[PATCH v2 2/2] cgroup/cpuset: prevent local partition activation over remote partition and sibling xcpus conflict

[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, two paths still allow overlapping exclusive CPU ownership between
a top-level local partition and a remote partition or sibling:

1. If a top-level cgroup A1 has cpuset.cpus.exclusive set, is first set to
   "root" while cpuset.cpus is empty (becoming "root invalid"), has a
   child A1/A2 enabled as a remote partition on those exclusive CPUs, and
   then receives CPUs via A1/cpuset.cpus, update_cpumasks_hier() 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:1866 at update_parent_effective_cpumask+0x54a/0x8a0

2. In cpus_excl_conflict(), when a valid local partition A1 uses implicit
   exclusive CPUs (cpuset.cpus set without cpuset.cpus.exclusive, so
   sibling->exclusive_cpus is empty while sibling->effective_xcpus is
   populated), a sibling cgroup B1 can still set cpuset.cpus.exclusive on
   the same CPUs because cpus_excl_conflict() only checks
   sibling->exclusive_cpus.

Fix these issues by:
- Checking trialcs->exclusive_cpus against subpartitions_cpus in
  validate_partition() when !is_partition_valid(cs).
- Falling back to sibling->effective_xcpus in cpus_excl_conflict() when
  sibling->exclusive_cpus is empty.
- Adding regression test cases for both paths 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
Signed-off-by: Hui Peng <benquike@gmail.com>
---
Changes in v2:
- Split from the remote_partition_enable() fix into patch 2/2.
- Check trialcs->exclusive_cpus (rather than effective_xcpus) in
  validate_partition() when !is_partition_valid(cs) so sibling local
  partitions with overlapping cpuset.cpus continue to share remaining
  available CPUs as expected by test_cpuset_prs.sh.
- Add regression tests to tools/testing/selftests/cgroup/test_cpuset_prs.sh.

 kernel/cgroup/cpuset.c                        | 9 ++++++++-
 tools/testing/selftests/cgroup/test_cpuset_prs.sh | 3 +++
 2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 7381fa8502e7..f46a51f07938 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -713,7 +713,10 @@ static inline bool cpus_excl_conflict(struct cpuset *trial, struct cpuset *sibli
 		return true;
 
 	/* Exclusive_cpus cannot intersect */
-	return cpumask_intersects(trial->exclusive_cpus, sibling->exclusive_cpus);
+	return cpumask_intersects(trial->exclusive_cpus,
+				  cpumask_empty(sibling->exclusive_cpus)
+				  ? sibling->effective_xcpus
+				  : sibling->exclusive_cpus);
 }
 
 static inline bool mems_excl_conflict(struct cpuset *cs1, struct cpuset *cs2)
@@ -2415,6 +2418,10 @@ 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..f62155cf6162 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"
+	" X1-3:P1 CX2-3:P2  .      .    C1-3     .      .      .     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 \
@@ -443,6 +445,7 @@ TEST_MATRIX=(
 
 	# Changes to cpuset.cpus.exclusive that violate exclusivity rule is rejected
 	"   C0-3     .      .    C4-5   X0-3     .      .     X3-5   1 A1:0-3|B1:4-5"
+	"  C0-3:P1   .      .    C4-5     .      .      .     X3-5   1 A1:0-3|B1:4-5 A1:P1|B1:P0"
 
 	# cpuset.cpus.exclusive cannot be set to a superset of sibling's cpuset.cpus
 	"   C0-3     .      .    C4-5   X3-5     .      .      .     1 A1:0-3|B1:4-5"
-- 
2.49.0