CVE-2026-98403 in Linux
Summary
by MITRE • 10/14/2026
In the Linux kernel, the following vulnerability has been resolved:
s390/boot: Avoid IPL parameter append past command line
A command line may occupy all but the terminating byte of COMMAND_LINE_SIZE. In that case append_ipl_block_parm() passes a zero size to the IPL parameter conversion helpers and points the destination one byte past early_command_line. The helpers subtract one from the unsigned size and write the converted parameter outside the command line buffer.
Convert the IPL parameter in the command line parsing buffer first. A parameter beginning with '=' can then replace the existing command line regardless of its length, while other parameters are appended only when space remains.
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Analysis
by VulDB Data Team • 10/14/2026
The vulnerability identified as a flaw in the s390 boot process within the Linux kernel involves an off-by-one memory write error during the initialization phase. Specifically, the function append_ipl_block_parm() is responsible for processing Initial Program Load parameters passed to the kernel. The core technical issue arises when the existing command line occupies all but one byte of the COMMAND_LINE_SIZE buffer. In this scenario, the calculation for available space results in a zero size being passed to the IPL parameter conversion helpers. However, these internal helper functions perform an unsigned arithmetic operation by subtracting one from the provided size value before writing data. Due to integer underflow on an unsigned type, a size of zero becomes a very large positive number, causing the function to write converted parameters well beyond the boundaries of the early_command_line buffer into adjacent memory regions.
This out-of-bounds write constitutes a classic heap or stack overflow depending on where the command line buffer is allocated in memory during boot. Such an error can lead to data corruption of critical kernel structures located immediately after the command line buffer, potentially causing system instability, crashes, or unpredictable behavior during the early stages of system startup. In more severe scenarios involving specific memory layouts and attacker-controlled input parameters, this flaw could theoretically be exploited to execute arbitrary code with kernel privileges, although exploitation is complicated by the fact that this occurs before most security mechanisms are fully initialized. The vulnerability highlights a critical lack of bounds checking in low-level boot-time parsing routines where assumptions about available space were not rigorously validated against actual buffer limits.
From a classification perspective, this issue aligns with CWE-193 Off-by-One Error and CWE-787 Out-of-bounds Write. These categories describe situations where memory access occurs one byte past the end of an allocated block or beyond its intended boundaries due to incorrect boundary calculations. The operational impact is primarily centered on system reliability during boot, as corrupted kernel data structures can prevent successful initialization of subsystems that depend on accurate command line parsing. This affects the integrity and availability aspects of the CIA triad for systems relying on s390 architecture with specific IPL parameter configurations.
Mitigation strategies involve applying the upstream Linux kernel patch that restructures how IPL parameters are handled. The fix ensures that conversion occurs within a dedicated buffer first, allowing safe validation before any writes to critical memory areas. For administrators unable to immediately update their kernels, careful management of boot loader configuration is advised to ensure command lines do not approach maximum size limits when combined with additional IPL parameters. Regularly updating the kernel and monitoring for patches related to s390 architecture vulnerabilities are essential steps in maintaining system security against such low-level memory corruption flaws.