Re: [PATCH][RESEND] KVM: nVMX: Fix vmptrld fail and vmwrite error when L1 goes down

[Wanpeng Li]

Hi Marcelo,
On Fri, Jul 11, 2014 at 05:03:34PM -0300, Marcelo Tosatti wrote:
>On Fri, Jul 11, 2014 at 12:22:17PM +0800, Wanpeng Li wrote:
>> This bug can be trigger by L1 goes down directly w/ enable_shadow_vmcs.
>> 
>> [ 6413.158950] kvm: vmptrld           (null)/780000000000 failed
>> [ 6413.158954] vmwrite error: reg 401e value 4 (err 1)
>> [ 6413.158957] CPU: 0 PID: 4840 Comm: qemu-system-x86 Tainted: G           OE 3.16.0kvm+ #2
>> [ 6413.158958] Hardware name: Dell Inc. OptiPlex 9020/0DNKMN, BIOS A05 12/05/2013
>> [ 6413.158959]  0000000000000003 ffff880210c9fb58 ffffffff81741de9 ffff8800d7433f80
>> [ 6413.158960]  ffff880210c9fb68 ffffffffa059fa08 ffff880210c9fb78 ffffffffa05938bf
>> [ 6413.158962]  ffff880210c9fba8 ffffffffa059a97f ffff8800d7433f80 0000000000000003
>> [ 6413.158963] Call Trace:
>> [ 6413.158968]  [<ffffffff81741de9>] dump_stack+0x45/0x56
>> [ 6413.158972]  [<ffffffffa059fa08>] vmwrite_error+0x2c/0x2e [kvm_intel]
>> [ 6413.158974]  [<ffffffffa05938bf>] vmcs_writel+0x1f/0x30 [kvm_intel]
>> [ 6413.158976]  [<ffffffffa059a97f>] free_nested.part.73+0x5f/0x170 [kvm_intel]
>> [ 6413.158978]  [<ffffffffa059ab13>] vmx_free_vcpu+0x33/0x70 [kvm_intel]
>> [ 6413.158991]  [<ffffffffa0360324>] kvm_arch_vcpu_free+0x44/0x50 [kvm]
>> [ 6413.158998]  [<ffffffffa0360f92>] kvm_arch_destroy_vm+0xf2/0x1f0 [kvm]
>> 
>> Commit 26a865 (KVM: VMX: fix use after free of vmx->loaded_vmcs) fix the use 
>> after free bug by move free_loaded_vmcs() before free_nested(), however, this 
>> lead to free loaded_vmcs->vmcs premature and vmptrld load a NULL pointer during 
>> sync shadow vmcs to vmcs12. In addition, vmwrite which used to disable shadow 
>> vmcs and reset VMCS_LINK_POINTER failed since there is no valid current-VMCS.
>> This patch fix it by skipping kfree vmcs02_list item for current-vmcs in 
>> nested_free_all_saved_vmcss() and kfree it after free_loaded_vmcs(). This can 
>> also avoid use after free bug.
>> 
>> Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxxxxxx>
>> ---
>>  arch/x86/kvm/vmx.c | 15 +++++++++++----
>>  1 file changed, 11 insertions(+), 4 deletions(-)
>> 
>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>> index 021d84a..6e427ac 100644
>> --- a/arch/x86/kvm/vmx.c
>> +++ b/arch/x86/kvm/vmx.c
>> @@ -5762,10 +5762,11 @@ static void nested_free_all_saved_vmcss(struct vcpu_vmx *vmx)
>>  {
>>  	struct vmcs02_list *item, *n;
>>  	list_for_each_entry_safe(item, n, &vmx->nested.vmcs02_pool, list) {
>> -		if (vmx->loaded_vmcs != &item->vmcs02)
>> +		if (vmx->loaded_vmcs != &item->vmcs02) {
>>  			free_loaded_vmcs(&item->vmcs02);
>> -		list_del(&item->list);
>> -		kfree(item);
>> +			list_del(&item->list);
>> +			kfree(item);
>> +		}
>>  	}
>>  	vmx->nested.vmcs02_num = 0;
>>  
>> @@ -7526,10 +7527,16 @@ static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu)
>>  static void vmx_free_vcpu(struct kvm_vcpu *vcpu)
>>  {
>>  	struct vcpu_vmx *vmx = to_vmx(vcpu);
>> +	struct vmcs02_list *item;
>>  
>>  	free_vpid(vmx);
>> -	free_loaded_vmcs(vmx->loaded_vmcs);
>>  	free_nested(vmx);
>
>static void free_nested(struct vcpu_vmx *vmx)
>{
>        if (!vmx->nested.vmxon)
>                return;
>        vmx->nested.vmxon = false;
>
>> +	free_loaded_vmcs(vmx->loaded_vmcs);
>> +	if (vmx->nested.vmxon)
>
>Can this codepath ever execute?
>

How about set vmx->nested.vmcs02_num = 1 in nested_free_all_saved_vmcss()
since the vmcs in vmcs02_pool which current used is not freed at this point 
and check vmx->nested.vmcs02_num == 1 in vmx_free_vcpu()? Otherwise, any 
better proposal is appreciated.

Regards,
Wanpeng Li 

>> +		list_for_each_entry(item, &vmx->nested.vmcs02_pool, list) {
>> +			list_del(&item->list);
>> +			kfree(item);
>> +		}
>
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