Re: [PATCH 0/2] module: some refactoring in module_sig_check()

[Joe Perches <joe@perches.com>]

On Tue, 2020-10-13 at 23:32 +0300, Sergey Shtylyov wrote:
> Here are 2 patches against the 'modules-next' branch of Jessica Yu's 'linux.git' repo.
> I'm doing some little refactoring in module_sig_check()...
> 
> [1/2] module: merge repetitive strings in module_sig_check()
> [2/2] module: unindent comments in module_sig_check()

I think this code is rather cryptic and could be made clearer.

How about:
---
 kernel/module.c | 51 ++++++++++++++++++++++++++-------------------------
 1 file changed, 26 insertions(+), 25 deletions(-)

diff --git a/kernel/module.c b/kernel/module.c
index c075a18103fb..98b3af96a5bd 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -2884,46 +2884,47 @@ static int module_sig_check(struct load_info *info, int flags)
 	 * Require flags == 0, as a module with version information
 	 * removed is no longer the module that was signed
 	 */
-	if (flags == 0 &&
-	    info->len > markerlen &&
-	    memcmp(mod + info->len - markerlen, MODULE_SIG_STRING, markerlen) == 0) {
+	if (flags == 0 && info->len > markerlen &&
+	    !memcmp(mod + info->len - markerlen, MODULE_SIG_STRING, markerlen)) {
 		/* We truncate the module to discard the signature */
 		info->len -= markerlen;
 		err = mod_verify_sig(mod, info);
+		if (!err) {
+			info->sig_ok = true;
+			return 0;
+		}
 	}
 
+	/*
+	 * We don't permit modules to be loaded into trusted kernels
+	 * without a valid signature on them, but if we're not enforcing,
+	 * some errors are non-fatal.
+	 */
 	switch (err) {
-	case 0:
-		info->sig_ok = true;
-		return 0;
-
-		/* We don't permit modules to be loaded into trusted kernels
-		 * without a valid signature on them, but if we're not
-		 * enforcing, certain errors are non-fatal.
-		 */
 	case -ENODATA:
-		reason = "Loading of unsigned module";
-		goto decide;
+		reason = "unsigned module";
+		break;
 	case -ENOPKG:
-		reason = "Loading of module with unsupported crypto";
-		goto decide;
+		reason = "module with unsupported crypto";
+		break;
 	case -ENOKEY:
-		reason = "Loading of module with unavailable key";
-	decide:
-		if (is_module_sig_enforced()) {
-			pr_notice("%s: %s is rejected\n", info->name, reason);
-			return -EKEYREJECTED;
-		}
-
-		return security_locked_down(LOCKDOWN_MODULE_SIGNATURE);
-
+		reason = "module with unavailable key";
+		break;
+	default:
 		/* All other errors are fatal, including nomem, unparseable
 		 * signatures and signature check failures - even if signatures
 		 * aren't required.
 		 */
-	default:
 		return err;
 	}
+
+	if (is_module_sig_enforced()) {
+		pr_notice("%s: loading of %s is rejected\n",
+			  info->name, reason);
+		return -EKEYREJECTED;
+	}
+
+	return security_locked_down(LOCKDOWN_MODULE_SIGNATURE);
 }
 #else /* !CONFIG_MODULE_SIG */
 static int module_sig_check(struct load_info *info, int flags)