module: Fix kernel panic when a symbol st_shndx is out of bounds

Overview

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]/

Affected

• Linux >= 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 < 5d16f519b6eb1d071807e57efe0df2baa8d32ad6
• Linux >= 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 < 4bbdb0e48176fd281c2b9a211b110db6fd94e175
• Linux >= 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 < 082f15d2887329e0f43fd3727e69365f5bfe5d2c
• Linux >= 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 < ec2b22a58073f80739013588af448ff6e2ab906f
• Linux >= 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 < ef75dc1401d8e797ee51559a0dd0336c225e1776
• Linux >= 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 < 6ba6957c640f58dc8ef046981a045da43e47ea23
• Linux >= 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 < f9d69d5e7bde2295eb7488a56f094ac8f5383b92
• Linux 2.6.12

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.