CVE-2026-80951 in Linux
Summary
by MITRE • 09/11/2026
In the Linux kernel, the following vulnerability has been resolved:
i3c: master: svc: bound IBI payload to the requested max_payload_len
svc_i3c_master_handle_ibi() reads the IBI payload from the RX FIFO into the IBI slot. The loop is bounded by the hardware FIFO size (SVC_I3C_FIFO_SIZE), not by the slot size.
slot->data points into the IBI pool, which i3c_generic_ibi_alloc_pool() sizes at max_payload_len per slot. svc_i3c_master_request_ibi() only rejects a max_payload_len larger than SVC_I3C_FIFO_SIZE, so a driver can request a smaller one. mctp-i3c requests 1. Each readsb() then copies the controller RXCOUNT bytes (up to 31) with no check against the slot size. A device that sends more bytes than the slot holds writes past slot->data, an out-of-bounds write into the IBI pool.
Bound the loop by dev->ibi->max_payload_len and clamp each read to the space left in the slot, the same way dw-i3c does. A device can still send more than the requested payload. Flush the leftover bytes from the RX FIFO so they do not leak into the next transfer.
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Analysis
by VulDB Data Team • 09/12/2026
The Linux kernel's I3C master service driver contains a critical out-of-bounds write vulnerability within the interrupt-based data reception routine, specifically in the svc_i3c_master_handle_ibi function. This flaw arises from an improper boundary check during the copying of incoming payload data from the hardware receive FIFO into memory slots allocated for In-Band Interrupts. The IBI mechanism allows devices to send small amounts of data without establishing a full connection, making it essential that the kernel strictly enforces size constraints on these transient transfers. However, the existing implementation incorrectly uses the maximum hardware FIFO capacity as the loop bound rather than the specific payload length requested and allocated for the current slot. This discrepancy creates a scenario where an I3C device can exploit this logic error to write data beyond the boundaries of its designated memory buffer.
The technical root cause lies in the mismatch between the allocation size and the copy operation limits. When an IBI transfer is initiated, the kernel allocates a specific amount of memory defined by max_payload_len via i3c_generic_ibi_alloc_pool. For instance, drivers like mctp-i3c may request significantly smaller payloads than the hardware's maximum FIFO depth. Despite this allocation constraint, svc_i3c_master_handle_ibi iterates through the RX FIFO using SVC_I3C_FIFO_SIZE as its limit. Each iteration performs a readb operation that copies up to thirty-one bytes into slot->data without verifying if there is sufficient space remaining in the allocated buffer. Consequently, if an I3C device transmits more data than max_payload_len permits, the kernel writes past the end of the slot's memory region. This results in heap-based out-of-bounds write corruption within the IBI pool, potentially overwriting adjacent kernel structures or other critical data segments.
From a security perspective, this vulnerability is classified under CWE-787: Out-of-Bounds Write and aligns with ATT&CK techniques related to privilege escalation through memory corruption attacks such as heap spraying or arbitrary code execution if an attacker can control the overwritten content. The operational impact of this flaw includes potential system instability, kernel panics due to corrupted metadata, and severe security risks including remote code execution if the vulnerability is triggered by a malicious I3C device connected to the bus. Since I3C interfaces are increasingly common in embedded systems for connecting sensors and peripherals, an attacker with physical access or control over an attached peripheral could exploit this flaw to compromise the host system's integrity.
The resolution involves modifying svc_i3c_master_handle_ibi to strictly bound the copy loop by dev->ibi->max_payload_len rather than the hardware FIFO size. Additionally, each read operation must be clamped to ensure it does not exceed the remaining space in the current slot, mirroring the safer implementation found in dw-i3c drivers. Crucially, after processing the allowed payload length, any leftover bytes residing in the RX FIFO must be explicitly flushed or discarded. This step is vital because failing to clear these excess bytes could cause them to leak into subsequent transfers, leading to data corruption and further instability. Implementing this fix ensures that the kernel respects application-level size constraints regardless of hardware capabilities, thereby eliminating the out-of-bounds write vector while maintaining correct protocol behavior for legitimate devices sending oversized payloads.