Re: [PATCH v3 1/2] KVM: nVMX: Fix incorrect preemption timer vmexit in nested guest

[Haozhong Zhang]

On 07/07/16 14:56, Wanpeng Li wrote:
> 2016-07-07 14:48 GMT+08:00 Haozhong Zhang <haozhong.zhang@xxxxxxxxx>:
> > On 07/07/16 11:46, 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;
> >
> > If patch 2 can avoid accidentally enabling preemption timer in vmcs02,
> > will this one still be needed?
> 
> After complete "L1 TSC deadline timer to trigger while L2 is running",
> L0's preemption timer fire when L2 is running can result in
> (is_guest_mode(vcpu) && nested_vmx_exit_handled(vcpu)) be true, right?
>

In prepare_vmcs02():

    exec_control &= ~PIN_BASED_VMX_PREEMPTION_TIMER;
    ...
    vmcs_write32(PIN_BASED_VM_EXEC_CONTROL, exec_control);

so preemption timer will never be enabled while L2 guest is running.

Haozhong