Re: [PATCH v4 1/2] KVM: nVMX: Fix incorrect preemption timer vmexit in nested guest

[yunhong jiang]

On Thu, 7 Jul 2016 14:29:45 +0200
Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:

> 
> 
> On 07/07/2016 14:18, 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.
> > 
> > Tested-by: Haozhong Zhang <haozhong.zhang@xxxxxxxxx>
> > 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;
> >  	}
> > 
> 
> Thanks, applied to kvm/queue.
> 
> Paolo

Just back from vocation and see this issue. Really sorry that I didn't consider
the nested VM situation when working on the original patch.

Paolo/Wanpeng/Haozhong, thanks for the patch.

--jyh