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

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

[Waiman Long]

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);
 }
 
-- 
2.18.1

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

[Juergen Gross]

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?

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


Juergen

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

[Waiman Long]

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

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

[Juergen Gross]

On 25/03/2019 19:03, Waiman Long wrote:
> 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().

I think I'd prefer that.


Juergen

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

[Waiman Long]

On 04/01/2019 02:38 AM, Juergen Gross wrote:
> On 25/03/2019 19:03, Waiman Long wrote:
>> 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().
> I think I'd prefer that.
>
>
> Juergen
>
It turns out that it may be caused by a software bug after all. You can
ignore this patch for now.

Thanks,
Longman