On Monday, 12 February 2024 11:26:24 CET syzbot wrote:
syzbot found the following issue on:
HEAD commit: 41bccc98fb79 Linux 6.8-rc2 git tree: git://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git for-kernelci console output: https://syzkaller.appspot.com/x/log.txt?x=14200118180000 kernel config: https://syzkaller.appspot.com/x/.config?x=451a1e62b11ea4a6 dashboard link: https://syzkaller.appspot.com/bug?extid=a6a4b5bb3da165594cff compiler: Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40 userspace arch: arm64
Unfortunately, I don't have any reproducer for this issue yet.
Downloadable assets: disk image: https://storage.googleapis.com/syzbot-assets/0772069e29cf/disk-41bccc98.raw.... vmlinux: https://storage.googleapis.com/syzbot-assets/659d3f0755b7/vmlinux-41bccc98.x... kernel image: https://storage.googleapis.com/syzbot-assets/7780a45c3e51/Image-41bccc98.gz....
IMPORTANT: if you fix the issue, please add the following tag to the commit: Reported-by: syzbot+a6a4b5bb3da165594cff@syzkaller.appspotmail.com
#syz test
This crash does not have a reproducer. I cannot test it.
From 5984ace8f8df7cf8d6f98ded0eebe7d962028992 Mon Sep 17 00:00:00 2001 From: Sven Eckelmann sven@narfation.org Date: Mon, 12 Feb 2024 13:10:33 +0100 Subject: [PATCH] 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 in each step 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 configurations 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
net/batman-adv/translation-table.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/net/batman-adv/translation-table.c b/net/batman-adv/translation-table.c index b95c36765d04..2243cec18ecc 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;
-- 2.39.2