[PATCH] module: fix [e_shstrndx].sh_size=0 OOB access

[Alexey Dobriyan <adobriyan@gmail.com>]

It is trivial to craft a module to trigger OOB access in this line:

	if (info->secstrings[strhdr->sh_size - 1] != '\0') {

BUG: unable to handle page fault for address: ffffc90000aa0fff
#PF: supervisor read access in kernel mode
#PF: error_code(0x0000) - not-present page
PGD 100000067 P4D 100000067 PUD 100066067 PMD 10436f067 PTE 0
Oops: 0000 [#1] PREEMPT SMP PTI
CPU: 7 PID: 1215 Comm: insmod Not tainted 5.18.0-rc5-00007-g9bf578647087-dirty #10
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.14.0-4.fc34 04/01/2014
RIP: 0010:load_module+0x19b/0x2391

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
---

 kernel/module.c |    4 ++++
 1 file changed, 4 insertions(+)

--- a/kernel/module.c
+++ b/kernel/module.c
@@ -3033,6 +3033,10 @@ static int elf_validity_check(struct load_info *info)
 	 * strings in the section safe.
 	 */
 	info->secstrings = (void *)info->hdr + strhdr->sh_offset;
+	if (strhdr->sh_size == 0) {
+		pr_err("empty section name table\n");
+		goto no_exec;
+	}
 	if (info->secstrings[strhdr->sh_size - 1] != '\0') {
 		pr_err("ELF Spec violation: section name table isn't null terminated\n");
 		goto no_exec;