[PATCH] cpufreq: cppc_cpufreq: prevent crash on reading freqdomain_cpus

[Chris Hyser <chris.hyser@oracle.com>]

From: chris hyser <chris.hyser@oracle.com>

While running stress-ng --sysfs on an ARM system following a cpu offline,
we encountered the following NULL pointer dereference in the cppc_cpufreq
scaling driver:

[ 1003.576816] Call trace:
[ 1003.579255]  _find_next_bit+0x20/0xc8
[ 1003.582909]  cpufreq_show_cpus+0x78/0xf4
[ 1003.586830]  show_freqdomain_cpus+0x20/0x30 [cppc_cpufreq]
[ 1003.592318]  show+0x4c/0x78
[ 1003.595104]  sysfs_kf_seq_show+0x9

This is the exact issue described in commit e25303676e18 ("cpufreq:
acpi_cpufreq: prevent crash on reading freqdomain_cpus") with the fix
described there also solving this issue. I tracked the root cause to the
following: a scaling driver which provides a struct freq_attr sysfs
attributes array passed via struct cpufreq_driver during driver
registration, has .init() and .exit() functions and does _not_ provide
.online()/.offline() routines. cpufreq core creates the attributes, but
does not remove them even though .exit() frees the underlying memory. The
core functions and most drivers have corresponding NULL data pointer
checks.

Signed-off-by: Chris Hyser <chris.hyser@oracle.com>
---
 drivers/cpufreq/cppc_cpufreq.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/cpufreq/cppc_cpufreq.c b/drivers/cpufreq/cppc_cpufreq.c
index 24eaf0e..4210353 100644
--- a/drivers/cpufreq/cppc_cpufreq.c
+++ b/drivers/cpufreq/cppc_cpufreq.c
@@ -876,6 +876,9 @@ static ssize_t show_freqdomain_cpus(struct cpufreq_policy *policy, char *buf)
 {
 	struct cppc_cpudata *cpu_data = policy->driver_data;
 
+	if (unlikely(!cpu_data))
+		return -ENODEV;
+
 	return cpufreq_show_cpus(cpu_data->shared_cpu_map, buf);
 }
 cpufreq_freq_attr_ro(freqdomain_cpus);
-- 
1.8.3.1