[riot-notifications] [RIOT-OS/RIOT] gnrc_ipv6_nib: fix acquire race on gnrc_ipv6_nib_get_next_hop_l2addr() (#16450)

Martine Lenders notifications at github.com
Wed May 5 18:45:11 CEST 2021



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### Contribution description
When two threads use `gnrc_ipv6_nib_get_next_hop_l2addr()` to determine a next hop (e.g. when there is both an IPv6 sender and a 6LoWPAN fragment forwarder), a race condition may happen, where one thread acquires the NIB and the other acquires the network interface resulting in a deadlock. By releasing the NIB (if acquired) before trying to acquire the network interface and re-acquiring the NIB after the network interface is acquired, this is fixed.

An example for how the race condition might come to pass is that first, the IPv6 thread, acquires both the `netif` and the NIB for a look-up of an off-link (global) address:

https://github.com/RIOT-OS/RIOT/blob/6e4434a76049356d5c24be606084cd38abc6d432/sys/net/gnrc/network_layer/ipv6/nib/nib.c#L198-L199

and then continues to https://github.com/RIOT-OS/RIOT/blob/6e4434a76049356d5c24be606084cd38abc6d432/sys/net/gnrc/network_layer/ipv6/nib/nib.c#L261-L269

However, when the 6LoWPAN thread (with a fragment forwarding module like `gnrc_sixlowpan_frag_minfwd` or `gnrc_sixlowpan_frag_sfr` via the VRB [here]()) during that also makes a lookup but with a specific `netif != NULL` (same as the new `netif` in IPv6) in L198, it will get stuck on L199, since the IPv6 thread still hold the lock on the NIB, while then waiting on the `netif` in 268. I did not really observe this with the link-local variant

https://github.com/RIOT-OS/RIOT/blob/6e4434a76049356d5c24be606084cd38abc6d432/sys/net/gnrc/network_layer/ipv6/nib/nib.c#L210-L215

but in theory it can also happen there.

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### Testing procedure
Both `tests/gnrc_ipv6_nibu` and `tests/gnrc_ipv6_nib_6ln` should still pass and `gnrc_networking` should still work. I found this by running experiments very similar to https://github.com/5G-I3/IEEE-LCN-2019/, but with a higher send interval (250-750ms), for SFR with congestion control. Without this fix I ran into the race condition quite often with low congestion windows. With this fix, I was not able to reproduce the race condition yet.
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### Issues/PRs references
None.
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You can view, comment on, or merge this pull request online at:

  https://github.com/RIOT-OS/RIOT/pull/16450

-- Commit Summary --

  * gnrc_ipv6_nib: fix acquire race on gnrc_ipv6_nib_get_next_hop_l2addr()

-- File Changes --

    M sys/net/gnrc/network_layer/ipv6/nib/nib.c (22)

-- Patch Links --

https://github.com/RIOT-OS/RIOT/pull/16450.patch
https://github.com/RIOT-OS/RIOT/pull/16450.diff

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