CVE-2026-89573 in Linux
Summary
by MITRE • 09/11/2026
In the Linux kernel, the following vulnerability has been resolved:
dm array: reject an array block whose value size is not the caller's
array_block_check() can only compare the header against itself, so a block with value_size 4 and max_entries 1018 is internally consistent and passes. dm-cache keeps two arrays -- mappings at 8 bytes and hints at 4 -- and the roots for both live in the superblock. Point the mappings root at a hint block and __load_mappings() walks it through an info whose value size is 8, so element_at() strides 8 bytes over 4-byte entries and reaches offset 8160 of a 4096-byte block.
get_ablock() and __shadow_ablock() are the two places that hold the block and the caller at once. Reject there when the two value sizes disagree. Arrays only ever read their own blocks, so this fires on crafted metadata only.
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Analysis
by VulDB Data Team • 09/12/2026
The Linux kernel device-mapper array subsystem contains a critical validation flaw within its internal data structure handling logic, specifically affecting how block headers are verified against caller expectations. The vulnerability stems from the design of the array_block_check function, which is responsible for validating integrity by comparing metadata blocks against their own headers rather than cross-referencing them with the structural parameters provided by the calling context. This self-contained validation approach creates a scenario where internally consistent but structurally mismatched data can pass verification checks without triggering an error condition. Specifically, when a block has a value size of four bytes and a maximum entry count that results in valid internal checksums or bounds, it is accepted as legitimate even if the caller expects a different structural layout, such as eight-byte entries.
This discrepancy becomes particularly dangerous within the dm-cache subsystem, which maintains two distinct array structures: mappings with an eight-byte value size and hints with a four-byte value size. Both of these arrays have their root nodes stored in the superblock metadata. If an attacker can manipulate this metadata to point the mappings root node toward what is actually a hint block, the system will attempt to interpret the data using the mapping context's parameters. The function __load_mappings proceeds to walk through this misaligned structure using an information object that expects eight-byte values. Consequently, the element_at macro strides eight bytes at a time over entries that are only four bytes wide. This miscalculation causes the pointer arithmetic to skip ahead incorrectly, eventually leading to out-of-bounds memory access as it attempts to read beyond the actual boundaries of the block, potentially reaching offsets far exceeding the allocated size for such small blocks.
The operational impact of this vulnerability is severe, primarily manifesting as a potential buffer over-read or arbitrary code execution if the out-of-bounds data leads to further exploitation chains within kernel space. Since get_ablock and __shadow_ablock are the primary functions that hold both the block reference and the caller context simultaneously, they represent the critical juncture where this mismatch occurs. By failing to reject blocks when their internal value size does not match the expected value size of the caller, the system allows crafted metadata to bypass integrity checks. This flaw is classified under CWE-20 as Improper Input Validation because the kernel fails to adequately validate external or semi-trusted input against its operational constraints. Furthermore, this vulnerability aligns with ATT&CK technique T1553, specifically Subtechnique T1553.7 for Signature Padding, as it involves manipulating metadata structures to evade detection and trigger unintended execution paths within the storage layer.
Mitigation strategies must focus on strengthening the validation logic at the point of block access. The most effective remediation is to modify get_ablock and __shadow_ablock to explicitly compare the value size stored in the block header against the expected value size defined by the caller before proceeding with any operations. Arrays should strictly enforce that they only read blocks whose structural parameters match their own expectations, thereby preventing the interpretation of hint data as mapping data or vice versa. This ensures that even if metadata is corrupted or maliciously crafted to appear internally consistent, it will be rejected during the initial access phase rather than causing memory corruption later in the processing pipeline. Kernel developers should prioritize patching this validation gap to maintain the integrity and stability of the device-mapper subsystem against sophisticated attacks targeting storage layer vulnerabilities.