[PATCH v4 2/3] cgroup/cpuset: prevent activating local partition over remote one

[Hui Peng <benquike@gmail.com>]

Commit 6da580ec656a ("cgroup/cpuset: Don't allow creation of local
partition over a remote one") added a check in update_prstate() to reject
creating a top-level local partition whose cs->exclusive_cpus intersects
subpartitions_cpus.

However, if a top-level cgroup A1 has a child A1/A2 enabled as a remote
partition on a subset of its exclusive CPUs (X2-3 on A1, CX2-3:P2 on
A1/A2), and A1 is then set to "root" (P1, becoming "root invalid" via
update_prstate() because its exclusive CPUs intersect subpartitions_cpus)
followed by expanding its CPUs to include additional non-remote CPUs
(CX1-3), update_cpumask() / update_exclusive_cpumask() calls
validate_partition() without going through update_prstate(). Because
validate_partition() does not check trialcs->exclusive_cpus against
subpartitions_cpus, A1 transitions from "root invalid" to a valid local
partition over the existing remote partition A1/A2 and triggers:

  WARNING: kernel/cgroup/cpuset.c:1943 at update_parent_effective_cpumask+0x189b/0x1fd0

Check trialcs->exclusive_cpus against subpartitions_cpus in
validate_partition() when !is_partition_valid(cs), and add a regression
test case to tools/testing/selftests/cgroup/test_cpuset_prs.sh.

Tested in QEMU on Linux 7.3.0-rc3 using
tools/testing/selftests/cgroup/test_cpuset_prs.sh.

Fixes: 6da580ec656a ("cgroup/cpuset: Don't allow creation of local partition over a remote one")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Hui Peng <benquike@gmail.com>
---
 kernel/cgroup/cpuset.c                            | 4 ++++
 tools/testing/selftests/cgroup/test_cpuset_prs.sh | 2 ++
 2 files changed, 6 insertions(+)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 7381fa8502e7..7c5b4a8f1f3a 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -2415,6 +2415,10 @@ static enum prs_errcode validate_partition(struct cpuset *cs, struct cpuset *tri
 	if (cpumask_empty(trialcs->effective_xcpus))
 		return PERR_INVCPUS;
 
+	if ((parent == &top_cpuset) && !is_partition_valid(cs) &&
+	    cpumask_intersects(trialcs->exclusive_cpus, subpartitions_cpus))
+		return PERR_REMOTE;
+
 	if (prstate_housekeeping_conflict(trialcs->partition_root_state,
 					  trialcs->effective_xcpus))
 		return PERR_HKEEPING;
diff --git a/tools/testing/selftests/cgroup/test_cpuset_prs.sh b/tools/testing/selftests/cgroup/test_cpuset_prs.sh
index b732078bf319..4a451350ad84 100755
--- a/tools/testing/selftests/cgroup/test_cpuset_prs.sh
+++ b/tools/testing/selftests/cgroup/test_cpuset_prs.sh
@@ -321,6 +321,8 @@ TEST_MATRIX=(
 	" C1-3:P2  X4:P2    .      .      .    X3:P2    .      .     0 A1:1-2|XA1:1-3|A2:3:XA2:3 A1:P2|A2:P2 1-3"
 	" C0-3:P2    .      .    C4-6   C0-4     .      .      .     0 A1:0-4|B1:5-6 A1:P2|B1:P0"
 	" C0-3:P2    .      .    C4-6 C0-4:C0-3  .      .      .     0 A1:0-3|B1:4-6 A1:P2|B1:P0 0-3"
+	"   X2-3  CX2-3:P2  .      .  P1:CX1-3   .      .      .     0 A1:1|A2:2-3|XA2:2-3 \
+								       A1:P-1|A2:P2 2-3"
 
 	# Local partition invalidation tests
 	" C0-3:X1-3:P2 C1-3:X2-3:P2 C2-3:X3:P2 \
-- 
2.49.0