module: add stop-grap sanity check on module memcpy()

The integrity of the struct module we load is important, and although
our ELF validator already checks that the module section must match
struct module, add a stop-gap check before we memcpy() the final minted
module. This also makes those inspecting the code what the goal is.

While at it, clarify the goal behind updating the sh_addr address.
The current comment is pretty misleading.
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>

kernel/module/main.c

@@ -2213,7 +2213,8 @@ static int move_module(struct module *mod, struct load_info *info)
 {
 	int i;
 	void *ptr;
-	enum mod_mem_type t;
+	enum mod_mem_type t = 0;
+	int ret = -ENOMEM;
 
 	for_each_mod_mem_type(type) {
 		if (!mod->mem[type].size) {
@@ -2249,9 +2250,26 @@ static int move_module(struct module *mod, struct load_info *info)
 
 		dest = mod->mem[type].base + (shdr->sh_entsize & SH_ENTSIZE_OFFSET_MASK);
 
-		if (shdr->sh_type != SHT_NOBITS)
+		if (shdr->sh_type != SHT_NOBITS) {
+			/*
+			 * Our ELF checker already validated this, but let's
+			 * be pedantic and make the goal clearer. We actually
+			 * end up copying over all modifications made to the
+			 * userspace copy of the entire struct module.
+			 */
+			if (i == info->index.mod &&
+			   (WARN_ON_ONCE(shdr->sh_size != sizeof(struct module)))) {
+				ret = -ENOEXEC;
+				goto out_enomem;
+			}
 			memcpy(dest, (void *)shdr->sh_addr, shdr->sh_size);
-		/* Update sh_addr to point to copy in image. */
+		}
+		/*
+		 * Update the userspace copy's ELF section address to point to
+		 * our newly allocated memory as a pure convenience so that
+		 * users of info can keep taking advantage and using the newly
+		 * minted official memory area.
+		 */
 		shdr->sh_addr = (unsigned long)dest;
 		pr_debug("\t0x%lx %s\n",
 			 (long)shdr->sh_addr, info->secstrings + shdr->sh_name);
@@ -2261,7 +2279,7 @@ static int move_module(struct module *mod, struct load_info *info)
 out_enomem:
 	for (t--; t >= 0; t--)
 		module_memory_free(mod->mem[t].base, t);
-	return -ENOMEM;
+	return ret;
 }
 
 static int check_export_symbol_versions(struct module *mod)