ext4: fix BUG_ON() when directory entry has invalid rec_len

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

ext4: fix BUG_ON() when directory entry has invalid rec_len

The rec_len field in the directory entry has to be a multiple of 4. A

corrupted filesystem image can be used to hit a BUG() in

ext4_rec_len_to_disk(), called from make_indexed_dir().

------------[ cut here ]------------

kernel BUG at fs/ext4/ext4.h:2413!

...

RIP: 0010:make_indexed_dir+0x53f/0x5f0

...

Call Trace:

<TASK>

? add_dirent_to_buf+0x1b2/0x200

ext4_add_entry+0x36e/0x480

ext4_add_nondir+0x2b/0xc0

ext4_create+0x163/0x200

path_openat+0x635/0xe90

do_filp_open+0xb4/0x160

? __create_object.isra.0+0x1de/0x3b0

? _raw_spin_unlock+0x12/0x30

do_sys_openat2+0x91/0x150

__x64_sys_open+0x6c/0xa0

do_syscall_64+0x3c/0x80

entry_SYSCALL_64_after_hwframe+0x46/0xb0

The fix simply adds a call to ext4_check_dir_entry() to validate the

directory entry, returning -EFSCORRUPTED if the entry is invalid.