mm/damon/dbgfs: protect targets destructions with kdamond_lock

commit 3479641 upstream.

DAMON debugfs interface iterates current monitoring targets in
'dbgfs_target_ids_read()' while holding the corresponding
'kdamond_lock'.  However, it also destructs the monitoring targets in
'dbgfs_before_terminate()' without holding the lock.  This can result in
a use_after_free bug.  This commit avoids the race by protecting the
destruction with the corresponding 'kdamond_lock'.

Link: https://lkml.kernel.org/r/[email protected]
Reported-by: Sangwoo Bae <[email protected]>
Fixes: 4bc0595 ("mm/damon: implement a debugfs-based user space interface")
Signed-off-by: SeongJae Park <[email protected]>
Cc: <[email protected]>	[5.15.x]
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

mm/damon/dbgfs.c

@@ -309,10 +309,12 @@ static int dbgfs_before_terminate(struct damon_ctx *ctx)
 	if (!targetid_is_pid(ctx))
 		return 0;
 
+	mutex_lock(&ctx->kdamond_lock);
 	damon_for_each_target_safe(t, next, ctx) {
 		put_pid((struct pid *)t->id);
 		damon_destroy_target(t);
 	}
+	mutex_unlock(&ctx->kdamond_lock);
 	return 0;
 }