Public bug reported:
On Ubuntu 26.04 with kernel 7.0.0-34-generic, traffic from an LXD
instance attached to the Ubuntu Fan bridge (lxdfan0, created by
MicroCloud when OVN is not used) triggers the warning "Voluntary context
switch within RCU read-side critical section!" followed by RCU stalls,
hung tasks and finally a complete host hang. It happened twice: once
triggered by a container’s systemd-resolved and once by a VM’s vhost-net
thread.
The Ubuntu Fan patch in vxlan_xmit_one() jumps to tx_free from inside
the rcu_read_lock() section without unlocking. This is the bug fixed in
2024 under LP #2064176; the fix is missing from the 7.0 kernel source
(verified in 7.0.0-38.38 and 7.0.0-39.39).
Ubuntu kernel, package linux (Ubuntu Fan patch in
drivers/net/vxlan/vxlan_core.c)
** Affects: linux (Ubuntu)
Importance: Undecided
Status: New
** Tags: resolute
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https://bugs.launchpad.net/bugs/2170968
Title:
Ubuntu 7.0 kernel: Fan VXLAN discard path leaks the RCU read lock,
causing RCU stalls and a full host hang
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