CVE-2026-31521

Description

In the Linux kernel, the following vulnerability has been resolved: module: Fix kernel panic when a symbol st_shndx is out of bounds The module loader doesn't check for bounds of the ELF section index in simplify_symbols(): for (i = 1; i < symsec->sh_size / sizeof(Elf_Sym); i++) { const char *name = info->strtab + sym[i].st_name; switch (sym[i].st_shndx) { case SHN_COMMON: [...] default: /* Divert to percpu allocation if a percpu var. */ if (sym[i].st_shndx == info->index.pcpu) secbase = (unsigned long)mod_percpu(mod); else /** HERE --> **/ secbase = info->sechdrs[sym[i].st_shndx].sh_addr; sym[i].st_value += secbase; break; } } A symbol with an out-of-bounds st_shndx value, for example 0xffff (known as SHN_XINDEX or SHN_HIRESERVE), may cause a kernel panic: BUG: unable to handle page fault for address: ... RIP: 0010:simplify_symbols+0x2b2/0x480 ... Kernel panic - not syncing: Fatal exception This can happen when module ELF is legitimately using SHN_XINDEX or when it is corrupted. Add a bounds check in simplify_symbols() to validate that st_shndx is within the valid range before using it. This issue was discovered due to a bug in llvm-objcopy, see relevant discussion for details [1]. [1] https://lore.kernel.org/linux-modules/[email protected]/

CVSS Details

CVSS Score

5.5

Severity

MEDIUM

CVSS Vector

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

Configurations (Affected Products)

cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:* - VULNERABLE

Linux Kernel (稳定版分支，具体受影响版本请参考Git补丁提交记录)

PoC / Exploit Code

⚠ For Security Research Only

The following code is for security research and authorized testing only.

python

/*
 * Conceptual PoC for CVE-2026-31521
 * This Python script modifies a valid Kernel Module (.ko) to trigger the vulnerability.
 * It corrupts the st_shndx field of a symbol to 0xffff (SHN_XINDEX).
 */

import struct
import sys

def create_poc_module(input_file, output_file):
    with open(input_file, 'rb') as f:
        data = bytearray(f.read())

    # Note: In a real scenario, we must parse the ELF section headers to find
    # the exact offset of the symbol table (.symtab) and the specific symbol entry.
    # This is a conceptual representation of the modification.
    
    # Assume we found the offset of st_shndx for a symbol (e.g., offset 0x1000)
    # We set it to 0xffff to trigger the out-of-bounds access in simplify_symbols().
    # st_shndx is a 2-byte field (uint16_t) at the end of the 64-bit Elf_Sym structure.
    
    # symbol_st_shndx_offset = 0x1234 (Example offset)
    # struct.pack_into('<H', data, symbol_st_shndx_offset, 0xffff)
    
    print(f"[+] Conceptually modifying {input_file} to set st_shndx=0xffff")
    print(f"[+] Saving to {output_file}")
    
    # For the purpose of this output, we just copy the file as a placeholder
    # because calculating the real offset requires a full ELF parser.
    with open(output_file, 'wb') as f:
        f.write(data)

    print("[*] To trigger: insmod", output_file)
    print("[*] Expected Result: Kernel panic - not syncing: Fatal exception")

if __name__ == "__main__":
    if len(sys.argv) < 3:
        print("Usage: python poc.py <input.ko> <output_poc.ko>")
    else:
        create_poc_module(sys.argv[1], sys.argv[2])

References

[1] CVE.org https://www.cve.org/CVERecord?id=CVE-2026-31521
[2] NVD NIST https://nvd.nist.gov/vuln/detail/CVE-2026-31521
[3] CVE Details https://www.cvedetails.com/cve/CVE-2026-31521/
[4] VulDB https://vuldb.com/cve/CVE-2026-31521
[5] https://git.kernel.org/stable/c/082f15d2887329e0f43fd3727e69365f5bfe5d2c
[6] https://git.kernel.org/stable/c/4bbdb0e48176fd281c2b9a211b110db6fd94e175
[7] https://git.kernel.org/stable/c/5d16f519b6eb1d071807e57efe0df2baa8d32ad6
[8] https://git.kernel.org/stable/c/6ba6957c640f58dc8ef046981a045da43e47ea23
[9] https://git.kernel.org/stable/c/ec2b22a58073f80739013588af448ff6e2ab906f
[10] https://git.kernel.org/stable/c/ef75dc1401d8e797ee51559a0dd0336c225e1776
[11] https://git.kernel.org/stable/c/f9d69d5e7bde2295eb7488a56f094ac8f5383b92

Raw JSON Data

JSON

{"cve": {"id": "CVE-2026-31521", "sourceIdentifier": "416baaa9-dc9f-4396-8d5f-8c081fb06d67", "published": "2026-04-22T14:16:51.930", "lastModified": "2026-04-28T18:26:58.433", "vulnStatus": "Analyzed", "cveTags": [], "descriptions": [{"lang": "en", "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nmodule: Fix kernel panic when a symbol st_shndx is out of bounds\n\nThe module loader doesn't check for bounds of the ELF section index in\nsimplify_symbols():\n\n for (i = 1; i < symsec->sh_size / sizeof(Elf_Sym); i++) {\n\t\tconst char *name = info->strtab + sym[i].st_name;\n\n\t\tswitch (sym[i].st_shndx) {\n\t\tcase SHN_COMMON:\n\n\t\t[...]\n\n\t\tdefault:\n\t\t\t/* Divert to percpu allocation if a percpu var. */\n\t\t\tif (sym[i].st_shndx == info->index.pcpu)\n\t\t\t\tsecbase = (unsigned long)mod_percpu(mod);\n\t\t\telse\n /** HERE --> **/\t\tsecbase = info->sechdrs[sym[i].st_shndx].sh_addr;\n\t\t\tsym[i].st_value += secbase;\n\t\t\tbreak;\n\t\t}\n\t}\n\nA symbol with an out-of-bounds st_shndx value, for example 0xffff\n(known as SHN_XINDEX or SHN_HIRESERVE), may cause a kernel panic:\n\n BUG: unable to handle page fault for address: ...\n RIP: 0010:simplify_symbols+0x2b2/0x480\n ...\n Kernel panic - not syncing: Fatal exception\n\nThis can happen when module ELF is legitimately using SHN_XINDEX or\nwhen it is corrupted.\n\nAdd a bounds check in simplify_symbols() to validate that st_shndx is\nwithin the valid range before using it.\n\nThis issue was discovered due to a bug in llvm-objcopy, see relevant\ndiscussion for details [1].\n\n[1] https://lore.kernel.org/linux-modules/[email protected]/"}], "metrics": {"cvssMetricV31": [{"source": "[email protected]", "type": "Primary", "cvssData": {"version": "3.1", "vectorString": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H", "baseScore": 5.5, "baseSeverity": "MEDIUM", "attackVector": "LOCAL", "attackComplexity": "LOW", "privilegesRequired": "LOW", "userInteraction": "NONE", "scope": "UNCHANGED", "confidentialityImpact": "NONE", "integrityImpact": "NONE", "availabilityImpact": "HIGH"}, "exploitabilityScore": 1.8, "impactScore": 3.6}]}, "weaknesses": [{"source": "[email protected]", "type": "Primary", "description": [{"lang": "en", "value": "CWE-787"}]}], "configurations": [{"nodes": [{"operator": "OR", "negate": false, "cpeMatch": [{"vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*", "versionStartIncluding": "2.6.12.1", "versionEndExcluding": "5.15.203", "matchCriteriaId": "A0A14F27-2B54-495E-9E83-FD1A2D3AC85C"}, {"vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*", "versionStartIncluding": "5.16", "versionEndExcluding": "6.1.168", "matchCriteriaId": "E2DDDCA1-6DAB-4018-B920-8F045DDD8D3B"}, {"vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*", "versionStartIncluding": "6.2", "versionEndExcluding": "6.6.131", "matchCriteriaId": "CE6ED4D4-0046-4573-BFA9-D64143B6A89F"}, {"vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*", "versionStartIncluding": "6.7", "versionEndExcluding": "6.12.80", "matchCriteriaId": "97EB19EC-A11E-49C6-9D2F-6F6EC6CB98B6"}, {"vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*", "versionStartIncluding": "6.13", "versionEndExcluding": "6.18.21", "matchCriteriaId": "ED39847A-3B46-4729-B7CA-B2C30B9FA8FE"}, {"vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*", "versionStartIncluding": "6.19", "versionEndExcluding": "6.19.11", "matchCriteriaId": "4CA2E747-A9EC-4518-9AA2-B4247FC748B7"}, {"vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:2.6.12:-:*:*:*:*:*:*", "matchCriteriaId": "6F62EECE-8FB1-4D57-85D8-CB9E23CF313C"}, {"vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:2.6.12:rc2:*:*:*:*:*:*", "matchCriteriaId": "4F76C298-81DC-43E4-8FC9-DC005A2116EF"}, {"vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:2.6.12:rc3:*:*:*:*:*:*", "matchCriteriaId": "0AB349B2-3F78-4197-882B-90ADB3BF645A"}, {"vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:2.6.12:rc4:*:*:*:*:*:*", "matchCriteriaId": "6AC88830-A9BC-4607-B572-A4B502FC9FD0"}, {"vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:2.6.12:rc5:*:*:*:*:*:*", "matchCriteriaId": "476CB3A5-D022-4F13-AAEF-CB6A5785516A"}, {"vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:7.0:rc1:*:*:*:*:*:*", "matchCriteriaId": "F253B622-8837-4245-BCE5-A7BF8FC76A16"}, {"vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:7.0:rc2:*:*:*:*:*:*", "matchCriteriaId": "4AE85AD8-4641-4E7C-A2F4-305E2CD9EE64"}]}]}], "references": [{"url": "https://git.kernel.org/stable/c/082f15d2887329e0f43fd3727e69365f5bfe5d2c", "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67", "tags": ["Patch"]}, {"url": "https://git.kernel.org/stable/c/4bbdb0e48176fd281c2b9a211b110db6fd94e175", "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67", "tags": ["Patch"]}, {"url": "https:/ ... (truncated)