[ovs-dev] [ovs-discuss] Patch for PACKET_OUT getting deleted twice crash

Tony van der Peet tony.vanderpeet at gmail.com
Tue Jun 15 00:28:50 UTC 2021


Yes, I can create that patch.

Tony

On Sat, Jun 12, 2021 at 5:24 AM Ilya Maximets <i.maximets at ovn.org> wrote:

> On 6/9/21 3:12 PM, Aaron Conole wrote:
> > Ilya Maximets <i.maximets at ovn.org> writes:
> >
> >> On 6/7/21 3:59 PM, Ilya Maximets wrote:
> >>> On 6/7/21 3:09 PM, Aaron Conole wrote:
> >>>> Ilya Maximets <i.maximets at ovn.org> writes:
> >>>>
> >>>>>> Here is a patch with both a test and a fix.
> >>>>
> >>>> Thanks so much!  It's nice to get fixes, but I think it's really great
> >>>> when test cases come along with them.
> >>>>
> >>>>> Hi.  Thanks for working n this!
> >>>>>
> >>>>> CC: ovs-dev
> >>>>>
> >>>>>> Not submitting as a formal
> >>>>>> patch because I would like some feedback on whether 1) maintainers
> feel
> >>>>>> this is worth fixing and
> >>>>>
> >>>>> I can reproduce the crash with your test.  Basically, actions in
> userspace
> >>>>> datapath may drop packets if something goes wrong.  'meter' action
> just
> >>>>> seems to be the most explicit variant.  So, I think, this is
> definitely
> >>>>> worth fixing as some other condition might trigger this crash on
> packet-out
> >>>>> as well.
> >>>>>
> >>>>> ==2568112==ERROR: AddressSanitizer: heap-use-after-free
> >>>>> on address 0x61600000699c at pc 0x000000573860 bp 0x7ffebc6cc880 sp
> 0x7ffebc6cc878
> >>>>> READ of size 1 at 0x61600000699c thread T0
> >>>>>     #0 0x57385f in dp_packet_delete lib/dp-packet.h:242:16
> >>>>>     #1 0x57372c in ofproto_packet_out_uninit ofproto/ofproto.c:3562:5
> >>>>>     #2 0x585e77 in handle_packet_out ofproto/ofproto.c:3722:5
> >>>>>     #3 0x583801 in handle_single_part_openflow
> ofproto/ofproto.c:8499:16
> >>>>>     #4 0x570c9c in handle_openflow ofproto/ofproto.c:8686:21
> >>>>>     #5 0x611781 in ofconn_run ofproto/connmgr.c:1329:13
> >>>>>     #6 0x6112ed in connmgr_run ofproto/connmgr.c:356:9
> >>>>>     #7 0x56fdf4 in ofproto_run ofproto/ofproto.c:1891:5
> >>>>>     #8 0x545ec0 in bridge_run__ vswitchd/bridge.c:3251:9
> >>>>>     #9 0x5456a5 in bridge_run vswitchd/bridge.c:3310:5
> >>>>>     #10 0x55f5b1 in main vswitchd/ovs-vswitchd.c:127:9
> >>>>>     #11 0x7f85bfe09081 in __libc_start_main
> (/lib64/libc.so.6+0x27081)
> >>>>>     #12 0x46d00d in _start (vswitchd/ovs-vswitchd+0x46d00d)
> >>>>>
> >>>>>> 2) whether this is the way to fix it.
> >>>>>
> >>>>> See inline.
> >>>>>
> >>>>>>
> >>>>>> I have tried to make the most minimal change possible, but this
> means that
> >>>>>> there might be paths through the code that give unexpected
> behaviour (which
> >>>>>> in the worst case would be a memory leak I suppose).
> >>>>>>
> >>>>>> Tony
> >>>>>>
> >>>>>> diff --git a/lib/dp-packet.h b/lib/dp-packet.h
> >>>>>> index 246be14d0..5e0dabe67 100644
> >>>>>> --- a/lib/dp-packet.h
> >>>>>> +++ b/lib/dp-packet.h
> >>>>>> @@ -739,6 +739,7 @@ struct dp_packet_batch {
> >>>>>>     size_t count;
> >>>>>>     bool trunc; /* true if the batch needs truncate. */
> >>>>>>     bool do_not_steal; /* Indicate that the packets should not be
> stolen.
> >>>>>> */
> >>>>>> +    bool packet_out; /* Indicate single packet is PACKET_OUT */
> >>>>>>     struct dp_packet *packets[NETDEV_MAX_BURST];
> >>>>>> };
> >>>>>>
> >>>>>> @@ -748,6 +749,7 @@ dp_packet_batch_init(struct dp_packet_batch
> *batch)
> >>>>>>     batch->count = 0;
> >>>>>>     batch->trunc = false;
> >>>>>>     batch->do_not_steal = false;
> >>>>>> +    batch->packet_out = false;
> >>>>>> }
> >>>>>>
> >>>>>> static inline void
> >>>>>> diff --git a/lib/dpif-netdev.c b/lib/dpif-netdev.c
> >>>>>> index 650e67ab3..deba4a94a 100644
> >>>>>> --- a/lib/dpif-netdev.c
> >>>>>> +++ b/lib/dpif-netdev.c
> >>>>>> @@ -4170,6 +4170,7 @@ dpif_netdev_execute(struct dpif *dpif, struct
> >>>>>> dpif_execute *execute)
