CVE-2024-49882 in Linux
Summary
by MITRE • 10/21/2024
In the Linux kernel, the following vulnerability has been resolved:
ext4: fix double brelse() the buffer of the extents path
In ext4_ext_try_to_merge_up(), set path[1].p_bh to NULL after it has been
released, otherwise it may be released twice. An example of what triggers this is as follows:
split2 map split1 |--------|-------|--------|
ext4_ext_map_blocks ext4_ext_handle_unwritten_extents ext4_split_convert_extents // path->p_depth == 0 ext4_split_extent // 1. do split1 ext4_split_extent_at |ext4_ext_insert_extent | ext4_ext_create_new_leaf | ext4_ext_grow_indepth | le16_add_cpu(&neh->eh_depth, 1) | ext4_find_extent | // return -ENOMEM |// get error and try zeroout |path = ext4_find_extent | path->p_depth = 1 |ext4_ext_try_to_merge | ext4_ext_try_to_merge_up | path->p_depth = 0 | brelse(path[1].p_bh) ---> not set to NULL here
|// zeroout success // 2. update path ext4_find_extent // 3. do split2 ext4_split_extent_at ext4_ext_insert_extent ext4_ext_create_new_leaf ext4_ext_grow_indepth le16_add_cpu(&neh->eh_depth, 1) ext4_find_extent path[0].p_bh = NULL;
path->p_depth = 1 read_extent_tree_block ---> return err // path[1].p_bh is still the old value
ext4_free_ext_path ext4_ext_drop_refs // path->p_depth == 1 brelse(path[1].p_bh) ---> brelse a buffer twice
Finally got the following WARRNING when removing the buffer from lru:
============================================ VFS: brelse: Trying to free free buffer WARNING: CPU: 2 PID: 72 at fs/buffer.c:1241 __brelse+0x58/0x90 CPU: 2 PID: 72 Comm: kworker/u19:1 Not tainted 6.9.0-dirty #716 RIP: 0010:__brelse+0x58/0x90 Call Trace: __find_get_block+0x6e7/0x810 bdev_getblk+0x2b/0x480 __ext4_get_inode_loc+0x48a/0x1240 ext4_get_inode_loc+0xb2/0x150 ext4_reserve_inode_write+0xb7/0x230 __ext4_mark_inode_dirty+0x144/0x6a0 ext4_ext_insert_extent+0x9c8/0x3230 ext4_ext_map_blocks+0xf45/0x2dc0 ext4_map_blocks+0x724/0x1700 ext4_do_writepages+0x12d6/0x2a70 [...]
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Analysis
by VulDB Data Team • 01/19/2026
The vulnerability identified as CVE-2024-49882 resides within the Linux kernel's ext4 file system implementation and specifically affects the buffer management mechanisms during extent tree operations. This issue manifests as a double brelse() operation, which occurs when a buffer head reference is released twice, leading to potential system instability and memory corruption. The flaw is particularly significant because it operates at the kernel level where improper memory handling can result in system crashes or security vulnerabilities. The vulnerability is categorized under CWE-415 as an improper handling of memory allocation and deallocation, and it aligns with ATT&CK technique T1068 which involves privilege escalation through kernel exploits. The issue stems from the ext4_ext_try_to_merge_up() function where buffer head references are not properly cleared after being released, creating a scenario where the same buffer can be freed multiple times.
The technical root cause of this vulnerability involves the improper management of extent path buffer references during complex extent tree modifications. When the ext4 file system encounters conditions requiring extent splitting and merging operations, it maintains a path structure that tracks buffer head references through various levels of the extent tree. During the execution of ext4_split_extent_at() and related functions, the code path attempts to split extents at different points in the tree, which requires creating new leaf nodes and updating the extent tree structure. The problem occurs when ext4_ext_try_to_merge_up() releases a buffer head reference at path[1].p_bh without setting it to NULL, leaving a dangling reference that persists through subsequent operations. This particular sequence of operations is triggered during memory allocation failures and recovery attempts, where the system tries to zero out extent blocks but encounters errors that lead to an inconsistent state. The vulnerability is exacerbated by the fact that the system continues processing even after encountering allocation failures, leading to a cascade of operations that eventually results in the double release of the same buffer head.
The operational impact of this vulnerability extends beyond simple system crashes to potentially enable privilege escalation and denial of service conditions within the Linux kernel environment. When the double brelse() operation occurs, it triggers kernel warnings that indicate attempts to free already freed buffers, which can corrupt kernel memory structures and lead to unpredictable behavior. The warning message indicates that the system is attempting to free a buffer that has already been released, which is a critical error in kernel memory management. This type of vulnerability can be exploited by malicious actors to cause system instability, potentially leading to complete system crashes or allowing for privilege escalation attacks that could compromise the integrity of the entire system. The vulnerability affects systems running Linux kernel versions where the ext4 file system is actively used for storage operations, particularly in environments where extensive file system operations and memory allocation failures are common. The attack surface is broad as any application or system process that performs extent tree operations within ext4 file systems could potentially trigger this condition.
Mitigation strategies for CVE-2024-49882 should focus on both immediate patching and system hardening approaches. The primary resolution involves applying the kernel patch that ensures path[1].p_bh is set to NULL after buffer release operations in the ext4_ext_try_to_merge_up() function, preventing the double release condition. System administrators should prioritize updating to kernel versions that contain the fix, typically those including the specific commit that addresses the buffer management issue. Additionally, monitoring systems should be implemented to detect and alert on buffer management warnings that might indicate this vulnerability is being exploited. The patch addresses the root cause by ensuring proper reference management during extent tree operations and prevents the cascading errors that lead to double buffer releases. Organizations should also implement regular kernel updates and vulnerability scanning procedures to maintain system integrity and prevent exploitation of similar buffer management issues. Security teams should consider implementing runtime protections and anomaly detection systems that can identify unusual buffer management patterns that might indicate exploitation attempts. The fix aligns with security best practices by ensuring proper resource management and preventing common memory corruption vulnerabilities that could be leveraged for privilege escalation attacks.