linux-kernel - [RFT PATCH v5 1/3] KVM: nVMX: avoid incorrect preemption timer vmexit in nested guest

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Date:	Fri,  8 Jul 2016 14:02:11 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:	Wanpeng Li <wanpeng.li@...mail.com>,
	Wanpeng Li <kernellwp@...il.com>,
	Radim Krčmář <rkrcmar@...hat.com>,
	Yunhong Jiang <yunhong.jiang@...el.com>,
	Jan Kiszka <jan.kiszka@...mens.com>,
	Haozhong Zhang <haozhong.zhang@...el.com>
Subject: [RFT PATCH v5 1/3] KVM: nVMX: avoid incorrect preemption timer vmexit in nested guest

From: Wanpeng Li <wanpeng.li@...mail.com>

The 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 returning true for preemption timer vmexit.  Then,
the L1 preemption timer vmexit is captured and be treated as a L2 preemption
timer vmexit, causing NULL pointer dereferences or worse in the L1 guest's
vmexit handler:

    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.

Return false since preemption timer vmexits must never be reflected to L2.

Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
---
 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 e564fa2c7ac8..f6e5cc679898 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;
 	}
-- 
1.8.3.1