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

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Date:	Thu, 7 Jul 2016 15:25:15 +0800
From:	Haozhong Zhang <haozhong.zhang@...el.com>
To:	Wanpeng Li <kernellwp@...il.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	kvm <kvm@...r.kernel.org>, Wanpeng Li <wanpeng.li@...mail.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Radim Krčmář <rkrcmar@...hat.com>,
	Yunhong Jiang <yunhong.jiang@...el.com>,
	Jan Kiszka <jan.kiszka@...mens.com>
Subject: Re: [PATCH v3 1/2] KVM: nVMX: Fix incorrect preemption timer vmexit
 in nested guest

On 07/07/16 15:07, Wanpeng Li wrote:
> 2016-07-07 15:02 GMT+08:00 Haozhong Zhang <haozhong.zhang@...el.com>:
> > On 07/07/16 14:56, Wanpeng Li wrote:
> >> 2016-07-07 14:48 GMT+08:00 Haozhong Zhang <haozhong.zhang@...el.com>:
> >> > On 07/07/16 11:46, Wanpeng Li wrote:
> >> >> From: Wanpeng Li <wanpeng.li@...mail.com>
> >> >>
> >> >> 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@...el.com>
> >> >> Cc: Paolo Bonzini <pbonzini@...hat.com>
> >> >> Cc: Radim Krčmář <rkrcmar@...hat.com>
> >> >> Cc: Yunhong Jiang <yunhong.jiang@...el.com>
> >> >> Cc: Jan Kiszka <jan.kiszka@...mens.com>
> >> >> Cc: Haozhong Zhang <haozhong.zhang@...el.com>
> >> >> Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
> >> >> ---
> >> >> 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.
> 
> It is not true after Paolo's patch.
>

OK, I didn't quite follow discussions before v3 and just noticed
Paolo's patch in v2 comments is to enable L1 preemption timer while L2
is running. Then this one looks reasonable.

Thanks,
Haozhong