module: don't ignore vermagic string if module doesn't have modversions

Linus found a logic bug: we ignore the version number in a module's
vermagic string if we have CONFIG_MODVERSIONS set, but modversions
also lets through a module with no __versions section for modprobe
--force (with tainting, but still).

We should only ignore the start of the vermagic string if the module
actually *has* crcs to check.  Rather than (say) having an
entertaining hissy fit and creating a config option to work around the
buggy code.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

init/Kconfig

@@ -845,9 +845,9 @@ config MODULE_FORCE_LOAD
 	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.
+	  Allow loading of modules without version information (ie. modprobe
+	  --force).  Forced module loading sets the 'F' (forced) taint flag and
+	  is usually a really bad idea.
 
 config MODULE_UNLOAD
 	bool "Module unloading"

kernel/module.c

@@ -957,11 +957,14 @@ static inline int check_modstruct_version(Elf_Shdr *sechdrs,
 	return check_version(sechdrs, versindex, "struct_module", mod, crc);
 }
 
-/* First part is kernel version, which we ignore. */
-static inline int same_magic(const char *amagic, const char *bmagic)
+/* First part is kernel version, which we ignore if module has crcs. */
+static inline int same_magic(const char *amagic, const char *bmagic,
+			     bool has_crcs)
 {
+	if (has_crcs) {
 		amagic += strcspn(amagic, " ");
 		bmagic += strcspn(bmagic, " ");
+	}
 	return strcmp(amagic, bmagic) == 0;
 }
 #else
@@ -981,7 +984,8 @@ static inline int check_modstruct_version(Elf_Shdr *sechdrs,
 	return 1;
 }
 
-static inline int same_magic(const char *amagic, const char *bmagic)
+static inline int same_magic(const char *amagic, const char *bmagic,
+			     bool has_crcs)
 {
 	return strcmp(amagic, bmagic) == 0;
 }
@@ -1874,7 +1878,7 @@ static struct module *load_module(void __user *umod,
 		err = try_to_force_load(mod, "magic");
 		if (err)
 			goto free_hdr;
-	} else if (!same_magic(modmagic, vermagic)) {
+	} else if (!same_magic(modmagic, vermagic, versindex)) {
 		printk(KERN_ERR "%s: version magic '%s' should be '%s'\n",
 		       mod->name, modmagic, vermagic);
 		err = -ENOEXEC;