# CVE-2024-40955: linux: In the Linux kernel, the following vulnerability has been resolved:  ext4: fix slab-out-of-bounds in ext4_mb_find_goo...

CVE-2024-40955 is fixed in linux on 1 platform, first by DEBIAN-CVE-2024-40955, released July 12, 2024. It is not reported as exploited. Patch Radar priority: Routine.

- Web page: https://patchblox.com/security/cve/CVE-2024-40955
- Priority: Routine
- CVSS: 5.5 (medium)
- EPSS: 0.0024
- CISA KEV: not listed
- Exploited (vendor report): no
- Publicly disclosed: no
- Published: Jul 12, 2024

## 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.

## Where it's fixed

| Platform | Product | Fixed in | Fix | Released |
|---|---|---|---|---|
| Linux | linux Debian 13.5 (trixie) | 6.9.7-1 | [DEBIAN-CVE-2024-40955](https://patchblox.com/security/data/fix/linux/debian-cve-2024-40955.md) | Jul 12, 2024 |
| Linux | linux Debian 14 | 6.9.7-1 | [DEBIAN-CVE-2024-40955](https://patchblox.com/security/data/fix/linux/debian-cve-2024-40955.md) | Jul 12, 2024 |

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