CVE-2024-40955 in Linux
Summary
by MITRE • 07/12/2024
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.
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Analysis
by VulDB Data Team • 08/09/2025
The vulnerability described in CVE-2024-40955 affects the Linux kernel's ext4 file system implementation and represents a slab-out-of-bounds read condition within the memory management subsystem. This flaw occurs in the ext4_mb_find_good_group_avg_frag_lists function, which is part of the block allocation algorithm responsible for managing free space in ext4 file systems. The issue manifests when a malicious actor or misconfigured system sets the mb_group_prealloc parameter to an excessively large value, specifically 2147483647, which exceeds the bounds of the internal data structures used for fragment list management.
The technical root cause lies in the improper validation of user-supplied input through the sysfs interface. When the s_mb_group_prealloc parameter is set to a value that results in a fragment list index calculation exceeding the allocated memory boundaries, the kernel attempts to access memory outside the intended slab allocation. The call trace demonstrates that this occurs during the block allocation process, specifically when the kernel tries to determine the best available group for allocation. The function mb_avg_fragment_size_order calculates an order value of 29 based on the fragment length, but the s_mb_avg_fragment_size array only contains 14 elements, leading to the out-of-bounds memory access. This vulnerability aligns with CWE-129, which covers improper validation of array indices, and CWE-787, which addresses out-of-bounds write operations.
The operational impact of this vulnerability is significant as it can lead to system instability, potential denial of service conditions, or even privilege escalation in certain scenarios. The vulnerability can be triggered through a simple sequence of commands involving filesystem creation, mounting, and sysfs parameter manipulation, making it accessible to attackers with basic system access. The KASAN (Kernel Address Sanitizer) report confirms that the out-of-bounds read occurs at a specific memory address, indicating that an attacker could potentially exploit this to cause kernel memory corruption or to gain unauthorized access to kernel data structures. The memory corruption could result in system crashes or allow for more sophisticated attacks that leverage the corrupted kernel memory state.
The mitigation strategy involves implementing proper input validation for the mb_group_prealloc attribute by introducing a new attribute type attr_clusters_in_group that restricts values to the valid range [0, sbi->s_clusters_per_group]. This approach ensures that the parameter values remain within the bounds of the allocated data structures. Additionally, the fix prevents the function mb_avg_fragment_size_order from returning order values greater than MB_NUM_ORDERS(sb), effectively limiting the maximum index that can be accessed in the fragment size lists. These changes align with ATT&CK technique T1068, which involves exploiting vulnerabilities in the kernel, and T1211, which covers privilege escalation through kernel exploits. The fix also reduces unnecessary loops in the allocation algorithm, improving both security and performance. This remediation approach follows the principle of least privilege and input validation, ensuring that all external inputs are properly validated before being used in kernel memory operations. The solution prevents the out-of-bounds access by constraining the parameter values to prevent calculation of invalid array indices, thereby maintaining the integrity of kernel memory management structures.