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

Aaron Conole aconole at redhat.com
Mon Jun 7 13:09:40 UTC 2021


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.  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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