ext4: fix slab-out-of-bounds in ext4_mb_find_good_group_avg_frag_lists()

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

ext4: fix slab-out-of-bounds in ext4_mb_find_good_group_avg_frag_lists()

We can trigger a slab-out-of-bounds with the following commands:

mkfs.ext4 -F /dev/$disk 10G

mount /dev/$disk /tmp/test

echo 2147483647 > /sys/fs/ext4/$disk/mb_group_prealloc

echo test > /tmp/test/file && sync

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

BUG: KASAN: slab-out-of-bounds in ext4_mb_find_good_group_avg_frag_lists+0x8a/0x200 [ext4]

Read of size 8 at addr ffff888121b9d0f0 by task kworker/u2:0/11

CPU: 0 PID: 11 Comm: kworker/u2:0 Tainted: GL 6.7.0-next-20240118 #521

Call Trace:

dump_stack_lvl+0x2c/0x50

kasan_report+0xb6/0xf0

ext4_mb_find_good_group_avg_frag_lists+0x8a/0x200 [ext4]

ext4_mb_regular_allocator+0x19e9/0x2370 [ext4]

ext4_mb_new_blocks+0x88a/0x1370 [ext4]

ext4_ext_map_blocks+0x14f7/0x2390 [ext4]

ext4_map_blocks+0x569/0xea0 [ext4]

ext4_do_writepages+0x10f6/0x1bc0 [ext4]

[...]

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

The flow of issue triggering is as follows:

// Set s_mb_group_prealloc to 2147483647 via sysfs

ext4_mb_new_blocks

ext4_mb_normalize_request

ext4_mb_normalize_group_request

ac->ac_g_ex.fe_len = EXT4_SB(sb)->s_mb_group_prealloc

ext4_mb_regular_allocator

ext4_mb_choose_next_group

ext4_mb_choose_next_group_best_avail

mb_avg_fragment_size_order

order = fls(len) - 2 = 29

ext4_mb_find_good_group_avg_frag_lists

frag_list = &sbi->s_mb_avg_fragment_size[order]

if (list_empty(frag_list)) // Trigger SOOB!

At 4k block size, the length of the s_mb_avg_fragment_size list is 14,

but an oversized s_mb_group_prealloc is set, causing slab-out-of-bounds

to be triggered by an attempt to access an element at index 29.

Add a new attr_id attr_clusters_in_group with values in the range

[0, sbi->s_clusters_per_group] and declare mb_group_prealloc as

that type to fix the issue. In addition avoid returning an order

from mb_avg_fragment_size_order() greater than MB_NUM_ORDERS(sb)

and reduce some useless loops.