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

2024-07-1212:31:58

Linux

www.cve.org

linux kernel

ext4

vulnerability fix

slab-out-of-bounds

sysfs

kasan

memory

block size

attribute fix

EPSS

Percentile

5.1%

JSON

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 &gt; /sys/fs/ext4/$disk/mb_group_prealloc
echo test &gt; /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.

CNA Affected

[
  {
    "product": "Linux",
    "vendor": "Linux",
    "defaultStatus": "unaffected",
    "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
    "programFiles": [
      "fs/ext4/mballoc.c",
      "fs/ext4/sysfs.c"
    ],
    "versions": [
      {
        "version": "7e170922f06b",
        "lessThan": "677ff4589f15",
        "status": "affected",
        "versionType": "git"
      },
      {
        "version": "7e170922f06b",
        "lessThan": "b829687ae122",
        "status": "affected",
        "versionType": "git"
      },
      {
        "version": "7e170922f06b",
        "lessThan": "13df4d44a3aa",
        "status": "affected",
        "versionType": "git"
      }
    ]
  },
  {
    "product": "Linux",
    "vendor": "Linux",
    "defaultStatus": "affected",
    "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
    "programFiles": [
      "fs/ext4/mballoc.c",
      "fs/ext4/sysfs.c"
    ],
    "versions": [
      {
        "version": "6.5",
        "status": "affected"
      },
      {
        "version": "0",
        "lessThan": "6.5",
        "status": "unaffected",
        "versionType": "custom"
      },
      {
        "version": "6.6.36",
        "lessThanOrEqual": "6.6.*",
        "status": "unaffected",
        "versionType": "custom"
      },
      {
        "version": "6.9.7",
        "lessThanOrEqual": "6.9.*",
        "status": "unaffected",
        "versionType": "custom"
      },
      {
        "version": "6.10",
        "lessThanOrEqual": "*",
        "status": "unaffected",
        "versionType": "original_commit_for_fix"
      }
    ]
  }
]