CVE-2026-64588 in Linux
Summary
by MITRE • 08/06/2026
In the Linux kernel, the following vulnerability has been resolved:
fuse-uring: fix data races on ring->ready
On weakly-ordered architectures, the store to fiq->ops can be reordered past the store to ring->ready, allowing a CPU that sees ring->ready == true via fuse_uring_ready() to dispatch requests through a stale fiq->ops pointer. Upgrade the store to smp_store_release() and the load in fuse_uring_ready() to smp_load_acquire() so that the preceding WRITE_ONCE(fiq->ops, ...) is visible to any CPU that observes ring->ready == true.
Additionally, fuse_uring_do_register() publishes ring->ready with WRITE_ONCE() but the fast-path check reads it with a plain load. This is a marked-vs-unmarked access that KCSAN will flag. Wrap it in READ_ONCE() to mark it without adding unnecessary ordering.
Also wrap the fc->ring load in fuse_uring_ready() in READ_ONCE() to prevent the compiler from reloading it between the NULL check and the dereference.
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Analysis
by VulDB Data Team • 08/06/2026
The vulnerability identified in the Linux kernel's fuse-uring implementation represents a critical data race condition that affects weakly-ordered architectures such as arm64 and some x86 configurations. This flaw manifests in the interaction between the file system's unified request handling mechanism and memory ordering semantics, creating potential for stale pointer dereferences that could lead to system instability or privilege escalation. The issue specifically impacts the fuse_uring_ready() function which serves as a fast-path check for determining when the io_uring ring is ready for processing requests, making it a critical component in the kernel's asynchronous I/O handling pipeline.
The core technical flaw stems from improper memory ordering during the initialization sequence of the fuse io_uring subsystem. When a CPU sets ring->ready to true using WRITE_ONCE() in the fuse_uring_do_register() function, subsequent CPUs observing this value through fuse_uring_ready() may see the updated ready flag before the associated fiq->ops pointer has been properly published. This violates memory ordering guarantees and creates a race condition where a CPU might dispatch requests using an outdated or uninitialized ops structure pointer. The vulnerability is classified as a data race under CWE-362 and represents a classic example of improper synchronization in concurrent programming environments. The weakness is particularly dangerous because it occurs during the bootstrapping phase of io_uring operations, potentially affecting all subsequent I/O processing.
The operational impact of this vulnerability extends beyond simple system instability to potential security implications that could be exploited by malicious actors. On weakly-ordered architectures, the reordering of memory operations can result in subtle but serious consequences where stale data is consumed by concurrent CPU cores. Attackers could potentially leverage this race condition to execute arbitrary code or escalate privileges by manipulating the timing of memory operations around the io_uring ring initialization. The vulnerability affects kernel versions that support fuse-uring functionality and particularly impacts systems where multiple CPU cores are actively processing I/O requests through the unified interface. This creates a vector for both denial-of-service attacks and privilege escalation attempts, making it a high-severity issue according to standard security frameworks.
The fix implemented addresses the memory ordering issues through proper smp_store_release() and smp_load_acquire() operations that enforce appropriate synchronization semantics between CPU cores. The solution specifically upgrades the store operation to use smp_store_release() which ensures that all prior stores to fiq->ops are completed before ring->ready is set to true, while the load in fuse_uring_ready() now uses smp_load_acquire() to ensure that any subsequent memory operations are not reordered before the ready flag is observed. Additionally, the implementation incorporates READ_ONCE() wrappers around the ring->ready access and fc->ring dereference to prevent compiler optimizations from introducing additional race conditions. These changes align with the ATT&CK framework's defense evasion techniques by ensuring proper synchronization mechanisms are in place to prevent adversarial exploitation of memory ordering vulnerabilities. The fix also addresses KCSAN (Kernel Concurrency Sanitizer) warnings that would otherwise be triggered by marked-vs-unmarked access patterns, thereby improving kernel stability and security posture across all supported architectures.
The resolution demonstrates the importance of proper memory ordering in concurrent systems and highlights how seemingly simple race conditions can have significant security implications. By implementing appropriate synchronization primitives and compiler barriers, the fix ensures that the fuse-uring subsystem maintains consistency across different CPU architectures while preventing the types of memory reordering issues that could be exploited by attackers. This vulnerability serves as a reminder of the critical need for careful consideration of memory ordering semantics in kernel-level code, particularly when dealing with shared data structures accessed by multiple execution contexts. The mitigation strategy provides a comprehensive solution that addresses both the immediate race condition and prevents similar issues from arising in related code paths within the kernel's I/O subsystem.