# CVE-2026-31521: module: Fix kernel panic when a symbol st_shndx is out of bounds

Status: published. Published: 2026-04-22. Updated: 2026-11-05.

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/20251224005752.201911-1-ihor.solodrai@linux.dev/

## Remediation

No remediation reported in collected sources

## Affected products and versions

- Linux / Linux: ≥ 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2; < 5d16f519b6eb1d071807e57efe0df2baa8d32ad6 (affected, git); ≥ 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2; < 4bbdb0e48176fd281c2b9a211b110db6fd94e175 (affected, git); ≥ 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2; < 082f15d2887329e0f43fd3727e69365f5bfe5d2c (affected, git); ≥ 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2; < ec2b22a58073f80739013588af448ff6e2ab906f (affected, git); ≥ 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2; < ef75dc1401d8e797ee51559a0dd0336c225e1776 (affected, git); ≥ 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2; < 6ba6957c640f58dc8ef046981a045da43e47ea23 (affected, git); ≥ 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2; < f9d69d5e7bde2295eb7488a56f094ac8f5383b92 (affected, git). Source: CNA: Linux
- Linux / Linux: 2.6.12 (affected); ≥ 0; < 2.6.12 (unaffected, semver); ≥ 5.15.203; ≤ 5.15.* (unaffected, semver); ≥ 6.1.168; ≤ 6.1.* (unaffected, semver); ≥ 6.6.131; ≤ 6.6.* (unaffected, semver); ≥ 6.12.80; ≤ 6.12.* (unaffected, semver); ≥ 6.18.21; ≤ 6.18.* (unaffected, semver); ≥ 6.19.11; ≤ 6.19.* (unaffected, semver); ≥ 7.0; ≤ * (unaffected, original_commit_for_fix). Source: CNA: Linux
- Siemens / SIMATIC S7-1500 CPU 1518-4 PN/DP MFP: ≥ V3.1.6; < * (affected, custom). Source: ADP: siemens-SADP
- Siemens / SIMATIC S7-1500 CPU 1518-4 PN/DP MFP: ≥ V3.1.5; < * (affected, custom). Source: ADP: siemens-SADP
- Siemens / SIMATIC S7-1500 CPU 1518-4 PN/DP MFP: ≥ V3.1.7; < * (affected, custom). Source: ADP: siemens-SADP
- Siemens / SIMATIC S7-1500 CPU 1518F-4 PN/DP MFP: ≥ V3.1.6; < * (affected, custom). Source: ADP: siemens-SADP
- Siemens / SIMATIC S7-1500 CPU 1518F-4 PN/DP MFP: ≥ V3.1.5; < * (affected, custom). Source: ADP: siemens-SADP
- Siemens / SIMATIC S7-1500 CPU 1518F-4 PN/DP MFP: ≥ V3.1.7; < * (affected, custom). Source: ADP: siemens-SADP
- Siemens / SIPLUS S7-1500 CPU 1518-4 PN/DP MFP: ≥ V3.1.6; < * (affected, custom). Source: ADP: siemens-SADP
- Siemens / SIPLUS S7-1500 CPU 1518-4 PN/DP MFP: ≥ V3.1.5; < * (affected, custom). Source: ADP: siemens-SADP
- Siemens / SIPLUS S7-1500 CPU 1518-4 PN/DP MFP: ≥ V3.1.7; < * (affected, custom). Source: ADP: siemens-SADP

## Severity and prioritization

- CISA KEV: No listing in collected sources

- EPSS: 0.13% as of 2026-11-05; percentile 2.54%

- SSVC: exploitation none; automatable no; technical impact partial; source ADP: CISA-ADP

- Weaknesses: Not reported

## Exploit evidence

No exploit evidence in collected sources. This is not proof of no exploitation.

## References

- https://git.kernel.org/stable/c/5d16f519b6eb1d071807e57efe0df2baa8d32ad6 (CNA: Linux)

- https://git.kernel.org/stable/c/4bbdb0e48176fd281c2b9a211b110db6fd94e175 (CNA: Linux)

- https://git.kernel.org/stable/c/082f15d2887329e0f43fd3727e69365f5bfe5d2c (CNA: Linux)

- https://git.kernel.org/stable/c/ec2b22a58073f80739013588af448ff6e2ab906f (CNA: Linux)

- https://git.kernel.org/stable/c/ef75dc1401d8e797ee51559a0dd0336c225e1776 (CNA: Linux)

- https://git.kernel.org/stable/c/6ba6957c640f58dc8ef046981a045da43e47ea23 (CNA: Linux)

- https://git.kernel.org/stable/c/f9d69d5e7bde2295eb7488a56f094ac8f5383b92 (CNA: Linux)

- https://cert-portal.siemens.com/productcert/html/ssa-082556.html (ADP: siemens-SADP)

- https://cert-portal.siemens.com/productcert/html/ssa-019113.html (ADP: siemens-SADP)

- https://cert-portal.siemens.com/productcert/html/ssa-457936.html (ADP: siemens-SADP)

## Provenance

- CNA: Linux: https://www.cve.org/CVERecord?id=CVE-2026-31521; retrieved 2026-11-05; source updated 2026-11-05

- FIRST EPSS: https://api.first.org/data/v1/epss?cve=CVE-2026-31521; retrieved 2026-11-05; source updated Not reported

Revision: 0d9c1d3b291b38607168b87f00b326f2410c926d038a3e2ed0212694dea63a6a

Canonical: https://y2.dev/cve/CVE-2026-31521

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