Re: [PATCH 1/2] KVM: lapic: fix access preemption timer stuff even if kernel_irqchip=off

Radim Krčmář
2016-08-03T10:12:12Z

2016-08-03 12:04+0800, Wanpeng Li:
> From: Wanpeng Li <wanpeng.li@hotmail.com>
>
> BUG: unable to handle kernel NULL pointer dereference at 000000000000008c
> IP: [<ffffffffc04e0180>] kvm_lapic_hv_timer_in_use+0x10/0x20 [kvm]
> PGD 0
> Oops: 0000 [#1] SMP
> Call Trace:
> kvm_arch_vcpu_load+0x86/0x260 [kvm]
> vcpu_load+0x46/0x60 [kvm]
> kvm_vcpu_ioctl+0x79/0x7c0 [kvm]
> ? __lock_is_held+0x54/0x70
> do_vfs_ioctl+0x96/0x6a0
> ? __fget_light+0x2a/0x90
> SyS_ioctl+0x79/0x90
> do_syscall_64+0x7c/0x1e0
> entry_SYSCALL64_slow_path+0x25/0x25
> RIP [<ffffffffc04e0180>] kvm_lapic_hv_timer_in_use+0x10/0x20 [kvm]
> RSP <ffff8800db1f3d70>
> CR2: 000000000000008c
> ---[ end trace a55fb79d2b3b4ee8 ]---
>
> This can be reproduced steadily by kernel_irqchip=off.
>
> We should not access preemption timer stuff if lapic is emulated in userspace.
> This patch fix it by avoiding access preemption timer stuff when kernel_irqchip=off.
>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Radim Krčmář <rkrcmar@redhat.com>
> Cc: Yunhong Jiang <yunhong.jiang@intel.com>
> Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
> ---

Reviewed-by: Radim Krčmář <rkrcmar@redhat.com>

> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> @@ -1348,6 +1348,9 @@ static void start_sw_tscdeadline(struct kvm_lapic *apic)
>
> bool kvm_lapic_hv_timer_in_use(struct kvm_vcpu *vcpu)

Btw, the "hv" in makes it look like it has something to do with Hyper-V.
What does "hv" stand for anyway? Hardware Virtualized?

Thanks.

> {
> + if (!lapic_in_kernel(vcpu))
> + return false;
> +
> return vcpu->arch.apic->lapic_timer.hv_timer_in_use;
> }
> EXPORT_SYMBOL_GPL(kvm_lapic_hv_timer_in_use);