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

[Liran Alon <LIRAN.ALON@ORACLE.COM>]

On 22/11/17 09:56, Wanpeng Li wrote:
> 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;
>
Funny bug. Great analysis.
Reviewed-by: Liran Alon <liran.alon@oracle.com>