> On 25 Jan 2020, at 21:03, Jason <jgg@xxxxxxxx> wrote:
>
> On Thu, Jan 23, 2020 at 04:55:21PM +0100, Håkon Bugge wrote:
>> Using CX-3 virtual functions, either from a bare-metal machine or
>> pass-through from a VM, MAD packets are proxied through the PF driver.
>>
>> Since the VF drivers have separate name spaces for MAD Transaction Ids
>> (TIDs), the PF driver has to re-map the TIDs and keep the book keeping
>> in a cache.
>>
>> Following the RDMA Connection Manager (CM) protocol, it is clear when
>> an entry has to evicted from the cache. When a DREP is sent from
>> mlx4_ib_multiplex_cm_handler(), id_map_find_del() is called. Similar
>> when a REJ is received by the mlx4_ib_demux_cm_handler(),
>> id_map_find_del() is called.
>>
>> This function wipes out the TID in use from the IDR or XArray and
>> removes the id_map_entry from the table.
>>
>> In short, it does everything except the topping of the cake, which is
>> to remove the entry from the list and free it. In other words, for the
>> REJ case enumerated above, one id_map_entry will be leaked.
>>
>> For the other case above, a DREQ has been received first. The
>> reception of the DREQ will trigger queuing of a delayed work to delete
>> the id_map_entry, for the case where the VM doesn't send back a DREP.
>>
>> In the normal case, the VM _will_ send back a DREP, and
>> id_map_find_del() will be called.
>>
>> But this scenario introduces a secondary leak. First, when the DREQ is
>> received, a delayed work is queued. The VM will then return a DREP,
>> which will call id_map_find_del(). As stated above, this will free the
>> TID used from the XArray or IDR. Now, there is window where that
>> particular TID can be re-allocated, lets say by an outgoing REQ. This
>> TID will later be wiped out by the delayed work, when the function
>> id_map_ent_timeout() is called. But the id_map_entry allocated by the
>> outgoing REQ will not be de-allocated, and we have a leak.
>>
>> Both leaks are fixed by removing the id_map_find_del() function and
>> only using schedule_delayed(). Of course, a check in
>> schedule_delayed() to see if the work already has been queued, has
>> been added.
>>
>> Another benefit of always using the delayed version for deleting
>> entries, is that we do get a TimeWait effect; a TID no longer in use,
>> will occupy the XArray or IDR for CM_CLEANUP_CACHE_TIMEOUT time,
>> without any ability of being re-used for that time period.
>>
>> Signed-off-by: Håkon Bugge <haakon.bugge@xxxxxxxxxx>
>> ---
>
> Applied to for-next, I think we are probably done with -rc right now, and it
> doesn't have a fixes line or stable cc anyhow.
>
> I added
>
> Fixes: 3cf69cc8dbeb ("IB/mlx4: Add CM paravirtualization")
>
> Though
Indeed ;-) Internal orcl review revealed two use-after-free cases in the commit.
When sending (or receiving) a DREQ, we queue delayed work to clean up and free the id_map_entry. Now, if the delay in the delayed work queueing is in close proximity with the DREP coming back from the network (or received from the VM to be sent out on the wire), there is a tiny window where the DREP may find its id_map_entry, but before schedule_deleayed() is called, the delayed work kicks in and deletes the id_map_entry. The now freed id_map_entry will be dereferenced in schedule_delayed().
The fix is quite simple, do not call schedule_delayed() for DREPs, since work already have been queued when the DREQs were handled.
Expect a fix shortly.
Thxs, Håkon
>
> Jason
