Re: [PATCH v3] KVM: x86: Fix potential preemption when get the current kvmclock timestamp

[Paolo Bonzini]

On 12/05/2017 03:12, Wanpeng Li wrote:
> From: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
> 
>  BUG: using __this_cpu_read() in preemptible [00000000] code: qemu-system-x86/2809
>  caller is __this_cpu_preempt_check+0x13/0x20
>  CPU: 2 PID: 2809 Comm: qemu-system-x86 Not tainted 4.11.0+ #13
>  Call Trace:
>   dump_stack+0x99/0xce
>   check_preemption_disabled+0xf5/0x100
>   __this_cpu_preempt_check+0x13/0x20
>   get_kvmclock_ns+0x6f/0x110 [kvm]
>   get_time_ref_counter+0x5d/0x80 [kvm]
>   kvm_hv_process_stimers+0x2a1/0x8a0 [kvm]
>   ? kvm_hv_process_stimers+0x2a1/0x8a0 [kvm]
>   ? kvm_arch_vcpu_ioctl_run+0xac9/0x1ce0 [kvm]
>   kvm_arch_vcpu_ioctl_run+0x5bf/0x1ce0 [kvm]
>   kvm_vcpu_ioctl+0x384/0x7b0 [kvm]
>   ? kvm_vcpu_ioctl+0x384/0x7b0 [kvm]
>   ? __fget+0xf3/0x210
>   do_vfs_ioctl+0xa4/0x700
>   ? __fget+0x114/0x210
>   SyS_ioctl+0x79/0x90
>   entry_SYSCALL_64_fastpath+0x23/0xc2
>  RIP: 0033:0x7f9d164ed357
>   ? __this_cpu_preempt_check+0x13/0x20
> 
> This can be reproduced by run kvm-unit-tests/hyperv_stimer.flat w/ 
> CONFIG_PREEMPT and CONFIG_DEBUG_PREEMPT enabled.
> 
> Safe access to per-CPU data requires a couple of constraints, though: the 
> thread working with the data cannot be preempted and it cannot be migrated 
> while it manipulates per-CPU variables. If the thread is preempted, the 
> thread that replaces it could try to work with the same variables; migration 
> to another CPU could also cause confusion. However there is no preemption 
> disable when reads host per-CPU tsc rate to calculate the current kvmclock 
> timestamp.
> 
> This patch fixes it by utilizing get_cpu/put_cpu pair to guarantee both 
> __this_cpu_read() and rdtsc() are not preempted.
> 
> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
> Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
> ---
> v2 -> v3:
>  * update patch description
> v1 -> v2:
>  * utilize get_cpu/put_cpu pair to protect both __this_cpu_read and rdtsc()
> 
>  arch/x86/kvm/x86.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index b54125b..3b5fc7e 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1763,6 +1763,7 @@ u64 get_kvmclock_ns(struct kvm *kvm)
>  {
>  	struct kvm_arch *ka = &kvm->arch;
>  	struct pvclock_vcpu_time_info hv_clock;
> +	u64 ret;
>  
>  	spin_lock(&ka->pvclock_gtod_sync_lock);
>  	if (!ka->use_master_clock) {
> @@ -1774,10 +1775,17 @@ u64 get_kvmclock_ns(struct kvm *kvm)
>  	hv_clock.system_time = ka->master_kernel_ns + ka->kvmclock_offset;
>  	spin_unlock(&ka->pvclock_gtod_sync_lock);
>  
> +	/* both __this_cpu_read() and rdtsc() should be on the same cpu */
> +	get_cpu();
> +
>  	kvm_get_time_scale(NSEC_PER_SEC, __this_cpu_read(cpu_tsc_khz) * 1000LL,
>  			   &hv_clock.tsc_shift,
>  			   &hv_clock.tsc_to_system_mul);
> -	return __pvclock_read_cycles(&hv_clock, rdtsc());
> +	ret = __pvclock_read_cycles(&hv_clock, rdtsc());
> +
> +	put_cpu();
> +
> +	return ret;
>  }
>  
>  static void kvm_setup_pvclock_page(struct kvm_vcpu *v)
> 

Reviewed-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx