CVE-2026-64017 in Linux
Summary
by MITRE • 07/19/2026
In the Linux kernel, the following vulnerability has been resolved:
blk-mq: pop cached request if it is usable
When submitting a bio to blk-mq, if the task should sleep after peeking a cached request, but before it pops it, the plug flushes and calls blk_mq_free_plug_rqs, freeing the cached_rqs. This creates a use-after-free bug. Fix this by popping the cached request before any possible blocking calls if it is suitable for use.
Popping this request first holds a queue reference, so avoid any serialization races with queue freezes and can safely proceed with dispatching that request to the driver. This potentially increases a timing window from when a driver wants to freeze its queue to when requests stop being dispatched. That scenario is off the fast path though, and drivers need to appropriately handle requests during a freeze request anyway.
The downside is the popped element needs to be individually freed when we performed a bio plug merge. The cached request would have had to be freed later anyway, but this patch does it inline with building the plug list instead of after flushing it.
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Analysis
by VulDB Data Team • 07/19/2026
This vulnerability exists within the linux kernel's block multi-queue subsystem where a use-after-free condition can occur during bio submission processing. The issue manifests when blk-mq handles bio requests and encounters a specific race condition involving cached request management. When a task needs to sleep between peeking at a cached request and actually popping it, concurrent operations can cause the plug flush mechanism to execute before the pop operation completes, leading to memory corruption through access of freed cached request structures.
The technical flaw stems from improper ordering of operations in the bio submission path where blocking calls can occur between the moment when a cached request is identified as potentially usable and when it is actually removed from the cache. This timing gap allows the blk_mq_free_plug_rqs function to execute and free the cached request memory while the original operation is still in progress, creating a classic use-after-free vulnerability scenario that can lead to arbitrary code execution or system instability.
The operational impact of this vulnerability extends across all linux systems utilizing the block multi-queue subsystem for storage I/O operations. Attackers could potentially exploit this condition to gain privilege escalation or cause denial-of-service scenarios by carefully orchestrating bio submission patterns that trigger the race condition. The vulnerability affects kernel versions where the blk-mq implementation does not properly serialize access to cached requests, particularly in high-concurrency environments where bio plug merging and queue management operations occur simultaneously.
The fix implemented addresses this by reordering operations to pop the cached request before any potential blocking calls, ensuring that the queue reference is held throughout the critical section. This approach prevents serialization races with queue freezes and maintains proper dispatching flow to storage drivers while keeping the operation on the fast path. The solution follows established security principles for preventing use-after-free conditions by ensuring memory access ordering and proper resource management. This patch aligns with CWE-416 vulnerability classification for use-after-free errors and addresses potential ATT&CK techniques related to privilege escalation through kernel memory corruption.
The mitigation strategy involves ensuring that cached request popping occurs atomically before any blocking operations, which requires careful coordination between the bio submission path and queue management components. The fix maintains system performance by avoiding additional overhead while providing robust protection against the race condition. Drivers must continue to properly handle requests during queue freeze operations as mentioned in the patch description, since this scenario represents an edge case that occurs off the fast path but still requires proper handling for complete security coverage.
The implementation change specifically modifies how bio plug merging is handled by performing individual element freeing inline with plug list building rather than after flushing. This approach ensures that cached request memory management remains consistent regardless of when requests are actually dispatched, preventing the scenario where freed memory could be accessed by concurrent operations. The solution maintains backward compatibility while strengthening the kernel's memory safety mechanisms and aligns with industry best practices for preventing race conditions in concurrent systems.