[PATCH] KVM: VMX: Fix vmx->nested freeing when no SMI handler

[Wanpeng Li <kernellwp@gmail.com>]

From: Wanpeng Li <wanpeng.li@hotmail.com>

Reported by syzkaller:

	------------[ cut here ]------------
	WARNING: CPU: 5 PID: 2939 at arch/x86/kvm/vmx.c:3844 free_loaded_vmcs+0x77/0x80 [kvm_intel]
	CPU: 5 PID: 2939 Comm: repro Not tainted 4.14.0+ #26
	RIP: 0010:free_loaded_vmcs+0x77/0x80 [kvm_intel]
	Call Trace:
	 vmx_free_vcpu+0xda/0x130 [kvm_intel]
	 kvm_arch_destroy_vm+0x192/0x290 [kvm]
	 kvm_put_kvm+0x262/0x560 [kvm]
	 kvm_vm_release+0x2c/0x30 [kvm]
	 __fput+0x190/0x370
	 task_work_run+0xa1/0xd0
	 do_exit+0x4d2/0x13e0
	 do_group_exit+0x89/0x140
	 get_signal+0x318/0xb80
	 do_signal+0x8c/0xb40
	 exit_to_usermode_loop+0xe4/0x140
	 syscall_return_slowpath+0x206/0x230
	 entry_SYSCALL_64_fastpath+0x98/0x9a

The syzkaller testcase will execute VMXON/VMLAUCH instructions, so the 
vmx->nested stuff is populated, it will also issue KVM_SMI ioctl. However, 
the testcase is just a simple c program and not be lauched by something 
like seabios which implements smi_handler. Commit 05cade71cf (KVM: nSVM: 
fix SMI injection in guest mode) gets out of guest mode and set nested.vmxon 
to false for the duration of SMM according to SDM 34.14.1 "leave VMX 
operation" upon entering SMM. We can't alloc/free the vmx->nested stuff 
each time when entering/exiting SMM since it will induce more overhead. So 
the function vmx_pre_enter_smm() marks nested.vmxon false even if vmx->nested 
stuff is still populated. What it expected is em_rsm() can mark nested.vmxon 
to be true again. However, the smi_handler/rsm will not execute since there 
is no something like seabios in this scenario. The function free_nested() 
fails to free the vmx->nested stuff since the vmx->nested.vmxon is false 
which results in the above warning.

This patch fixes it by also considering the no SMI handler case, luckily 
vmx->nested.smm.vmxon is marked according to the value of vmx->nested.vmxon 
in vmx_pre_enter_smm(), we can take advantage of it and free vmx->nested 
stuff when L1 goes down.

Reported-by: Dmitry Vyukov <dvyukov@google.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Fixes: 05cade71cf (KVM: nSVM: fix SMI injection in guest mode)
Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
---
 arch/x86/kvm/vmx.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index dccc0f7..ed22425 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -7372,7 +7372,7 @@ static inline void nested_release_vmcs12(struct vcpu_vmx *vmx)
  */
 static void free_nested(struct vcpu_vmx *vmx)
 {
-	if (!vmx->nested.vmxon)
+	if (!vmx->nested.vmxon && !vmx->nested.smm.vmxon)
 		return;
 
 	vmx->nested.vmxon = false;
-- 
2.7.4