> >>>>>>
> >>>>>>     dp_packet_batch_init_packet(&pp, execute->packet);
> >>>>>>     pp.do_not_steal = true;
> >>>>>> +    pp.packet_out = execute->packet_out;
> >>>>>>     dp_netdev_execute_actions(pmd, &pp, false, execute->flow,
> >>>>>>                               execute->actions,
> execute->actions_len);
> >>>>>
> >>>>> There is already a dirty hack named "do_not_steal" that was
> introduced,
> >>>>> I guess, exactly to avoid crash in the conntrack code that could
> drop/steal
> >>>>> the packet just like meter action.  And it seems that here in
> >>>>> dpif_netdev_execute() is the only problematic entry point as all
> other
> >>>>> normal paths expects that packet might be destroyed.
> >>>>>
> >>>>> The problem was, I suppose, introduced when we tried to unify
> semantics
> >>>>> of "may_steal" flag by turning it into "should_steal".  But it seems
> that
> >>>>> in this function we really need to prohibit stealing of the packet
> since
> >>>>> ofproto layer still owns it regardless of the result of execution.
> >>>>>
> >>>>> I don't think that we need one more flag here, but we have several
> options
> >>>>> how to fix the crash:
> >>>>>
> >>>>> 1. Start honoring batch->do_not_steal flag in all actions that may
> result
> >>>>>    in packet drops.  As the original idea of having 'do_not_steal'
> flag for
> >>>>>    a batch is very hacky, I'd like to not do that.
> >>>>
> >>>> +1
> >>>>
> >>>>> 2. Try to propagate information that packet was deleted up to
> ofproto layer,
> >>>>>    i.e. make handle_packet_out() aware of that.  Will, probably, be
> not that
> >>>>>    easy to do.
> >>>>
> >>>> I had a look at doing this, but as you note it is quite intrusive, and
> >>>> we need to make changes all over.
> >>>>
> >>>>> 3. This function (dpif_netdev_execute) is not on a hot path in
> userspace
> >>>>>    datapath, IIUC.  It might be that it's just easier to remove the
> >>>>>    'do_not_steal' flag entirely, clone the packet here and call the
> >>>>>    dp_netdev_execute_actions() with should_steal=true.
> >>>>>    This sounds like the best solution for me, unless I overlooked
> some
> >>>>>    scenario, where this code is on a hot path.
> >>>>
> >>>> I like this approach.
> >>>>
> >>>>> Thoughts?
> >>>>>
> >>>>> Aaron, you have a patch[1] to remove 'do_not_steal' flag, so fix for
> this issue
> >>>>> will, likely, touch the same parts of the code.  What do you think
> about this
> >>>>> issue and possible solutions?
> >>>>
> >>>> I guess we should do the same thing we do in other places, ie: default
> >>>> assume that the packet cannot be 'stolen' and we should clone our own
> >>>> copies.
> >>>
> >>> I'm not sure that I understood this correctly, but the idea was that
> default
> >>> assumption is that packet can be stolen at any point of datapath
> processing
> >>> and higher layers should deal with this.
> >
> > Re-reading what I wrote, I don't either.  :-/  I agree with what you've
> > written.
> >
> >>> For the IP fragmentation handling this will mean that ipf will just
> take a
> >>> packet directly from the original batch without copying, so the
> original
> >>> batch will not have this packet anymore and ipf is allowed to free it
> at
> >>> any point in time, because now it owns this packet.
> >>> This aligns with the "should_steal" semantics, as any function called
> with
> >>> "should_steal=true" must take the ownership of the packet.  If the
> function
> >>> called with "should_steal=false" it still allowed to take ownership of
> some
> >>> packets from the batch and caller must be prepared for that.
> >>> If some function in datapath has no "should_steal" argument, it should
> be
> >>> treated as a function with "should_steal=false".   This applies to both
> >>> conntrack_execute() and dp_netdev_run_meter(), also to
> netdev_pop_header()
> >>> and so on.
> >>
