On 2019/8/15 17:35, Jan Kara Wrote: > On Thu 15-08-19 16:16:31, 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. >> >> Signed-off-by: zhangyi (F) <yi.zhang@xxxxxxxxxx> >> --- >> Changes since v3: >> - add comments before ext4_setup_system_zone() and >> ext4_release_system_zone() to explain why we need to serializes update >> sbi->system_blks pointer. >> - Fix block validity checking logic changes in v3. > > Thanks for the patch! The patch looks good. Just some language fixes in the > new comments below. You can add: > > Reviewed-by: Jan Kara <jack@xxxxxxx> > >> +/* >> + * Build system zone rbtree which is used for block validity checking. >> + * >> + * Note that system_blks pointer should be serializes updated at remount >> + * time even under sb->s_umount semaphore protection, due to it can be >> + * racing with ext4_data_block_valid() reading the system_blks rbtree at >> + * the same time. > > I'd rephrase this paragraph a bit to be easier to understand: > > The update of system_blks pointer in this function is protected by > sb->s_umount semaphore. However we have to be careful as we can be racing > with ext4_data_block_valid() calls reading system_blks rbtree protected > only by RCU. That's why we first build the rbtree and then swap it in place. > >> -/* Called when the filesystem is unmounted */ >> +/* >> + * Called when the filesystem is unmounted or when remounting it with >> + * noblock_validity specified. >> + * >> + * Note that system_blks pointer should be serializes updated and do >> + * the actual freeing after the RCU grace period at remount time even >> + * under sb->s_umount semaphore protection, due to it can be racing with >> + * ext4_data_block_valid() reading the system_blks rbtree at the same >> + * time. >> + */ > > Similarly here I'd phrase the last paragraph as: > > The update of system_blks pointer in this function is protected by > sb->s_umount semaphore. However we have to be careful as we can be racing > with ext4_data_block_valid() calls reading system_blks rbtree protected > only by RCU. So we first clear the system_blks pointer and then free the > rbtree only after RCU grace period expires. > Yes, it looks better, will do. Thanks, Yi.
