Description

In the Linux kernel, the following vulnerability has been resolved: s390/zcrypt: Improve CCA CPRB length and overflow checks The xcrb_msg_to_type6cprb_msgx() function lacks proper input validation, creating security vulnerabilities: 1. Integer overflow after CEIL4 alignment: Signed int variables could overflow during 4-byte boundary alignment, causing undersized buffer allocations or incorrect bounds checking. 2. Missing minimum size validation: The CPRBX structure is copied from userspace without verifying sufficient buffer length. Undersized buffers cause uninitialized memory access when reading structure fields like cprbx.cprb_len and cprbx.domain. 3. Arithmetic overflow in sum calculations: Adding control block and data block sizes could overflow, bypassing size checks and enabling buffer overflows. Fix by using size_t for length calculations, adding U32_MAX boundary checks after alignment, validating minimum control block size before copying from userspace, and detecting sum calculation overflows.

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "7.8",
  "pubDate": "2026-08-26T15:17:08.977Z",
  "pubdate": "2026-08-26T15:17:08.977Z",
  "executiveSummary": "The Linux kernel s390 zcrypt subsystem is susceptible to multiple integer overflow and memory safety vulnerabilities within the xcrb_msg_to_type6cprb_msgx function.\nThese vulnerabilities stem from insufficient input validation and arithmetic errors during the processing of user-supplied CPRBX structures.\nAn unprivileged local attacker can exploit these flaws to trigger undersized buffer allocations, perform out-of-bounds memory access, or induce heap-based buffer overflows.\nThe primary risk involves the potential for local privilege escalation or kernel memory corruption, which could lead to system instability or arbitrary code execution within the kernel context.\nExploitation requires the ability to interact with the zcrypt device driver from a local user space process.\nThe vulnerability highlights failures in handling user-controlled length inputs and arithmetic operations on architectural boundaries.",
  "technicalDetails": "The root cause lies in the flawed implementation of the xcrb_msg_to_type6cprb_msgx function within the Linux kernel s390 zcrypt CCA CPRB processing logic. The implementation fails to safely handle length calculations derived from userspace inputs.\nSpecifically, the code performs CEIL4 alignment on signed integer variables. Without proper bounds checking, these arithmetic operations are prone to integer overflow. When a large value is supplied, the overflow results in a wrapped, smaller-than-expected value, leading to inadequate buffer allocation for subsequent memory operations.\nFurthermore, the function lacks a critical minimum size validation for the incoming CPRBX structure. The system copies data from userspace directly into kernel structures without verifying if the provided buffer meets the minimum required length. This leads to scenarios where fields such as cprbx.cprb_len and cprbx.domain are read from uninitialized or out-of-bounds kernel memory, resulting in information disclosure or incorrect downstream logic processing.\nA third issue involves arithmetic overflows occurring during the summation of control block and data block sizes. By manipulating these inputs, an attacker can bypass size validation checks. This bypass allows for the allocation of buffers that are too small to hold the intended data, triggering a heap-based buffer overflow when the kernel proceeds to write the user-provided data into these undersized memory regions.\nThe attack flow proceeds as follows: 1) An attacker opens the zcrypt device file. 2) The attacker crafts a malicious CPRBX structure designed to trigger integer overflows during the alignment phase or sum calculations. 3) The kernel driver receives this structure and performs the flawed arithmetic, leading to an incorrect allocation size. 4) The driver performs a copy operation from userspace to kernel space, overflowing the allocated heap buffer or reading from incorrect memory locations. 5) Successful exploitation results in the corruption of kernel heap metadata, potential execution of arbitrary code, or the exposure of sensitive kernel stack/heap data."
}