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

[Haozhong Zhang]

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.

Tested-by: Haozhong Zhang <haozhong.zhang@xxxxxxxxx>