CVE-2024-42305

In the Linux kernel, the following vulnerability has been resolved:

ext4: check dot and dotdot of dx_root before making dir indexed

Syzbot reports a issue as follows:

============================================

BUG: unable to handle page fault for address: ffffed11022e24fe

PGD 23ffee067 P4D 23ffee067 PUD 0

Oops: Oops: 0000 [#1] PREEMPT SMP KASAN PTI

CPU: 0 PID: 5079 Comm: syz-executor306 Not tainted 6.10.0-rc5-g55027e689933 #0

Call 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 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 to 1.

Therefore add the ext4_check_dx_root() helper function to add more sanity

checks to dot and dotdot before starting the conversion to avoid the above

issue.