module: Harden STRICT_MODULE_RWX

We're very close to enforcing W^X memory, refuse to load modules that
violate this principle per construction.

[jeyu: move module_enforce_rwx_sections under STRICT_MODULE_RWX as per discussion]
Link: http://lore.kernel.org/r/20200403171303.GK20760@hirez.programming.kicks-ass.net
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Jessica Yu <jeyu@kernel.org>

kernel/module.c

@@ -2052,9 +2052,28 @@ static void module_enable_nx(const struct module *mod)
 	frob_writable_data(&mod->init_layout, set_memory_nx);
 }
 
+static int module_enforce_rwx_sections(Elf_Ehdr *hdr, Elf_Shdr *sechdrs,
+				       char *secstrings, struct module *mod)
+{
+	const unsigned long shf_wx = SHF_WRITE|SHF_EXECINSTR;
+	int i;
+
+	for (i = 0; i < hdr->e_shnum; i++) {
+		if ((sechdrs[i].sh_flags & shf_wx) == shf_wx)
+			return -ENOEXEC;
+	}
+
+	return 0;
+}
+
 #else /* !CONFIG_STRICT_MODULE_RWX */
 /* module_{enable,disable}_ro() stubs are in module.h */
 static void module_enable_nx(const struct module *mod) { }
+static int module_enforce_rwx_sections(Elf_Ehdr *hdr, Elf_Shdr *sechdrs,
+				       char *secstrings, struct module *mod)
+{
+	return 0;
+}
 #endif /*  CONFIG_STRICT_MODULE_RWX */
 
 #ifdef CONFIG_LIVEPATCH
@@ -3385,6 +3404,11 @@ static struct module *layout_and_allocate(struct load_info *info, int flags)
 	if (err < 0)
 		return ERR_PTR(err);
 
+	err = module_enforce_rwx_sections(info->hdr, info->sechdrs,
+					  info->secstrings, info->mod);
+	if (err < 0)
+		return ERR_PTR(err);
+
 	/* We will do a special allocation for per-cpu sections later. */
 	info->sechdrs[info->index.pcpu].sh_flags &= ~(unsigned long)SHF_ALLOC;