powerpc/powernv/memtrace: Fix crashing the kernel when enabling concu…

…rrently

commit d671894 upstream.

It's very easy to crash the kernel right now by simply trying to
enable memtrace concurrently, hammering on the "enable" interface

loop.sh:
  #!/bin/bash

  dmesg --console-off

  while true; do
          echo 0x40000000 > /sys/kernel/debug/powerpc/memtrace/enable
  done

[root@localhost ~]# loop.sh &
[root@localhost ~]# loop.sh &

Resulting quickly in a kernel crash. Let's properly protect using a
mutex.

Fixes: 9d5171a ("powerpc/powernv: Enable removal of memory for in memory tracing")
Cc: stable@vger.kernel.org# v4.14+
Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201111145322.15793-3-david@redhat.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

arch/powerpc/platforms/powernv/memtrace.c

@@ -30,6 +30,7 @@ struct memtrace_entry {
 	char name[16];
 };
 
+static DEFINE_MUTEX(memtrace_mutex);
 static u64 memtrace_size;
 
 static struct memtrace_entry *memtrace_array;
@@ -290,6 +291,7 @@ static int memtrace_online(void)
 
 static int memtrace_enable_set(void *data, u64 val)
 {
+	int rc = -EAGAIN;
 	u64 bytes;
 
 	/*
@@ -302,25 +304,31 @@ static int memtrace_enable_set(void *data, u64 val)
 		return -EINVAL;
 	}
 
+	mutex_lock(&memtrace_mutex);
+
 	/* Re-add/online previously removed/offlined memory */
 	if (memtrace_size) {
 		if (memtrace_online())
-			return -EAGAIN;
+			goto out_unlock;
 	}
 
-	if (!val)
-		return 0;
+	if (!val) {
+		rc = 0;
+		goto out_unlock;
+	}
 
 	/* Offline and remove memory */
 	if (memtrace_init_regions_runtime(val))
-		return -EINVAL;
+		goto out_unlock;
 
 	if (memtrace_init_debugfs())
-		return -EINVAL;
+		goto out_unlock;
 
 	memtrace_size = val;
-
-	return 0;
+	rc = 0;
+out_unlock:
+	mutex_unlock(&memtrace_mutex);
+	return rc;
 }
 
 static int memtrace_enable_get(void *data, u64 *val)