psi: allow unprivileged users with CAP_SYS_RESOURCE to write psi files

Currently only root can write files under /proc/pressure. Relax this to
allow tasks running as unprivileged users with CAP_SYS_RESOURCE to be
able to write to these files.
Signed-off-by: Josh Hunt <johunt@akamai.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Link: https://lkml.kernel.org/r/20210402025833.27599-1-johunt@akamai.com

kernel/sched/psi.c

@@ -1061,19 +1061,27 @@ static int psi_cpu_show(struct seq_file *m, void *v)
 	return psi_show(m, &psi_system, PSI_CPU);
 }
 
+static int psi_open(struct file *file, int (*psi_show)(struct seq_file *, void *))
+{
+	if (file->f_mode & FMODE_WRITE && !capable(CAP_SYS_RESOURCE))
+		return -EPERM;
+
+	return single_open(file, psi_show, NULL);
+}
+
 static int psi_io_open(struct inode *inode, struct file *file)
 {
-	return single_open(file, psi_io_show, NULL);
+	return psi_open(file, psi_io_show);
 }
 
 static int psi_memory_open(struct inode *inode, struct file *file)
 {
-	return single_open(file, psi_memory_show, NULL);
+	return psi_open(file, psi_memory_show);
 }
 
 static int psi_cpu_open(struct inode *inode, struct file *file)
 {
-	return single_open(file, psi_cpu_show, NULL);
+	return psi_open(file, psi_cpu_show);
 }
 
 struct psi_trigger *psi_trigger_create(struct psi_group *group,
@@ -1353,9 +1361,9 @@ static int __init psi_proc_init(void)
 {
 	if (psi_enable) {
 		proc_mkdir("pressure", NULL);
-		proc_create("pressure/io", 0, NULL, &psi_io_proc_ops);
-		proc_create("pressure/memory", 0, NULL, &psi_memory_proc_ops);
-		proc_create("pressure/cpu", 0, NULL, &psi_cpu_proc_ops);
+		proc_create("pressure/io", 0666, NULL, &psi_io_proc_ops);
+		proc_create("pressure/memory", 0666, NULL, &psi_memory_proc_ops);
+		proc_create("pressure/cpu", 0666, NULL, &psi_cpu_proc_ops);
 	}
 	return 0;
 }