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

[Ihor Solodrai <ihor.solodrai@linux.dev>]

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/

Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
---
 kernel/module/main.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/kernel/module/main.c b/kernel/module/main.c
index 710ee30b3bea..5bf456fad63e 100644
--- a/kernel/module/main.c
+++ b/kernel/module/main.c
@@ -1568,6 +1568,13 @@ static int simplify_symbols(struct module *mod, const struct load_info *info)
 			break;
 
 		default:
+			if (sym[i].st_shndx >= info->hdr->e_shnum) {
+				pr_err("%s: Symbol %s has an invalid section index %u (max %u)\n",
+				       mod->name, name, sym[i].st_shndx, info->hdr->e_shnum - 1);
+				ret = -ENOEXEC;
+				break;
+			}
+
 			/* Divert to percpu allocation if a percpu var. */
 			if (sym[i].st_shndx == info->index.pcpu)
 				secbase = (unsigned long)mod_percpu(mod);
-- 
2.52.0