[PATCH 5.15.y] s390/cpum_cf: Check session mask before cpu offline

[Sasha Levin <sashal@kernel.org>]

From: Sumanth Korikkar <sumanthk@linux.ibm.com>

[ Upstream commit 99a95985dfc85eecad97f0991a2401d2fcef294c ]

cfset_offline_cpu() stops counter sets for every active session without
checking whether the outgoing cpu belongs to the session mask.

For example, run an hwctr session on cpu0 and a cpu bound perf event on
cpu1:

lshwc -l 9999 -i 5 0:b
perf stat -C 1 -e cpum_cf/CPU_CYCLES/ sleep 120
chcpu -d 1
chcpu -e 1

Although the hwctr session does not include cpu1, cfset_offline_cpu()
decrements its counter set reference count. The subsequent perf teardown
decrements it again, causing it to underflow.

The perf event keeps the percpu state alive across hotplug. When cpu1
comes online, the hwctr session is started there and the reference count
moves from -1 to 0. Stopping the hwctr session then decrements it to -1
again, so the zero triggered counter set disable path is skipped.

Check the session mask before stopping counter sets on the outgoing cpu.
This prevents the unmatched decrement and the resulting reference count
underflow.

Fixes: a029a4eab39e ("s390/cpumf: Allow concurrent access for CPU Measurement Counter Facility")
Reported-by: Christian Borntraeger <borntraeger@linux.ibm.com>
Cc: stable@vger.kernel.org # v5.14+
Signed-off-by: Sumanth Korikkar <sumanthk@linux.ibm.com>
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
[ sashal: adapted the per-session CPU-mask check to the existing if condition for the single global cfset_request. ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 arch/s390/kernel/perf_cpum_cf.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/s390/kernel/perf_cpum_cf.c b/arch/s390/kernel/perf_cpum_cf.c
index d2a2a18b55808..f7b270d2404cc 100644
--- a/arch/s390/kernel/perf_cpum_cf.c
+++ b/arch/s390/kernel/perf_cpum_cf.c
@@ -1223,7 +1223,8 @@ int cfset_offline_cpu(unsigned int cpu)
 	struct cfset_call_on_cpu_parm p;
 
 	mutex_lock(&cfset_ctrset_mutex);
-	if (cfset_request.ctrset) {
+	if (cfset_request.ctrset &&
+	    cpumask_test_cpu(cpu, &cfset_request.mask)) {
 		p.sets = cfset_request.ctrset;
 		cfset_ioctl_off(&p);
 		cpumask_clear_cpu(cpu, &cfset_request.mask);
-- 
2.53.0