[PATCH] f2fs: Add mutex to prevent gc task from being accessed before initialization

[Edward Adam Davis]

syzbot report null-ptr-deref Write in f2fs_stop_gc_thread, because 
shutdown and remount can cause multi-path entry into f2fs_stop_gc_thread,
resulting in incorrect access to f2fs_gc_task before it is properly
initialized.

Reported-and-tested-by: syzbot+1a8e2b31f2ac9bd3d148@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=1a8e2b31f2ac9bd3d148
Signed-off-by: Edward Adam Davis <eadavis@xxxxxx>
---
 fs/f2fs/gc.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/fs/f2fs/gc.c b/fs/f2fs/gc.c
index 6066c6eecf41..a624e8271fbc 100644
--- a/fs/f2fs/gc.c
+++ b/fs/f2fs/gc.c
@@ -27,6 +27,7 @@ static struct kmem_cache *victim_entry_slab;
 
 static unsigned int count_bits(const unsigned long *addr,
 				unsigned int offset, unsigned int len);
+static DEFINE_MUTEX(gc_lock);
 
 static int gc_thread_func(void *data)
 {
@@ -203,10 +204,13 @@ int f2fs_start_gc_thread(struct f2fs_sb_info *sbi)
 
 void f2fs_stop_gc_thread(struct f2fs_sb_info *sbi)
 {
-	struct f2fs_gc_kthread *gc_th = sbi->gc_thread;
+	struct f2fs_gc_kthread *gc_th;
 
-	if (!gc_th)
+	guard(mutex)(&gc_lock);
+	gc_th = sbi->gc_thread;
+	if (!gc_th || gc_th->f2fs_gc_task < PAGE_OFFSET)
 		return;
+
 	kthread_stop(gc_th->f2fs_gc_task);
 	wake_up_all(&gc_th->fggc_wq);
 	kfree(gc_th);
-- 
2.43.0