Re: [PATCH 1/2] virtio-ccw: fix virtio_set_ind_atomic

[Christian Borntraeger <borntraeger@de.ibm.com>]

On 16.06.20 08:33, Cornelia Huck wrote:
> On Tue, 16 Jun 2020 07:58:53 +0200
> Christian Borntraeger <borntraeger@de.ibm.com> wrote:
> 
>> On 16.06.20 06:50, Halil Pasic wrote:
>>> The atomic_cmpxchg() loop is broken because we occasionally end up with
>>> old and _old having different values (a legit compiler can generate code
>>> that accessed *ind_addr again to pick up a value for _old instead of
>>> using the value of old that was already fetched according to the
>>> rules of the abstract machine). This means the underlying CS instruction
>>> may use a different old (_old) than the one we intended to use if
>>> atomic_cmpxchg() performed the xchg part.
>>>
>>> Let us use volatile to force the rules of the abstract machine for
>>> accesses to *ind_addr. Let us also rewrite the loop so, we that the
>>> new old is used to compute the new desired value if the xchg part
>>> is not performed.
>>>
>>> Signed-off-by: Halil Pasic <pasic@linux.ibm.com>
>>> Reported-by: Andre Wild <Andre.Wild1@ibm.com>
>>> Fixes: 7e7494627f ("s390x/virtio-ccw: Adapter interrupt support.")
>>> ---
>>>  hw/s390x/virtio-ccw.c | 18 ++++++++++--------
>>>  1 file changed, 10 insertions(+), 8 deletions(-)
>>>
>>> diff --git a/hw/s390x/virtio-ccw.c b/hw/s390x/virtio-ccw.c
>>> index c1f4bb1d33..3c988a000b 100644
>>> --- a/hw/s390x/virtio-ccw.c
>>> +++ b/hw/s390x/virtio-ccw.c
>>> @@ -786,9 +786,10 @@ static inline VirtioCcwDevice *to_virtio_ccw_dev_fast(DeviceState *d)
>>>  static uint8_t virtio_set_ind_atomic(SubchDev *sch, uint64_t ind_loc,
>>>                                       uint8_t to_be_set)
>>>  {
>>> -    uint8_t ind_old, ind_new;
>>> +    uint8_t expected, actual;
>>>      hwaddr len = 1;
>>> -    uint8_t *ind_addr;
>>> +    /* avoid  multiple fetches */
>>> +    uint8_t volatile *ind_addr;
>>>  
>>>      ind_addr = cpu_physical_memory_map(ind_loc, &len, true);
>>>      if (!ind_addr) {
>>> @@ -796,14 +797,15 @@ static uint8_t virtio_set_ind_atomic(SubchDev *sch, uint64_t ind_loc,
>>>                       __func__, sch->cssid, sch->ssid, sch->schid);
>>>          return -1;
>>>      }
>>> +    actual = *ind_addr;
>>>      do {
>>> -        ind_old = *ind_addr;  
>>
>> to make things easier to understand. Adding a barrier in here also fixes the issue.
>> Reasoning follows below:
>>
>>> -        ind_new = ind_old | to_be_set;  
>>
>> with an analysis from Andreas (cc)
>>
>>  #define atomic_cmpxchg__nocheck(ptr, old, new)    ({                    \   
>>  
>>      typeof_strip_qual(*ptr) _old = (old);                               \   
>>  
>>      (void)__atomic_compare_exchange_n(ptr, &_old, new, false,           \   
>>  
>>                                __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST);      \   
>>  
>>      _old;                                                               \   
>>  
>>  })
>>  
>> ind_old is copied into _old in the macro. Instead of doing the copy from the
>> register the compiler reloads the value from memory. The result is that _old
>> and ind_old end up having different values. _old in r1 with the bits set
>> already and ind_old in r10 with the bits cleared. _old gets updated by CS
>> and matches ind_old afterwards - both with the bits being 0. So the !=
>> compare is false and the loop is left without having set any bits.
>>
>>
>> Paolo (to),
>> I am asking myself if it would be safer to add a barrier or something like
>> this in the macros in include/qemu/atomic.h. 

Having said this, I think that the refactoring from Halil (to re-use actual) 
also makes sense independent of the fix. 
> 
> I'm also wondering whether this has been seen on other architectures as
> well? There are also some callers in non-s390x code, and dealing with
> this in common code would catch them as well.
> 
>>
>>
>>
>>
>>> -    } while (atomic_cmpxchg(ind_addr, ind_old, ind_new) != ind_old);
>>> -    trace_virtio_ccw_set_ind(ind_loc, ind_old, ind_new);
>>> -    cpu_physical_memory_unmap(ind_addr, len, 1, len);
>>> +        expected = actual;
>>> +        actual = atomic_cmpxchg(ind_addr, expected, expected | to_be_set);
>>> +    } while (actual != expected);
>>> +    trace_virtio_ccw_set_ind(ind_loc, actual, actual | to_be_set);
>>> +    cpu_physical_memory_unmap((void *)ind_addr, len, 1, len);
>>>  
>>> -    return ind_old;
>>> +    return actual;
>>>  }
>>>  
>>>  static void virtio_ccw_notify(DeviceState *d, uint16_t vector)
>>>   
>>
>>
>