sched/debug: Fix memory corruption caused by multiple small reads of flags

Reading /proc/sys/kernel/sched_domain/cpu*/domain0/flags mutliple times
with small reads causes oopses with slub corruption issues because the kfree is
free'ing an offset from a previous allocation. Fix this by adding in a new
pointer 'buf' for the allocation and kfree and use the temporary pointer tmp
to handle memory copies of the buf offsets.

Fixes: 5b9f8ff7 ("sched/debug: Output SD flag names rather than their values")
Reported-by: Jeff Bastian <jbastian@redhat.com>
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
Link: https://lkml.kernel.org/r/20201029151103.373410-1-colin.king@canonical.com

kernel/sched/debug.c

@@ -251,7 +251,7 @@ static int sd_ctl_doflags(struct ctl_table *table, int write,
 	unsigned long flags = *(unsigned long *)table->data;
 	size_t data_size = 0;
 	size_t len = 0;
-	char *tmp;
+	char *tmp, *buf;
 	int idx;
 
 	if (write)
@@ -269,17 +269,17 @@ static int sd_ctl_doflags(struct ctl_table *table, int write,
 		return 0;
 	}
 
-	tmp = kcalloc(data_size + 1, sizeof(*tmp), GFP_KERNEL);
-	if (!tmp)
+	buf = kcalloc(data_size + 1, sizeof(*buf), GFP_KERNEL);
+	if (!buf)
 		return -ENOMEM;
 
 	for_each_set_bit(idx, &flags, __SD_FLAG_CNT) {
 		char *name = sd_flag_debug[idx].name;
 
-		len += snprintf(tmp + len, strlen(name) + 2, "%s ", name);
+		len += snprintf(buf + len, strlen(name) + 2, "%s ", name);
 	}
 
-	tmp += *ppos;
+	tmp = buf + *ppos;
 	len -= *ppos;
 
 	if (len > *lenp)
@@ -294,7 +294,7 @@ static int sd_ctl_doflags(struct ctl_table *table, int write,
 	*lenp = len;
 	*ppos += len;
 
-	kfree(tmp);
+	kfree(buf);
 
 	return 0;
 }