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

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Date:   Wed, 22 Nov 2017 11:07:39 +0200
From:   Liran Alon <LIRAN.ALON@...CLE.COM>
To:     Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
CC:     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



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.

Regards,
-Liran