Make forced module loading optional

The kernel module loader used to be much too happy to allow loading of
modules for the wrong kernel version by default.  For example, if you
had MODVERSIONS enabled, but tried to load a module with no version
info, it would happily load it and taint the kernel - whether it was
likely to actually work or not!

Generally, such forced module loading should be considered a really
really bad idea, so make it conditional on a new config option
(MODULE_FORCE_LOAD), and make it default to off.

If somebody really wants to force module loads, that's their problem,
but we should not encourage it.  Especially as it happened to me by
mistake (ie regular unversioned Fedora modules getting loaded) causing
lots of strange behavior.
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

init/Kconfig

@@ -825,6 +825,15 @@ menuconfig MODULES
 
 	  If unsure, say Y.
 
+config MODULE_FORCE_LOAD
+	bool "Forced module loading"
+	depends on MODULES
+	default n
+	help
+	  This option allows loading of modules even if that would set the
+          'F' (forced) taint, due to lack of version info.  Which is
+	  usually a really bad idea.
+
 config MODULE_UNLOAD
 	bool "Module unloading"
 	depends on MODULES

kernel/module.c

@@ -890,6 +890,19 @@ static struct module_attribute *modinfo_attrs[] = {
 
 static const char vermagic[] = VERMAGIC_STRING;
 
+static int try_to_force_load(struct module *mod, const char *symname)
+{
+#ifdef CONFIG_MODULE_FORCE_LOAD
+	if (!(tainted & TAINT_FORCED_MODULE))
+		printk("%s: no version for \"%s\" found: kernel tainted.\n",
+		       mod->name, symname);
+	add_taint_module(mod, TAINT_FORCED_MODULE);
+	return 0;
+#else
+	return -ENOEXEC;
+#endif
+}
+
 #ifdef CONFIG_MODVERSIONS
 static int check_version(Elf_Shdr *sechdrs,
 			 unsigned int versindex,
@@ -914,18 +927,18 @@ static int check_version(Elf_Shdr *sechdrs,
 
 		if (versions[i].crc == *crc)
 			return 1;
-		printk("%s: disagrees about version of symbol %s\n",
-		       mod->name, symname);
 		DEBUGP("Found checksum %lX vs module %lX\n",
 		       *crc, versions[i].crc);
-		return 0;
+		goto bad_version;
 	}
-	/* Not in module's version table.  OK, but that taints the kernel. */
-	if (!(tainted & TAINT_FORCED_MODULE))
-		printk("%s: no version for \"%s\" found: kernel tainted.\n",
-		       mod->name, symname);
-	add_taint_module(mod, TAINT_FORCED_MODULE);
+
+	if (!try_to_force_load(mod, symname))
 		return 1;
+
+bad_version:
+	printk("%s: disagrees about version of symbol %s\n",
+	       mod->name, symname);
+	return 0;
 }
 
 static inline int check_modstruct_version(Elf_Shdr *sechdrs,
@@ -1853,9 +1866,9 @@ static struct module *load_module(void __user *umod,
 	modmagic = get_modinfo(sechdrs, infoindex, "vermagic");
 	/* This is allowed: modprobe --force will invalidate it. */
 	if (!modmagic) {
-		add_taint_module(mod, TAINT_FORCED_MODULE);
-		printk(KERN_WARNING "%s: no version magic, tainting kernel.\n",
-		       mod->name);
+		err = try_to_force_load(mod, "magic");
+		if (err)
+			goto free_hdr;
 	} else if (!same_magic(modmagic, vermagic)) {
 		printk(KERN_ERR "%s: version magic '%s' should be '%s'\n",
 		       mod->name, modmagic, vermagic);
@@ -2006,9 +2019,10 @@ static struct module *load_module(void __user *umod,
 	    (mod->num_gpl_future_syms && !gplfuturecrcindex) ||
 	    (mod->num_unused_syms && !unusedcrcindex) ||
 	    (mod->num_unused_gpl_syms && !unusedgplcrcindex)) {
-		printk(KERN_WARNING "%s: No versions for exported symbols."
-		       " Tainting kernel.\n", mod->name);
-		add_taint_module(mod, TAINT_FORCED_MODULE);
+		printk(KERN_WARNING "%s: No versions for exported symbols.\n", mod->name);
+		err = try_to_force_load(mod, "nocrc");
+		if (err)
+			goto cleanup;
 	}
 #endif
 	markersindex = find_sec(hdr, sechdrs, secstrings, "__markers");