Affected: Linux

In the Linux kernel, the following vulnerability has been resolved:

rcutorture: Fix ksoftirqd boosting timing and iteration

The RCU priority boosting can fail in two situations:

1) If (nr_cpus= > maxcpus=), which means if the total number of CPUs is higher than those brought online at boot, then torture_onoff() may later bring up CPUs that weren't online on boot. Now since rcutorture initialization only boosts the ksoftirqds of the CPUs that have been set online on boot, the CPUs later set online by torture_onoff won't benefit from the boost, making RCU priority boosting fail.

2) The ksoftirqd kthreads are boosted after the creation of rcu_torture_boost() kthreads, which opens a window large enough for these rcu_torture_boost() kthreads to wait (despite running at FIFO priority) for ksoftirqds that are still running at SCHED_NORMAL priority.

The issues can trigger for example with:

./kvm.sh --configs TREE01 --kconfig "CONFIG_RCU_BOOST=y"

[ 34.968561] rcu-torture: !!! [ 34.968627] ------------[ cut here ]------------ [ 35.014054] WARNING: CPU: 4 PID: 114 at kernel/rcu/rcutorture.c:1979 rcu_torture_stats_print+0x5ad/0x610 [ 35.052043] Modules linked in: [ 35.069138] CPU: 4 PID: 114 Comm: rcu_torture_sta Not tainted 5.18.0-rc1 #1 [ 35.096424] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.14.0-0-g155821a-rebuilt.opensuse.org 04/01/2014 [ 35.154570] RIP: 0010:rcu_torture_stats_print+0x5ad/0x610 [ 35.198527] Code: 63 1b 02 00 74 02…