The following commit has been merged in the master branch: commit b1f532a3b1e6d2e5559c7ace49322922637a28aa Author: Sven Eckelmann sven@narfation.org Date: Mon Feb 12 13:58:33 2024 +0100

batman-adv: Avoid infinite loop trying to resize local TT

If the MTU of one of an attached interface becomes too small to transmit the local translation table then it must be resized to fit inside all fragments (when enabled) or a single packet.

But if the MTU becomes too low to transmit even the header + the VLAN specific part then the resizing of the local TT will never succeed. This can for example happen when the usable space is 110 bytes and 11 VLANs are on top of batman-adv. In this case, at least 116 byte would be needed. There will just be an endless spam of

batman_adv: batadv0: Forced to purge local tt entries to fit new maximum fragment MTU (110)

in the log but the function will never finish. Problem here is that the timeout will be halved all the time and will then stagnate at 0 and therefore never be able to reduce the table even more.

There are other scenarios possible with a similar result. The number of BATADV_TT_CLIENT_NOPURGE entries in the local TT can for example be too high to fit inside a packet. Such a scenario can therefore happen also with only a single VLAN + 7 non-purgable addresses - requiring at least 120 bytes.

While this should be handled proactively when:

* interface with too low MTU is added * VLAN is added * non-purgeable local mac is added * MTU of an attached interface is reduced * fragmentation setting gets disabled (which most likely requires dropping attached interfaces)

not all of these scenarios can be prevented because batman-adv is only consuming events without the the possibility to prevent these actions (non-purgable MAC address added, MTU of an attached interface is reduced). It is therefore necessary to also make sure that the code is able to handle also the situations when there were already incompatible system configuration are present.

Cc: stable@vger.kernel.org Fixes: a19d3d85e1b8 ("batman-adv: limit local translation table max size") Reported-by: syzbot+a6a4b5bb3da165594cff@syzkaller.appspotmail.com Signed-off-by: Sven Eckelmann sven@narfation.org Signed-off-by: Simon Wunderlich sw@simonwunderlich.de

diff --git a/net/batman-adv/translation-table.c b/net/batman-adv/translation-table.c index b95c36765d045..2243cec18ecc8 100644 --- a/net/batman-adv/translation-table.c +++ b/net/batman-adv/translation-table.c @@ -3948,7 +3948,7 @@ void batadv_tt_local_resize_to_mtu(struct net_device *soft_iface)

spin_lock_bh(&bat_priv->tt.commit_lock);

- while (true) { + while (timeout) { table_size = batadv_tt_local_table_transmit_size(bat_priv); if (packet_size_max >= table_size) break;