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

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Date:   Wed, 22 Nov 2017 17:31:46 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Liran Alon <LIRAN.ALON@...cle.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        kvm <kvm@...r.kernel.org>, Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Wanpeng Li <wanpeng.li@...mail.com>,
        Dmitry Vyukov <dvyukov@...gle.com>
Subject: Re: [PATCH] KVM: VMX: Fix vmx->nested freeing when no SMI handler

2017-11-22 17:07 GMT+08:00 Liran Alon <LIRAN.ALON@...cle.com>:
>
>
> On 22/11/17 10:45, Liran Alon wrote:
>>
>>
>>
>> On 22/11/17 09:56, Wanpeng Li wrote:
>>>
>>> From: Wanpeng Li <wanpeng.li@...mail.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@...gle.com>
>>> Cc: Paolo Bonzini <pbonzini@...hat.com>
>>> Cc: Radim Krčmář <rkrcmar@...hat.com>
>>> Cc: Dmitry Vyukov <dvyukov@...gle.com>
>>> Fixes: 05cade71cf (KVM: nSVM: fix SMI injection in guest mode)
>>> Signed-off-by: Wanpeng Li <wanpeng.li@...mail.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@...cle.com>
>
> Actually, I would add one more thing to patch:
> I think we should also set "vmx->nested.smm.vmxon = false;" after
> "vmx->nested.vmxon = false;" to correctlyhandle the case VMXOFF is executed
> from SMI handler. Otherwise, when SMI handler executes RSM, we will reach
> vmx_pre_leave_smm() which will set again "vmx->nested.vmxon = true;" which I
> think shouldn't happen.

I didn't see a real scenario for this.

Regards,
Wanpeng Li