UBUNTU-CVE-2026-98542: Low severity ubuntu/linux-hwe-edge vulnerability

In the Linux kernel, the following vulnerability has been resolved: cgroup/cpuset: Return PERRNOCPUS in remotepartitionenable() on subpartitionscpus conflict When a remote partition is created underneath an existing local partition via a non-partition (PRSMEMBER) intermediate cgroup, updateprstate() sees parent->partitionrootstate == PRSMEMBER and calls remotepartitionenable(). Commit 86888c7bd117 ("cgroup/cpuset: Add warnings to catch inconsistency in exclusive CPUs") replaced the cpumaskintersects(tmp->newcpus, subpartitionscpus) error check in remotepartitionenable() with WARNONONCE(). As a result, remotepartitionenable() emits a warning and proceeds to enable the remote partition on CPUs that are already owned by the ancestor local partition in subpartitionscpus. This can be reproduced on Linux 7.3.0-rc3 with: mkdir -p /tmp/cg1 mount -t cgroup2 none /tmp/cg1 echo "+cpuset" > /tmp/cg1/cgroup.subtreecontrol mkdir /tmp/cg1/A echo 1 > /tmp/cg1/A/cpuset.cpus echo 1 > /tmp/cg1/A/cpuset.cpus.exclusive echo root > /tmp/cg1/A/cpuset.cpus.partition echo "+cpuset" > /tmp/cg1/A/cgroup.subtreecontrol mkdir /tmp/cg1/A/B echo 1 > /tmp/cg1/A/B/cpuset.cpus echo 1 > /tmp/cg1/A/B/cpuset.cpus.exclusive echo "+cpuset" > /tmp/cg1/A/B/cgroup.subtreecontrol mkdir /tmp/cg1/A/B/D echo 1 > /tmp/cg1/A/B/D/cpuset.cpus echo 1 > /tmp/cg1/A/B/D/cpuset.cpus.exclusive echo root > /tmp/cg1/A/B/D/cpuset.cpus.partition which triggers: WARNING: kernel/cgroup/cpuset.c:1594 at remotepartitionenable+0x1c1/0x300 and leaves both /tmp/cg1/A and /tmp/cg1/A/B/D as active root partitions claiming exclusive CPU 1. Fix this by returning PERRNOCPUS when tmp->newcpus intersects subpartitionscpus in remotepartitionenable(), matching the error code used by remotecpusupdate() for the same subpartitionscpus conflict, and add a regression test case to tools/testing/selftests/cgroup/testcpusetprs.sh. Tested in QEMU on Linux 7.3.0-rc3 using the reproducer above and tools/testing/selftests/cgroup/testcpusetprs.sh.