Re: [Xen-devel] [PATCH 2/2] xen/blkfront: Add BUG_ON to deal with misbehaving backends.

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 28.05.12 at 12:18, Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
wrote:
> On Fri, 25 May 2012, Konrad Rzeszutek Wilk wrote:
>> Part of the ring structure is the 'id' field which is under
>> control of the frontend. The frontend stamps it with "some"
>> value (this some in this implementation being a value less
>> than BLK_RING_SIZE), and when it gets a response expects
>> said value to be in the response structure. We have a check
>> for the id field when spolling new requests but not when
>> de-spolling responses.
>> 
>> We also add an extra check in add_id_to_freelist to make
>> sure that the 'struct request' was not NULL - as we cannot
>> pass a NULL to __blk_end_request_all, otherwise that crashes
>> (and all the operations that the response is dealing with
>> end up with __blk_end_request_all).
>> 
>> Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
>> ---
>>  drivers/block/xen-blkfront.c |    7 +++++++
>>  1 files changed, 7 insertions(+), 0 deletions(-)
>> 
>> diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
>> index 60eed4b..8e177ca 100644
>> --- a/drivers/block/xen-blkfront.c
>> +++ b/drivers/block/xen-blkfront.c
>> @@ -145,6 +145,7 @@ static void add_id_to_freelist(struct blkfront_info 
> *info,
>>                             unsigned long id)
>>  {
>>      info->shadow[id].req.u.rw.id  = info->shadow_free;
>> +    BUG_ON(info->shadow[id].request == NULL);

This only catches a small sub-portion of possible bad backend
behavior. Checking (as the very first thing in the function) e.g.

info->shadow[id].req.u.rw.id == id

would seem to cover a broader set (based on my recent looking
at similar mismatches apparently resulting from the qdisk
backend occasionally sending bad/duplicate responses).

But take this with the below applied here too.

>>      info->shadow[id].request = NULL;
>>      info->shadow_free = id;
>>  }
>> @@ -746,6 +747,12 @@ static irqreturn_t blkif_interrupt(int irq, void 
> *dev_id)
>>  
>>              bret = RING_GET_RESPONSE(&info->ring, i);
>>              id   = bret->id;
>> +            /*
>> +             * The backend has messed up and given us an id that we would
>> +             * never have given to it (we stamp it up to BLK_RING_SIZE -
>> +             * look in get_id_from_freelist.
>> +             */
>> +            BUG_ON(id >= BLK_RING_SIZE);
>>              req  = info->shadow[id].request;
>>  
>>              if (bret->operation != BLKIF_OP_DISCARD)
> 
> While we should certainly check whether bret->id is valid before
> using it, is it actually correct that the frontend BUGs in response of a
> backend bug?
> 
> If the IO doesn't involve the root disk, the guest might be able to
> function correctly without communicating with the backend at all.
> I think we should WARN and return error. Maybe also call blkfront_remove
> if we can.

I very much agree to this.

Jan


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