CVE-2026-63802 in Linux
Summary
by MITRE • 07/19/2026
In the Linux kernel, the following vulnerability has been resolved:
blk-cgroup: fix UAF in __blkcg_rstat_flush()
When multiple blkgs in the same blkcg are released concurrently, a use-after-free can occur. The race happens when one blkg's __blkcg_rstat_flush() removes another blkg's iostat entries via llist_del_all(). The second blkg sees an empty list and proceeds to free itself while the first is still iterating over its entries.
Move the flush from __blkg_release() (RCU callback) to blkg_release() (before call_rcu). This ensures the RCU grace period waits for any concurrent flush's rcu_read_lock() section to complete before freeing.
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Analysis
by VulDB Data Team • 07/19/2026
This vulnerability exists within the linux kernel's block cgroup subsystem where a use-after-free condition can occur during concurrent removal of block group entries. The issue manifests when multiple block groups within the same block cgroup are released simultaneously, creating a race condition that compromises system stability and security integrity.
The technical flaw resides in the timing of resource cleanup operations within the blk-cgroup implementation. Specifically, when __blkcg_rstat_flush() executes on one block group, it removes iostat entries from another block group using llist_del_all() function. This operation creates a scenario where the second block group's list appears empty to subsequent operations, allowing that block group to proceed with self-destruction while the first block group is still processing its entries. The race condition occurs because the cleanup sequence lacks proper synchronization between concurrent flush operations.
The operational impact of this vulnerability extends beyond simple system instability to potential security implications within kernel memory management. When multiple block groups are released concurrently, the use-after-free condition can lead to arbitrary code execution or privilege escalation opportunities for malicious actors who can manipulate block group creation and destruction patterns. This represents a critical vulnerability that affects the kernel's ability to maintain consistent state during resource cleanup operations.
The fix implemented addresses this issue by repositioning the flush operation from __blkg_release() which operates within an RCU callback context to blkg_release() which executes before call_rcu(). This modification ensures that the RCU grace period properly waits for any concurrent flush operations' rcu_read_lock() sections to complete before allowing memory deallocation. The solution aligns with established kernel programming practices for managing concurrent access patterns and RCU synchronization primitives, effectively preventing the race condition that led to the use-after-free scenario.
This vulnerability type maps directly to CWE-416 Use After Free, which describes situations where a program continues to use a pointer after it has been freed, leading to unpredictable behavior. The fix demonstrates proper application of the ATT&CK technique T1068, which involves exploiting weaknesses in system design or implementation to achieve privilege escalation through resource management flaws. The remediation ensures proper ordering of operations and synchronization between concurrent kernel threads managing block group resources, preventing unauthorized access patterns during memory deallocation sequences.