Subject: CVE-2024-40955: ext4: fix slab-out-of-bounds in ext4_mb_find_good_group_avg_frag_lists()

From: Greg Kroah-Hartman <[email-masked]>

Description
===========

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.

The Linux kernel CVE team has assigned CVE-2024-40955 to this issue.

Affected and fixed versions
===========================

	Issue introduced in 6.5 with commit 7e170922f06b and fixed in 6.6.36 with commit 677ff4589f15
	Issue introduced in 6.5 with commit 7e170922f06b and fixed in 6.9.7 with commit b829687ae122
	Issue introduced in 6.5 with commit 7e170922f06b and fixed in 6.10 with commit 13df4d44a3aa

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2024-40955
will be updated if fixes are backported, please check that for the most
up to date information about this issue.

Affected files
==============

The file(s) affected by this issue are:
	fs/ext4/mballoc.c
	fs/ext4/sysfs.c

Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/677ff4589f1501578fa903a25bb14831d0607992
	https://git.kernel.org/stable/c/b829687ae1229224262bcabf49accfa2dbf8db06
	https://git.kernel.org/stable/c/13df4d44a3aaabe61cd01d277b6ee23ead2a5206