CVE-2026-90090 in Linux
Summary
by MITRE • 09/17/2026
In the Linux kernel, the following vulnerability has been resolved:
Bluetooth: btmtksdio: Fix out-of-bounds DMA read in the TX path
btmtksdio_tx_packet() rounds the transfer size up to the SDIO block size of 256 bytes, but hands the host controller the SKB buffer as is:
err = sdio_writesb(bdev->func, MTK_REG_CTDR, skb->data, round_up(skb->len, MTK_SDIO_BLOCK_SIZE));
Only skb->len bytes hold packet data, so the controller reads up to 255 bytes of uninitialised memory and sends it to the device over the SDIO bus. Depending on how much tailroom slack the SKB allocation happens to carry, that read can also extend past the end of the buffer.
Compute the padded length up front, ensure the SKB has tailroom for it, and zero-fill the padding with skb_put_zero(). skb->len then covers the padding, so sdio_writesb() no longer needs to round up. byte_tx keeps counting the header and the payload only, and the error path restores the SKB so that the caller can requeue it.
Writing behind skb->tail is only safe because the driver owns the buffer, which "Bluetooth: btmtksdio: Take exclusive ownership of the SKB before TX" ensures.
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Analysis
by VulDB Data Team • 09/17/2026
The vulnerability identified in the Linux kernel Bluetooth subsystem specifically affects the MediaTek SDIO driver component known as btmtksdio. This issue manifests as an out-of-bounds DMA read within the transmission path, posing significant risks to system stability and data confidentiality. The root cause lies in a mismatch between how packet lengths are calculated for hardware transmission and how memory buffers are managed by the network stack. When preparing packets for transmission via SDIO, the driver rounds up the transfer size to align with the device's block size of 256 bytes. However, it passes the original socket buffer SKB directly to the sdio_writesb function without adjusting the buffer metadata or ensuring sufficient allocated space for this padded length. Consequently, the hardware controller performs a DMA read that extends beyond the actual packet data contained within skb->len. This results in reading up to 255 bytes of uninitialized kernel memory and transmitting it over the SDIO bus to the Bluetooth device.
From an operational perspective, this flaw allows sensitive information residing in adjacent kernel memory regions to be leaked through the wireless interface. Depending on the allocation characteristics of the SKB at runtime, particularly regarding tailroom slack, the out-of-bounds read may also extend past the end of the allocated buffer entirely. Such behavior constitutes a classic heap-based or stack-based buffer over-read vulnerability where uninitialised data is exposed to external interfaces. This aligns with CWE-125 Out-of-bounds Read and CWE-908 Use of Uninitialized Resource, as the driver accesses memory regions that are not intended for transmission. The exposure of kernel memory contents can potentially aid attackers in bypassing security mechanisms such as KASLR or extracting cryptographic keys if they reside in nearby allocations. Furthermore, accessing invalid memory ranges could trigger fault conditions leading to denial of service scenarios depending on the specific hardware and kernel configuration responses to illegal DMA accesses.
The remediation strategy implemented addresses both the immediate buffer management error and ensures proper data integrity during transmission. The fix involves computing the padded length upfront before initiating any I/O operations. It then verifies that the SKB possesses adequate tailroom to accommodate this rounded-up size. If necessary, padding bytes are explicitly zero-filled using skb_put_zero(), ensuring no uninitialized memory is transmitted. By updating skb->len to cover the full padded payload, subsequent calls to sdio_writesb operate strictly within valid bounds without needing internal rounding logic that previously caused the overflow. Additionally, byte_tx counters are adjusted to track only header and actual payload bytes for accurate accounting, while error paths correctly restore SKB state to allow requeuing by higher-level protocols.
This technical correction reinforces secure coding practices related to memory ownership and buffer handling in network drivers. The safety of writing beyond skb->tail is guaranteed because the driver explicitly takes exclusive ownership of the SKB prior to transmission processing, a prerequisite established by preceding patches ensuring proper lifecycle management. This scenario maps to ATT&CK technique T1059 Command and Scripting Interpreter if viewed through the lens of potential exploitation chains involving kernel code execution following information disclosure, although primarily it represents an information leak vector classified under CWE-200 Exposure of Sensitive Information to an Unauthorized Actor. The resolution ensures that all DMA operations remain within allocated boundaries, eliminating both confidentiality breaches and potential stability issues arising from illegal memory access patterns in the Bluetooth SDIO subsystem.