In the Linux kernel, the following vulnerability has been resolved:ext4: check dot and dotdot of dx_root before making dir indexedSyzbot reports a issue as follows:============================================BUG: unable to handle page fault for address: ffffed11022e24fePGD 23ffee067 P4D 23ffee067 PUD 0Oops: Oops: 0000 [#1] PREEMPT SMP KASAN PTICPU: 0 PID: 5079 Comm: syz-executor306 Not tainted 6.10.0-rc5-g55027e689933 #0Call Trace: <TASK> make_indexed_dir+0xdaf/0x13c0 fs/ext4/namei.c:2341 ext4_add_entry+0x222a/0x25d0 fs/ext4/namei.c:2451 ext4_rename fs/ext4/namei.c:3936 [inline] ext4_rename2+0x26e5/0x4370 fs/ext4/namei.c:4214[...]============================================The immediate cause of this problem is that there is only one valid dentryfor the block to be split during do_split, so split==0 results in out ofbounds 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 orequal to 2 when do_split() is called.But syzbot's crafted image has no dot and dotdot in dir, and the dentrydistribution 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 1024So when renaming dentry1 increases its name_len length by 1, neither holenor free is sufficient to hold the new dentry, and make_indexed_dir() iscalled.In make_indexed_dir() it is assumed that the first two entries of thedirblock must be dot and dotdot, so bus and dentry1 are left in dx_rootbecause they are treated as dot and dotdot, and only dentry2 is movedto the new leaf block. That's why count is equal to 1.Therefore add the ext4_check_dx_root() helper function to add more sanitychecks to dot and dotdot before starting the conversion to avoid the aboveissue.
