In the Linux kernel, the following vulnerability has been resolved:
ext4: check dot and dotdot of dx_root before making dir indexed
The immediate cause of this problem is that there is only one valid dentry for the block to be split during do_split, so split==0 results in out of bounds accesses to the map triggering the issue.
do_split
unsigned split
dx_make_map
count = 1
split = count/2 = 0;
continued = hash2 == map[split - 1].hash;
---> map[4294967295]
The maximum length of a filename is 255 and the minimum block size is 1024, so it is always guaranteed that the number of entries is greater than or equal to 2 when do_split() is called.
But syzbot's crafted image has no dot and dotdot in dir, and the dentry distribution in dirblock is as follows:
bus dentry1 hole dentry2 free |xx--|xx-------------|...............|xx-------------|...............| 0 12 (8+248)=256 268 256 524 (8+256)=264 788 236 1024
So when renaming dentry1 increases its name_len length by 1, neither hole nor free is sufficient to hold the new dentry, and make_indexed_dir() is called.
In make_indexed_dir() it is assumed that the first two entries of the dirblock must be dot and dotdot, so bus and dentry1 are left in dx_root because they are treated as dot and dotdot, and only dentry2 is moved to the new leaf block. That's why count is equal...