Re: [PATCH v3] KVM: nVMX: Fix incorrect preemption timer vmexit in nested guest

[Paolo Bonzini]

On 06/07/2016 15:26, Haozhong Zhang wrote:
> On 07/06/16 19:42, Wanpeng Li wrote:
>> From: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
>>
>> BUG: unable to handle kernel NULL pointer dereference at           (null)
>> IP: [<          (null)>]           (null)
>> PGD 0 
>> Oops: 0010 [#1] SMP
>> Call Trace:
>>  ? kvm_lapic_expired_hv_timer+0x47/0x90 [kvm]
>>  handle_preemption_timer+0xe/0x20 [kvm_intel]
>>  vmx_handle_exit+0x169/0x15a0 [kvm_intel]
>>  ? kvm_arch_vcpu_ioctl_run+0xd5d/0x19d0 [kvm]
>>  kvm_arch_vcpu_ioctl_run+0xdee/0x19d0 [kvm]
>>  ? kvm_arch_vcpu_ioctl_run+0xd5d/0x19d0 [kvm]
>>  ? vcpu_load+0x1c/0x60 [kvm]
>>  ? kvm_arch_vcpu_load+0x57/0x260 [kvm]
>>  kvm_vcpu_ioctl+0x2d3/0x7c0 [kvm]
>>  do_vfs_ioctl+0x96/0x6a0
>>  ? __fget_light+0x2a/0x90
>>  SyS_ioctl+0x79/0x90
>>  do_syscall_64+0x68/0x180
>>  entry_SYSCALL64_slow_path+0x25/0x25
>> Code:  Bad RIP value.
>> RIP  [<          (null)>]           (null)
>>  RSP <ffff8800b5263c48>
>> CR2: 0000000000000000
>> ---[ end trace 9c70c48b1a2bc66e ]---
>>
>> This can be reproduced readily by preemption timer enabled on L0 and disabled 
>> on L1.
>>
>> Preemption timer for nested VMX is emulated by hrtimer which is started on L2
>> entry, stopped on L2 exit and evaluated via the check_nested_events hook. However,
>> nested_vmx_exit_handled is always return true for preemption timer vmexit, then 
>> the L1 preemption timer vmexit is captured and be treated as a L2 preemption 
>> timer vmexit, incurr a nested vmexit dereference NULL pointer.
>>
>> This patch fix it by depending on check_nested_events to capture L2 preemption 
>> timer(emulated hrtimer) expire and nested vmexit.
>>
>> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
>> Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
>> Cc: Yunhong Jiang <yunhong.jiang@xxxxxxxxx>
>> Cc: Jan Kiszka <jan.kiszka@xxxxxxxxxxx>
>> Cc: Haozhong Zhang <haozhong.zhang@xxxxxxxxx>
>> Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
>> ---
>> v2 -> v3:
>>  * update patch subject
>> v1 -> v2:
>>  * fix typo in patch description
>>
>>  arch/x86/kvm/vmx.c | 2 ++
>>  1 file changed, 2 insertions(+)
>>
>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>> index 85e2f0a..29c16a8 100644
>> --- a/arch/x86/kvm/vmx.c
>> +++ b/arch/x86/kvm/vmx.c
>> @@ -8041,6 +8041,8 @@ static bool nested_vmx_exit_handled(struct kvm_vcpu *vcpu)
>>  		return nested_cpu_has2(vmcs12, SECONDARY_EXEC_XSAVES);
>>  	case EXIT_REASON_PCOMMIT:
>>  		return nested_cpu_has2(vmcs12, SECONDARY_EXEC_PCOMMIT);
>> +	case EXIT_REASON_PREEMPTION_TIMER:
>> +		return false;
>>  	default:
>>  		return true;
>>  	}
>> -- 
>> 1.9.1
>>
> 
> This patch also fixed the crash of L1 Xen with L2 HVM guest. Xen does
> not enable preemption timer for HVM guests, and will get panic if it
> receives a preemption timer vmexit.

Thanks!  I'm still not sure why the bit is set in the vmcs02 though...

Paolo