• zhangyi (F)'s avatar
    ext4: fix potential use after free after remounting with noblock_validity · 7727ae52
    zhangyi (F) authored
    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@syzkaller.appspotmail.com
    Signed-off-by: default avatarzhangyi (F) <yi.zhang@huawei.com>
    Signed-off-by: default avatarTheodore Ts'o <tytso@mit.edu>
    Reviewed-by: default avatarJan Kara <jack@suse.cz>
    7727ae52
block_validity.c 9.86 KB