CVE-2026-89882 in Linux
Summary
by MITRE • 09/16/2026
In the Linux kernel, the following vulnerability has been resolved:
media: rkvdec: hevc: guard INTER_REF_PIC_SET_PRED index underflow
st_ref_pic_set_prediction() computes the reference RPS index as st_rps_idx - (delta_idx_minus1 + 1) per HEVC spec equation 7-59. Both operands are u8, so when delta_idx_minus1 + 1 exceeds the current index the subtraction wraps and the subsequent array access at calculated_rps_st_sets[ref_rps_idx] reads far out of bounds.
A userspace V4L2 client that can open the RKVDEC m2m decoder can submit an EXT_SPS_ST_RPS control with INTER_REF_PIC_SET_PRED set and delta_idx_minus1 crafted to trigger the underflow.
Reject the entry early when the reference index would underflow.
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Analysis
by VulDB Data Team • 09/16/2026
The Linux kernel media subsystem contains a critical integer underflow vulnerability within the HEVC video decoder driver for Rockchip hardware, specifically in the rkvdec module. This flaw resides in the st_ref_pic_set_prediction function which is responsible for calculating the reference picture set index according to High Efficiency Video Coding specifications. The calculation involves subtracting delta_idx_minus1 plus one from a current reference picture set index. Both operands are defined as unsigned eight-bit integers, meaning they cannot represent negative values. When the value of delta_idx_minus1 plus one exceeds the magnitude of the current st_rps_idx variable, standard unsigned arithmetic rules dictate that the result wraps around to a large positive number rather than becoming negative. This mathematical behavior creates an out-of-bounds array access condition because the resulting index points far beyond the allocated bounds of the calculated_rps_st_sets array.
This vulnerability is exploitable by local userspace applications with sufficient privileges to interact with video decoding hardware through the Video4Linux2 interface. An attacker or malicious application can open the RKVDEC memory-to-memory decoder device and submit a control structure containing an EXT_SPS_ST_RPS parameter. By carefully crafting the INTER_REF_PIC_SET_PRED field, specifically by setting delta_idx_minus1 to a value that causes the subtraction operation to underflow, the attacker forces the kernel to compute an invalid array index. The subsequent memory read operation accesses arbitrary kernel memory locations based on this wrapped-around index, potentially leading to information disclosure of sensitive kernel data or causing system instability through corrupted memory reads.
The operational impact of this vulnerability extends beyond simple out-of-bounds access. In many scenarios, reading from unmapped or protected kernel pages will trigger a page fault exception, resulting in an immediate kernel panic and denial of service for the entire system. If the accessed memory region is mapped but contains sensitive data such as cryptographic keys, process credentials, or other security-critical structures, the vulnerability can be leveraged for information disclosure attacks. Furthermore, depending on how the retrieved value is subsequently used within the decoder logic, it might lead to further exploitation chains that could compromise system integrity. The lack of proper bounds checking before array indexing represents a fundamental failure in input validation and defensive programming practices within the kernel media driver codebase.
To mitigate this vulnerability, developers have implemented an early rejection mechanism for entries where the reference index calculation would result in underflow. This fix ensures that any control request attempting to trigger the arithmetic wrap-around condition is validated before array access occurs. System administrators should apply available kernel updates or patches provided by their distribution vendors that include this specific rkvdec driver correction. Additionally, organizations deploying video processing hardware with Rockchip chips should verify that their media server applications enforce strict validation of input parameters and limit privileges for userspace clients interacting with V4L2 devices to minimize the attack surface in case similar vulnerabilities exist elsewhere in the stack.
This vulnerability aligns with Common Weakness Enumeration category CWE-190, which describes integer overflow or wraparound resulting in a value that is larger than expected. The exploitation technique involves manipulating input values to cause arithmetic errors leading to out-of-bounds memory access, corresponding closely to CWE-787 and CWE-125 depending on the specific outcome of the read operation. In terms of attack patterns, this scenario reflects techniques associated with privilege escalation or denial of service through kernel exploitation, mapping to ATT&CK tactics related to initial access via local application execution followed by potential impact on system availability if a crash occurs. Proper input validation and bounds checking remain essential defenses against such integer arithmetic vulnerabilities in low-level systems programming.