MODSIGN: Move the magic string to the end of a module and eliminate the search

Emit the magic string that indicates a module has a signature after the
signature data instead of before it.  This allows module_sig_check() to
be made simpler and faster by the elimination of the search for the
magic string.  Instead we just need to do a single memcmp().

This works because at the end of the signature data there is the
fixed-length signature information block.  This block then falls
immediately prior to the magic number.

From the contents of the information block, it is trivial to calculate
the size of the signature data and thus the size of the actual module
data.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 9 insertions, 17 deletions

diff --git a/kernel/module.c b/kernel/module.c
index 0e2da86..6085f5e 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -2421,25 +2421,17 @@ static inline void kmemleak_load_module(const struct module *mod,
 
 #ifdef CONFIG_MODULE_SIG
 static int module_sig_check(struct load_info *info,
-			    const void *mod, unsigned long *len)
+			    const void *mod, unsigned long *_len)
 {
 	int err = -ENOKEY;
-	const unsigned long markerlen = sizeof(MODULE_SIG_STRING) - 1;
-	const void *p = mod, *end = mod + *len;
-
-	/* Poor man's memmem. */
-	while ((p = memchr(p, MODULE_SIG_STRING[0], end - p))) {
-		if (p + markerlen > end)
-			break;
-
-		if (memcmp(p, MODULE_SIG_STRING, markerlen) == 0) {
-			const void *sig = p + markerlen;
-			/* Truncate module up to signature. */
-			*len = p - mod;
-			err = mod_verify_sig(mod, *len, sig, end - sig);
-			break;
-		}
-		p++;
+	unsigned long markerlen = sizeof(MODULE_SIG_STRING) - 1;
+	unsigned long len = *_len;
+
+	if (len > markerlen &&
+	    memcmp(mod + len - markerlen, MODULE_SIG_STRING, markerlen) == 0) {
+		/* We truncate the module to discard the signature */
+		*_len -= markerlen;
+		err = mod_verify_sig(mod, _len);
 	}
 
 	if (!err) {