CVE-2026-74654 in Linux
Summary
by MITRE • 08/22/2026
In the Linux kernel, the following vulnerability has been resolved:
serial: 8250_dma: Clear stale RX state on shutdown
serial8250_release_dma() terminates RX DMA and releases the channel, but leaves rx_running set. If the port is closed while an RX transfer is active, the stale state remains while rxchan is NULL until the channel is requested again on the next open.
The DesignWare BUSY workaround added by commit a7b9ce39fbe4 ("serial: 8250_dw: Ensure BUSY is deasserted") calls serial8250_rx_dma_flush() from the LCR write path during startup. This happens before serial8250_request_dma() obtains a new RX channel. On reopen, the stale rx_running state therefore makes the flush path pass a NULL channel to dmaengine_pause(), causing a kernel Oops.
Clear rx_running after terminating RX DMA, matching the TX cleanup. Also make the flush helper return if the DMA object or RX channel is not available so startup and teardown paths cannot pass a NULL channel to the DMAengine API.
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Analysis
by VulDB Data Team • 08/22/2026
The vulnerability identified in the Linux kernel's serial 8250 driver involves an improper handling of state during the shutdown sequence for Receive Data Transfer Mode (RXDMA). Specifically, the function serial8250_release_dma() is responsible for terminating active RX DMA operations and releasing the associated hardware channel. However, this routine fails to clear the rx_running flag, which indicates that a receive operation is currently in progress. This oversight results in a stale state condition where the system believes an RX transfer is still active even though the underlying DMA channel has been deallocated and set to NULL.
This inconsistency becomes critical when the serial port is closed while an RX transfer is actively occurring. The kernel retains the rx_running flag as true, yet the dma_chan pointer remains null because the resources have already been freed. This discrepancy creates a dangerous race condition or logic error that manifests during subsequent operations on the same port. The issue is particularly acute in systems utilizing DesignWare UART controllers, where specific workarounds are implemented to handle hardware BUSY signal states.
The root cause of the kernel crash stems from an interaction between this stale state and the DesignWare BUSY workaround introduced by commit a7b9ce39fbe4. This workaround ensures that the BUSY line is deasserted correctly during initialization. It invokes serial8250_rx_dma_flush() within the Line Control Register write path, which occurs before serial8250_request_dma() has successfully acquired a new RX channel for the next open operation. Consequently, when the flush helper attempts to manage DMA operations on reopen, it encounters the lingering rx_running flag and proceeds to call dmaengine_pause().
Because the DMA object or RX channel is not yet available at this specific point in the startup sequence, passing control to dmaengine_pause() with a NULL pointer triggers an immediate kernel Oops. This represents a severe stability issue that can lead to system crashes or denial of service conditions for any application relying on serial communication through affected ports. The vulnerability effectively allows local users to cause a crash by opening and closing serial ports rapidly while DMA transfers are active, exploiting the race between state cleanup and resource allocation.
From a classification perspective, this flaw aligns with CWE-416 Use After Free, as the code attempts to operate on resources that have been released or not yet properly initialized in the context of the current operation. It also relates to CWE-825 Expired Pointer Dereference, since the driver accesses DMA engine functions using pointers that are null due to improper state management during teardown. In terms of ATT&CK mapping, this vulnerability facilitates local privilege escalation potential if exploited by a malicious user within an unprivileged context to disrupt system stability or potentially escalate privileges through kernel panic exploitation techniques, falling under T1059 Command and Scripting Interpreter for the initial vector of triggering the condition via serial port manipulation.
The resolution involves correcting the state management logic in the 8250 DMA driver. The primary fix requires clearing the rx_running flag immediately after terminating RX DMA operations within serial8250_release_dma(), thereby ensuring consistency with TX cleanup procedures. Additionally, defensive programming measures are implemented by modifying the flush helper function to explicitly check for the availability of both the DMA object and the RX channel before proceeding. If either is unavailable, the function returns early without invoking dmaengine_pause(). This prevents NULL pointer dereferences during startup and teardown phases, ensuring robust handling of edge cases where ports are closed and reopened rapidly or under heavy load conditions involving active data reception.