> >> Hmm, OVS_ACTION_ATTR_TUNNEL_POP implies recirculation, so
> netdev_pop_header()
> >> is not a fully valid example here.   Actions that implies recirculation
> or
> >> recirculates packets in any other way (e.g. OVS_ACTION_ATTR_USERSPACE)
> should
> >> clone packets before doing that if not asked to take ownership.
> >
> > The ownership of the dp_packet object needs to be well established.  Use
> > of should_steal could be okay, but as an example, we have something in
> > the packet batch but it doesn't actually get honored (which is why I
> > proposed removing it).
>
> Yeah.  It looks like we have more issues than I thought initially.
> I thought that ip fragmentation module actually steals packets from
> the original batch, but it doesn't.  Instead it tries to hack some
> poor form of reference counting with 'do_not_steal' flag.  This  also
> creates yet another logical difference with kernel conntrack implementation
> that actually steals fragments and therefore stops their processing
> until reassembled.
>
> In the end, I think we can't create a consolidated solution here for
> all problems at once.  At least, we can't do that easily.  So, what
> I will do is that I'll review and apply Aaron's fix for ipf:
>
> https://patchwork.ozlabs.org/project/openvswitch/patch/20210521175905.165779-1-aconole@redhat.com/
>
>
> And this bug with crash on packet_out with meters should be fixed by
> cloning the packet inside dpif_netdev_execute() and calling the
> dp_netdev_execute_actions() with should_steal=true.
>
> Tony, could you prepare a patch for this?
>
> Best regards, Ilya Maximets.
>
> >
> >>>> If we are worried about the time it takes to copy the dp_packet
> >>>> structure and buffer, we can always introduce a reference counting
> >>>> mechanism later as an optimization.
> >>>>
> >>>> I would just prefer to do clone, and then the functional area which
> >>>> needs to hold a reference to a valid packet buffer can delete when it
> >>>> makes sense.
> >>>>
> >>>> Hope it helps.
> >>>>
> >>>>> Best regards, Ilya Maximets.
> >>>>>
> >>>>>
> >>>>> [1]
> https://patchwork.ozlabs.org/project/openvswitch/patch/20210521175905.165779-1-aconole@redhat.com/
> >>>>>
> >>>>> Non-line-wrapped version of the test for convenience:
> >>>>>
> >>>>> diff --git a/tests/ofproto-dpif.at b/tests/ofproto-dpif.at
> >>>>> index 31064ed95..d01f438b8 100644
> >>>>> --- a/tests/ofproto-dpif.at
> >>>>> +++ b/tests/ofproto-dpif.at
> >>>>> @@ -2159,6 +2159,27 @@ meter:controller flow_count:0
> packet_in_count:8 byte_in_count:112 duration:0.0s
> >>>>>  OVS_VSWITCHD_STOP
> >>>>>  AT_CLEANUP
> >>>>>
> >>>>> +AT_SETUP([ofproto-dpif packet-out table meter drop qwe])
> >>>>> +OVS_VSWITCHD_START
> >>>>> +add_of_ports br0 1 2
> >>>>> +
> >>>>> +AT_CHECK([ovs-ofctl -O OpenFlow13 add-meter br0 'meter=1 pktps
> bands=type=drop rate=1'])
> >>>>> +AT_CHECK([ovs-ofctl -O OpenFlow13 add-flow br0 'in_port=1
> action=meter:1,output:2'])
> >>>>> +
> >>>>> +ovs-ofctl -O OpenFlow13 packet-out br0 "in_port=1
> packet=50540000000a50540000000908004500001c000000000011a4cd0a0101010a0101020001000400080000
> actions=resubmit(,0)"
> >>>>> +ovs-ofctl -O OpenFlow13 packet-out br0 "in_port=1
> packet=50540000000a50540000000908004500001c000000000011a4cd0a0101010a0101020001000400080000
> actions=resubmit(,0)"
> >>>>> +
> >>>>> +# Check that vswitchd hasn't crashed by dumping the meter added
> above
> >>>>> +AT_CHECK([ovs-ofctl -O OpenFlow13 dump-meters br0 | ofctl_strip],
> [0], [dnl
> >>>>> +OFPST_METER_CONFIG reply (OF1.3):
> >>>>> +meter=1 pktps bands=
> >>>>> +type=drop rate=1
> >>>>> +])
> >>>>> +
> >>>>> +OVS_VSWITCHD_STOP
> >>>>> +AT_CLEANUP
> >>>>> +
> >>>>> +
> >>>>>  AT_SETUP([ofproto-dpif - MPLS handling])
> >>>>>  OVS_VSWITCHD_START([dnl
> >>>>>     add-port br0 p1 -- set Interface p1 type=dummy
> >>>>> ---
> >>>>
> >>>
> >
>
>


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