUILD_BUG_ON(HF_SMM_MASK != X86EMUL_SMM_MASK);
 	BUILD_BUG_ON(HF_SMM_INSIDE_NMI_MASK != X86EMUL_SMM_INSIDE_NMI_MASK);
 
+	ctxt->interruptibility = 0;
+	ctxt->have_exception = false;
+	ctxt->exception.vector = -1;
+	ctxt->perm_ok = false;
+
 	init_decode_cache(ctxt);
 	vcpu->arch.emulate_regs_need_sync_from_vcpu = false;
 }
@@ -7563,11 +7568,6 @@ int x86_decode_emulated_instruction(struct kvm_vcpu *vcpu, int emulation_type,
 	    kvm_vcpu_check_breakpoint(vcpu, &r))
 		return r;
 
-	ctxt->interruptibility = 0;
-	ctxt->have_exception = false;
-	ctxt->exception.vector = -1;
-	ctxt->perm_ok = false;
-
 	ctxt->ud = emulation_type & EMULTYPE_TRAP_UD;
 
 	r = x86_decode_insn(ctxt, insn, insn_len);