CVE-2026-63804 in Linux
Summary
by MITRE • 07/19/2026
In the Linux kernel, the following vulnerability has been resolved:
gfs2: fix use-after-free in gfs2_qd_dealloc
gfs2_qd_dealloc(), called as an RCU callback from gfs2_qd_dispose(), accesses the superblock object sdp through qd->qd_sbd after freeing qd. It does so to decrement sd_quota_count and wake up sd_kill_wait.
However, by the time the RCU callback runs, gfs2_put_super() may have already freed sdp via free_sbd(). This can happen when gfs2_quota_cleanup() is called during unmount: it disposes of quota objects via call_rcu() and then waits on sd_kill_wait with a 60-second timeout. If the timeout expires, or if gfs2_gl_hash_clear() triggers additional qd_put() calls that schedule more RCU callbacks after the wait completes, gfs2_put_super() will proceed to free the superblock while RCU callbacks referencing it are still pending.
Add an rcu_barrier() before free_sbd() in gfs2_put_super() to ensure all pending RCU callbacks (including gfs2_qd_dealloc) have completed before the superblock is freed.
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Analysis
by VulDB Data Team • 07/19/2026
The vulnerability described represents a critical use-after-free condition in the Linux kernel's gfs2 file system implementation that can lead to system instability and potential privilege escalation. This issue occurs within the gfs2_qd_dealloc function which serves as an RCU (Read-Copy-Update) callback mechanism, demonstrating a classic race condition scenario where memory cleanup operations are not properly synchronized with ongoing reference operations. The flaw manifests when the gfs2_qd_dealloc function attempts to access the superblock object sdp through the qd->qd_sbd pointer after the quota descriptor qd has already been freed, creating a dangerous situation where stale memory references can cause unpredictable behavior.
The technical execution of this vulnerability involves a complex interplay between multiple kernel subsystems and synchronization mechanisms. When gfs2_quota_cleanup() is invoked during unmount operations, it schedules quota objects for disposal using call_rcu() which defers the actual cleanup to a later RCU grace period. However, the sequence of operations allows gfs2_put_super() to proceed with freeing the superblock object through free_sbd() while RCU callbacks that reference this freed memory are still pending in the system. This race condition is particularly dangerous because it occurs during normal file system unmount procedures, making it exploitable through routine system operations and potentially allowing attackers to cause system crashes or escalate privileges.
The operational impact of this vulnerability extends beyond simple system instability into potential security implications that align with CWE-416 Use After Free, a well-documented weakness in software security practices where freed memory is accessed. The issue affects systems running the gfs2 file system and can result in kernel oops, system crashes, or denial of service conditions that compromise system availability. The vulnerability's exploitation potential increases when considering that RCU callbacks are typically used for asynchronous cleanup operations, and this particular case demonstrates how improper synchronization between different cleanup phases can create dangerous memory access patterns.
The recommended mitigation strategy involves implementing an rcu_barrier() call before the superblock is freed in gfs2_put_super(), ensuring all pending RCU callbacks including gfs2_qd_dealloc have completed execution. This approach directly addresses the root cause by providing proper synchronization between the callback execution phase and the memory deallocation phase, preventing the scenario where stale references are accessed after memory has been freed. From an ATT&CK perspective, this vulnerability relates to privilege escalation techniques through kernel exploitation, specifically targeting system stability and availability as part of the privilege escalation and defense evasion domains.
The fix demonstrates proper kernel memory management practices that align with industry standards for concurrent programming in kernel space. The solution ensures that all RCU callbacks complete their execution before proceeding with superblock destruction, preventing use-after-free conditions that could be exploited by malicious actors. This type of vulnerability highlights the complexity of kernel-level synchronization and the critical importance of proper reference counting and memory management in distributed file systems where multiple components must coordinate during mount and unmount operations while maintaining system stability. The fix also emphasizes the importance of understanding RCU semantics and how asynchronous cleanup operations can interact with resource destruction phases in kernel subsystems, particularly in enterprise storage solutions that rely on gfs2 for clustered file system operations.