CVE-2026-31521

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]/