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SubjectRe: [PATCH] KVM: VMX: Fix vmx->nested freeing when no SMI handler
2017-11-22 16:45 GMT+08:00 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.

Yeah, very funny.

> Reviewed-by: Liran Alon <liran.alon@oracle.com>

Thanks for the review. :)

Regards,
Wanpeng Li

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