Subject: Re: [syzbot] [batman?] BUG: soft lockup in sys_sendmsg

From: Sven Eckelmann <[email-masked]>
Date: Mon, 12 Feb 2024 14:28:32 +0100

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.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/659d3f0755b7/vmlinux-41bccc98.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/7780a45c3e51/Image-41bccc98.gz.xz
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: [email-masked]
> 

#syz test

>From 5984ace8f8df7cf8d6f98ded0eebe7d962028992 Mon Sep 17 00:00:00 2001
From: Sven Eckelmann <[email-masked]>
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: [email-masked]
Fixes: a19d3d85e1b8 ("batman-adv: limit local translation table max size")
Reported-by: [email-masked]
Signed-off-by: Sven Eckelmann <[email-masked]>
---
 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