Hi! On Mon 22-03-21 23:24:23, Zhang Yi wrote: > We find a use after free problem when CONFIG_QUOTA is enabled, the detail of > this problem is below. > > mount_bdev() > ext4_fill_super() > sb->s_root = d_make_root(root); > ext4_orphan_cleanup() > sb->s_flags |= SB_ACTIVE; <--- 1. mark sb active > ext4_orphan_get() > ext4_truncate() > ext4_block_truncate_page() > mark_buffer_dirty <--- 2. dirty inode > iput() > iput_final <--- 3. put into lru list > ext4_mark_recovery_complete <--- 4. failed and return error > sb->s_root = NULL; > deactivate_locked_super() > kill_block_super() > generic_shutdown_super() > <--- 5. did not evict_inodes > put_super() > __put_super() > <--- 6. put super block > > Because of the truncated inodes was dirty and will write them back later, it > will trigger use after free problem. Now the question is why we need to set > SB_ACTIVE bit when enable CONFIG_QUOTA below? > > #ifdef CONFIG_QUOTA > /* Needed for iput() to work correctly and not trash data */ > sb->s_flags |= SB_ACTIVE; > > This code was merged long long ago in v2.6.6, IIUC, it may not affect > the quota statistics it we evict inode directly in the last iput. > In order to slove this UAF problem, I'm not sure is there any side effect > if we just remove this code, or remove SB_ACTIVE and call evict_inodes() > in the error path of ext4_fill_super(). > > Could you give some suggestions? That's a very good question. I do remember that I've added this code back then because otherwise orphan cleanup was loosing updates to quota files. But you're right that now I don't see how that could be happening and it would be nice if we could get rid of this hack (and even better if it also fixes the problem you've found). I guess I'll just try and test this change with various quota configurations to see whether something still breaks or not. Thanks report! Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
