CVE-2026-99227 in Linux
Summary
by MITRE • 11/02/2026
In the Linux kernel, the following vulnerability has been resolved:
io_uring/waitid: prevent cancel reference wrap
io_waitid packs the cancellation state into bit 31 of ->refs and uses bits 30:0 as a 31-bit reference counter:
#define IO_WAITID_CANCEL_FLAG BIT(31) #define IO_WAITID_REF_MASK GENMASK(30, 0)
__io_waitid_cancel() marks a request canceled, then increments the low bits of ->refs to claim completion ownership. When another path owns the request, the increment remains even though cancellation returns false. The request remains discoverable by cancellation, so repeated failed attempts can wrap the 31-bit reference field to zero. A later cancellation can claim ownership and complete the request a second time.
Use atomic_fetch_or() when setting the cancel flag and inspect the old flag value. The first cancel attempt continues to the existing ownership increment, preserving its wake and issuer handoff semantics. Later attempts see the cancel flag and return without incrementing the reference field.
This makes repeated failed cancellation attempts idempotent while leaving the first-cancel state transitions unchanged.
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Analysis
by VulDB Data Team • 11/02/2026
The Linux kernel io_uring subsystem contains a logic flaw in the waitid request handling mechanism that can lead to use-after-free conditions or double completion of requests due to integer reference counter wrapping. The vulnerability resides within the __io_waitid_cancel function, which is responsible for managing the cancellation state of asynchronous I/O operations. Specifically, this function packs the cancellation status into bit 31 of a refs field while utilizing bits thirty through zero as a thirty-one-bit reference counter. This design relies on precise atomicity and correct incrementation logic to ensure that request ownership transitions are handled safely without race conditions or arithmetic overflows.
The core technical flaw occurs when multiple threads attempt to cancel the same io_uring waitid request concurrently. When __io_waitid_cancel is invoked, it first marks the request as canceled by setting bit thirty-one and then increments the lower bits of the refs field to claim ownership for completion processing. However, if another execution path already holds ownership of the request, the cancellation attempt returns false indicating failure, yet the increment operation on the reference counter still proceeds. This results in a phantom increase in the reference count that does not correspond to actual active references. Over time, repeated failed cancellation attempts can cause this thirty-one-bit field to wrap around from its maximum value back to zero.
The operational impact of this integer overflow is severe because it allows subsequent cancellation attempts to incorrectly perceive the request as having no remaining references or being in a state where they can claim ownership. Consequently, a later cancellation attempt may successfully complete the request a second time while the original owner still holds valid pointers and expectations regarding the request lifecycle. This double completion scenario constitutes a critical use-after-free vulnerability, potentially leading to kernel memory corruption, privilege escalation, or system instability depending on how the freed resources are subsequently allocated and utilized by other parts of the operating system.
To mitigate this issue, the fix modifies __io_waitid_cancel to utilize atomic_fetch_or when setting the cancel flag rather than a simple set operation followed by an increment. This approach allows the function to inspect the old value of the flags before proceeding with any reference count adjustments. If the cancel bit was already set by a previous attempt, the function returns immediately without performing the additional increment on the reference counter. This change ensures that repeated failed cancellation attempts become idempotent, meaning they have no side effects beyond the initial state transition, while preserving the correct wake and issuer handoff semantics for the first successful cancellation.
From a classification perspective, this vulnerability aligns with CWE-190 Integer Overflow or Wraparound, as the root cause is the unbounded increment of a fixed-width integer field leading to unexpected zeroing due to wrap-around behavior. It also relates closely to CWE-416 Use After Free, since the double completion allows access to memory that may have been freed by the first completion handler. In terms of adversary tactics, this flaw could be leveraged within ATT&CK technique T1059 Command and Scripting Interpreter if an attacker can trigger repeated cancellation attempts on controlled io_uring requests to achieve arbitrary code execution through kernel exploitation primitives derived from the use-after-free condition.
System administrators should ensure that their Linux kernels are updated with patches addressing this specific io_uring waitid logic error. Since io_uring is a high-performance asynchronous I/O interface often used in critical infrastructure and cloud environments, unpatched systems remain vulnerable to local privilege escalation attacks if an attacker can create sufficient contention on cancellation paths for waitid requests. Regular security auditing of kernel versions and prompt application of vendor-provided updates are essential defenses against this class of concurrency-related memory safety vulnerabilities.