fs: Handle multiply claimed blocks more gracefully with mmb

When a metadata block is referenced by multiple inodes and tracked by
metadata bh infrastructure (which is forbidden and generally indicates
filesystem corruption), it can happen that mmb_mark_buffer_dirty() is
called for two different mmb structures in parallel. This can lead to a
corruption of mmb linked list. Handle that situation gracefully (at
least from mmb POV) by serializing on setting bh->b_mmb.

Reported-by: Ruikai Peng <[email protected]>
Signed-off-by: Jan Kara <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Christian Brauner <[email protected]>

Author: jankara

fs/buffer.c

@@ -719,8 +719,15 @@ void mmb_mark_buffer_dirty(struct buffer_head *bh,
 	mark_buffer_dirty(bh);
 	if (!bh->b_mmb) {
 		spin_lock(&mmb->lock);
+		/*
+		 * For a corrupted filesystem with multiply claimed blocks this
+		 * can fail. Avoid corrupting the linked list in that case.
+		 */
+		if (cmpxchg(&bh->b_mmb, NULL, mmb) != NULL) {
+			spin_unlock(&mmb->lock);
+			return;
+		}
 		list_move_tail(&bh->b_assoc_buffers, &mmb->list);
-		bh->b_mmb = mmb;
 		spin_unlock(&mmb->lock);
 	}
 }