On Tue, Aug 27, 2019 at 08:08:39PM +0800, zhangyi (F) wrote: > Remount process will release system zone which was allocated before if > "noblock_validity" is specified. If we mount an ext4 file system to two > mountpoints with default mount options, and then remount one of them > with "noblock_validity", it may trigger a use after free problem when > someone accessing the other one. > > # mount /dev/sda foo > # mount /dev/sda bar > > User access mountpoint "foo" | Remount mountpoint "bar" > | > ext4_map_blocks() | ext4_remount() > check_block_validity() | ext4_setup_system_zone() > ext4_data_block_valid() | ext4_release_system_zone() > | free system_blks rb nodes > access system_blks rb nodes | > trigger use after free | > > This problem can also be reproduced by one mountpint, At the same time, > add_system_zone() can get called during remount as well so there can be > racing ext4_data_block_valid() reading the rbtree at the same time. > > This patch add RCU to protect system zone from releasing or building > when doing a remount which inverse current "noblock_validity" mount > option. It assign the rbtree after the whole tree was complete and > do actual freeing after rcu grace period, avoid any intermediate state. > > Reported-by: syzbot+1e470567330b7ad711d5@xxxxxxxxxxxxxxxxxxxxxxxxx > Signed-off-by: zhangyi (F) <yi.zhang@xxxxxxxxxx> > Signed-off-by: Theodore Ts'o <tytso@xxxxxxx> > Reviewed-by: Jan Kara <jack@xxxxxxx> Applied, thanks! - Ted
