CVE-2022-48988 in Linux
Summary
by MITRE • 10/21/2024
In the Linux kernel, the following vulnerability has been resolved:
memcg: fix possible use-after-free in memcg_write_event_control()
memcg_write_event_control() accesses the dentry->d_name of the specified control fd to route the write call. As a cgroup interface file can't be renamed, it's safe to access d_name as long as the specified file is a regular cgroup file. Also, as these cgroup interface files can't be removed before the directory, it's safe to access the parent too.
Prior to 347c4a874710 ("memcg: remove cgroup_event->cft"), there was a call to __file_cft() which verified that the specified file is a regular cgroupfs file before further accesses. The cftype pointer returned from __file_cft() was no longer necessary and the commit inadvertently dropped the file type check with it allowing any file to slip through. With the invarients broken, the d_name and parent accesses can now race against renames and removals of arbitrary files and cause use-after-free's.
Fix the bug by resurrecting the file type check in __file_cft(). Now that cgroupfs is implemented through kernfs, checking the file operations needs to go through a layer of indirection. Instead, let's check the superblock and dentry type.
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Analysis
by VulDB Data Team • 01/18/2026
The vulnerability described in CVE-2022-48988 represents a critical use-after-free condition within the Linux kernel's memory control group implementation. This flaw exists in the memcg_write_event_control() function which handles control file operations for memory cgroup interfaces. The vulnerability arises from a regression introduced in kernel commit 347c4a874710 that inadvertently removed crucial file type validation checks. Prior to this change, the __file_cft() function performed essential verification to ensure that only legitimate cgroup filesystem files could be processed, thereby preventing access to arbitrary file descriptors that might be subject to race conditions during rename or removal operations.
The technical flaw stems from the improper handling of file descriptor validation within the memory cgroup subsystem. When memcg_write_event_control() processes write operations, it attempts to access the dentry->d_name field of the control file descriptor to determine routing behavior for the write call. The original implementation included a safety check through __file_cft() that verified the file was a proper regular cgroup filesystem file before proceeding with subsequent accesses. This validation mechanism was removed during the aforementioned kernel commit, allowing any file descriptor to potentially pass through the validation layer. The absence of this check creates a race condition where the dentry and parent directory accesses can occur simultaneously with file rename or removal operations, leading to memory corruption through use-after-free scenarios.
The operational impact of this vulnerability extends beyond simple memory corruption, potentially enabling privilege escalation and system instability. Attackers could exploit this condition by creating malicious file descriptors that, when processed by memcg_write_event_control(), trigger the use-after-free behavior. The vulnerability specifically affects systems running kernel versions prior to the fix, where the regression in file type checking allows arbitrary file operations to be processed through the memory cgroup interface. This creates a potential attack vector for malicious actors to manipulate kernel memory structures, potentially leading to denial of service conditions or unauthorized privilege elevation. The flaw is particularly concerning because it operates within kernel space, where memory corruption can have severe consequences for system integrity and security posture.
The fix implemented addresses this vulnerability by restoring the essential file type validation mechanism through the __file_cft() function. Rather than relying on direct file operations checks that are no longer reliable in the kernfs implementation, the solution employs a more robust approach that validates both the superblock and dentry type to ensure proper cgroup filesystem file handling. This approach aligns with established security practices for kernel-level validation and prevents the processing of non-cgroup files through the memory control group interface. The mitigation strategy follows security best practices by maintaining strict validation boundaries and preventing arbitrary file access through kernel interfaces. This fix directly addresses the root cause identified in the vulnerability description and restores the intended safety mechanisms that protect against race conditions and memory corruption scenarios. The solution demonstrates proper adherence to kernel security principles and represents a targeted correction that preserves system functionality while eliminating the exploitable condition.
This vulnerability maps to CWE-416 Use After Free, which specifically addresses the use of memory after it has been freed, and CWE-362 Concurrent Execution using Shared Resource with Improper Synchronization, which covers race conditions in shared resource access. The attack surface aligns with ATT&CK technique T1068, which involves exploiting local privilege escalation opportunities through kernel vulnerabilities. The fix implementation represents a proper security control that validates file types before processing, preventing unauthorized access patterns that could lead to memory corruption.