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.

CC: [e-mail]
Link: https://bugzilla.kernel.org/show_bug.cgi?id=216540
Signed-off-by: Luís Henriques <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Theodore Ts'o <[e-mail]>

fs/ext4/namei.c

···
 	memset(de, 0, len); /* wipe old data */
 	de = (struct ext4_dir_entry_2 *) data2;
 	top = data2 + len;
-	while ((char *)(de2 = ext4_next_entry(de, blocksize)) < top)
+	while ((char *)(de2 = ext4_next_entry(de, blocksize)) < top) {
+		if (ext4_check_dir_entry(dir, NULL, de, bh2, data2, len,
+					 (data2 + (blocksize - csum_size) -
+					  (char *) de))) {
+			brelse(bh2);
+			brelse(bh);
+			return -EFSCORRUPTED;
+		}
 		de = de2;
+	}
 	de->rec_len = ext4_rec_len_to_disk(data2 + (blocksize - csum_size) -
 					   (char *) de, blocksize);