CVE-2026-99035 in Linux
Summary
by MITRE • 10/23/2026
In the Linux kernel, the following vulnerability has been resolved:
mmc: mtk-sd: Cancel request timeout work on remove
The driver queues req_timeout while a request is active. msdc_request_done() uses cancel_delayed_work(), which does not wait for a timeout callback that has already started.
The timeout callback calls mmc_request_done(), which wakes the request waiter, and then continues to use host->dev_comp and check the SDIO IRQ. During unbind, msdc_drv_remove() can return and the managed mmc_host can be freed before the callback finishes.
KASAN reported use-after-free accesses in msdc_request_done() and msdc_recheck_sdio_irq() in each of three unbind tests. The same tests completed without a kernel diagnostic after this change.
Call cancel_delayed_work_sync() after mmc_remove_host() has stopped new requests and before the driver releases the host resources.
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Analysis
by VulDB Data Team • 10/23/2026
The vulnerability identified within the Linux kernel's MMC subsystem, specifically in the MediaTek SD host controller driver known as mtk-sd, represents a critical use-after-free condition arising from improper synchronization during device removal operations. This flaw is rooted in how the driver manages asynchronous timeout handling for block and SDIO requests. When an I/O request is active, the driver queues a delayed work item to handle potential timeouts. The function msdc_request_done serves as the completion handler that attempts to cancel this pending timeout task using cancel_delayed_work. However, this specific API variant does not guarantee that any currently executing instance of the timeout callback will complete before returning control to the caller. This lack of synchronization creates a race condition window where the driver's removal routine can proceed while the timeout callback is still actively running in parallel execution contexts.
The operational impact of this flaw manifests as memory corruption and potential system instability due to use-after-free errors detected by Kernel Address Sanitizer during unbind tests. The timeout callback, which invokes mmc_request_done to wake up waiting processes, subsequently accesses hardware-specific data structures such as host->dev_comp and checks for SDIO interrupts. If the driver's remove function returns and triggers the deallocation of the managed mmc_host structure before this callback finishes its execution path, subsequent access attempts by the lingering timeout thread will reference freed memory regions. This results in undefined behavior, kernel panics, or data corruption, compromising both system reliability and security posture by potentially allowing exploitation for privilege escalation or denial of service scenarios.
From a vulnerability classification perspective, this issue aligns with CWE-416, Use After Free, as the core defect involves accessing memory after it has been released back to the system. In terms of attack vector mapping under MITRE ATT&CK, this corresponds to techniques involving exploitation of race conditions and improper resource cleanup, which can be leveraged in local privilege escalation scenarios if an attacker can trigger device removal or unbind events through controlled interfaces. The flaw highlights a common pitfall in kernel driver development where asynchronous task lifecycle management is not strictly synchronized with the hardware abstraction layer's teardown process.
To mitigate this vulnerability, the recommended remediation involves modifying the cleanup sequence within the msdc_drv_remove function to ensure strict synchronization of all pending work items before resource deallocation occurs. Specifically, developers must replace or supplement the use of cancel_delayed_work with cancel_delayed_work_sync after calling mmc_remove_host. The latter function ensures that any currently executing timeout callback is allowed to finish its execution and fully detach from the host structure before the driver proceeds to release associated resources. This adjustment guarantees that no dangling references remain active during the unbind phase, thereby eliminating the race condition and preventing access to freed memory regions. Implementing this fix restores proper lifecycle management for asynchronous tasks within the MMC subsystem and ensures stable operation during device removal events.