Re: [PATCH] x86/paravirt: Guard against invalid cpu # in pv_vcpu_is_preempted()

[Waiman Long <longman@redhat.com>]

On 03/25/2019 12:40 PM, Juergen Gross wrote:
> On 25/03/2019 16:57, Waiman Long wrote:
>> It was found that passing an invalid cpu number to pv_vcpu_is_preempted()
>> might panic the kernel in a VM guest. For example,
>>
>> [    2.531077] Oops: 0000 [#1] SMP PTI
>>   :
>> [    2.532545] Hardware name: Red Hat KVM, BIOS 0.5.1 01/01/2011
>> [    2.533321] RIP: 0010:__raw_callee_save___kvm_vcpu_is_preempted+0x0/0x20
>>
>> To guard against this kind of kernel panic, check is added to
>> pv_vcpu_is_preempted() to make sure that no invalid cpu number will
>> be used.
>>
>> Signed-off-by: Waiman Long <longman@redhat.com>
>> ---
>>  arch/x86/include/asm/paravirt.h | 6 ++++++
>>  1 file changed, 6 insertions(+)
>>
>> diff --git a/arch/x86/include/asm/paravirt.h b/arch/x86/include/asm/paravirt.h
>> index c25c38a05c1c..4cfb465dcde4 100644
>> --- a/arch/x86/include/asm/paravirt.h
>> +++ b/arch/x86/include/asm/paravirt.h
>> @@ -671,6 +671,12 @@ static __always_inline void pv_kick(int cpu)
>>  
>>  static __always_inline bool pv_vcpu_is_preempted(long cpu)
>>  {
>> +	/*
>> +	 * Guard against invalid cpu number or the kernel might panic.
>> +	 */
>> +	if (WARN_ON_ONCE((unsigned long)cpu >= nr_cpu_ids))
>> +		return false;
>> +
>>  	return PVOP_CALLEE1(bool, lock.vcpu_is_preempted, cpu);
>>  }
> Can this really happen without being a programming error?

This shouldn't happen without a programming error, I think. In my case,
it was caused by a race condition leading to use-after-free of the cpu
number. However, my point is that error like that shouldn't cause the
kernel to panic.

> Basically you'd need to guard all percpu area accesses to foreign cpus
> this way. Why is this one special?

It depends. If out-of-bound access can only happen with obvious
programming error, I don't think we need to guard against them. In this
case, I am not totally sure if the race condition that I found may
happen with existing code or not. To be prudent, I decide to send this
patch out.

The race condition that I am looking at is as follows:

  CPU 0                         CPU 1
  -----                         -----
up_write:
  owner = NULL;
  <release-barrier>
  count = 0;

<rcu-free task structure>
 
                          rwsem_can_spin_on_owner:
                            rcu_read_lock();
                            read owner;
                              :
                            vcpu_is_preempted(owner->cpu);
                              :
                            rcu_read_unlock()

When I tried to merge the owner into the count (clear the owner after
the barrier), I can reproduce the crash 100% when booting up the kernel
in a VM guest. However, I am not sure if the configuration above is safe
and is just very hard to reproduce.

Alternatively, I can also do the cpu check before calling
vcpu_is_preempted().

Cheers,
Longman