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>

2 files changed, 13 insertions, 9 deletions

diff --git a/init/Kconfig b/init/Kconfig
index 3b5adbf228c7..6135d07f31ec 100644
--- a/init/Kconfig
+++ b/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"
diff --git a/kernel/module.c b/kernel/module.c
index 2584c0e2762d..f5e9491ef7ac 100644
--- a/kernel/module.c
+++ b/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)
 {
-        amagic += strcspn(amagic, " ");
-        bmagic += strcspn(bmagic, " ");
+        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;