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

In the Linux kernel, the following vulnerability has been resolved:

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

The module loader doesn't check for bounds of the ELF section index in simplifysymbols():

for (i = 1; i < symsec->shsize / sizeof(ElfSym); i++) { const char name = info->strtab + sym[i].stname;

switch (sym[i].stshndx) { case SHNCOMMON:

[...]

default: / Divert to percpu allocation if a percpu var. / if (sym[i].stshndx == info->index.pcpu) secbase = (unsigned long)modpercpu(mod); else / HERE --> / secbase = info->sechdrs[sym[i].stshndx].shaddr; sym[i].stvalue += secbase; break; } }

A symbol with an out-of-bounds stshndx value, for example 0xffff (known as SHNXINDEX or SHNHIRESERVE), may cause a kernel panic:

BUG: unable to handle page fault for address: ... RIP: 0010:simplifysymbols+0x2b2/0x480 ... Kernel panic - not syncing: Fatal exception

This can happen when module ELF is legitimately using SHNXINDEX or when it is corrupted.

Add a bounds check in simplifysymbols() to validate that stshndx 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/