[PATCH] MODSIGN: Discard previous signature when signing modules

[PATCH] MODSIGN: Discard previous signature when signing modules

[Michal Marek]

The format only supports one signature, so discard any previous
signature before signing the module.

Signed-off-by: Michal Marek <mmarek@suse.cz>
---
 scripts/sign-file |   62 ++++++++++++++++++++++++++++++++++++----------------
 1 files changed, 43 insertions(+), 19 deletions(-)

diff --git a/scripts/sign-file b/scripts/sign-file
index 2b7c448..b390dc3 100755
--- a/scripts/sign-file
+++ b/scripts/sign-file
@@ -60,6 +60,23 @@ sub read_file($)
     return $contents;
 }
 
+sub openssl_pipe($$) {
+	my ($input, $cmd) = @_;
+	my ($pid, $res);
+
+	$pid = open2(*read_from, *write_to, $cmd) || die $cmd;
+	binmode write_to;
+	print write_to $input || die "pipe to $cmd";
+	close(write_to) || die "pipe to $cmd";
+
+	binmode read_from;
+	read(read_from, $res, 4096) || die "pipe from $cmd";
+	close(read_from) || die "pipe from $cmd";
+	waitpid($pid, 0) || die;
+	die "$cmd died: $?" if ($? >> 8);
+	return $res;
+}
+
 ###############################################################################
 #
 # First of all, we have to parse the X.509 certificate to find certain details
@@ -350,6 +367,24 @@ if ($dgst eq "sha1") {
     die "Unknown hash algorithm: $dgst\n";
 }
 
+my $unsigned_module = read_file($module);
+
+my $magic_number = "~Module signature appended~\n";
+my $magic_len = length($magic_number);
+my $info_len = 12;
+
+# Truncate existing signarure, if any
+if (substr($unsigned_module, -$magic_len) eq $magic_number) {
+	my $info = substr($unsigned_module, -$magic_len - $info_len, $info_len);
+	my ($name_len, $key_len, $sig_len) = unpack("xxxCCxxxN", $info);
+	my $subtract = $name_len + $key_len + $sig_len + $info_len + $magic_len;
+	if ($subtract > length($unsigned_module)) {
+		die "$module: Existing signature is malformed\n";
+	}
+	$unsigned_module = substr($unsigned_module, 0,
+		length($unsigned_module) - $subtract);
+}
+
 my $signature;
 if ($signature_file) {
 	$signature = read_file($signature_file);
@@ -357,41 +392,30 @@ if ($signature_file) {
 	#
 	# Generate the digest and read from openssl's stdout
 	#
-	my $digest;
-	$digest = readpipe("openssl dgst -$dgst -binary $module") || die "openssl dgst";
+	my $digest = openssl_pipe($unsigned_module,
+		"openssl dgst -$dgst -binary");
 
 	#
 	# Generate the binary signature, which will be just the integer that
 	# comprises the signature with no metadata attached.
 	#
-	my $pid;
-	$pid = open2(*read_from, *write_to,
-		     "openssl rsautl -sign -inkey $private_key -keyform PEM") ||
-	    die "openssl rsautl";
-	binmode write_to;
-	print write_to $prologue . $digest || die "pipe to openssl rsautl";
-	close(write_to) || die "pipe to openssl rsautl";
-
-	binmode read_from;
-	read(read_from, $signature, 4096) || die "pipe from openssl rsautl";
-	close(read_from) || die "pipe from openssl rsautl";
-	waitpid($pid, 0) || die;
-	die "openssl rsautl died: $?" if ($? >> 8);
+	$signature = openssl_pipe($prologue . $digest,
+		     "openssl rsautl -sign -inkey $private_key -keyform PEM");
 }
 $signature = pack("n", length($signature)) . $signature,
 
 #
 # Build the signed binary
 #
-my $unsigned_module = read_file($module);
-
-my $magic_number = "~Module signature appended~\n";
-
 my $info = pack("CCCCCxxxN",
 		$algo, $hash, $id_type,
 		length($signers_name),
 		length($key_identifier),
 		length($signature));
+# Make sure that $info_len value used above matches reality
+if (length($info) != $info_len) {
+	die "Signature info size changed ($info_len -> @{[length($info)]}";
+}
 
 if ($verbose) {
     print "Size of unsigned module: ", length($unsigned_module), "\n";
-- 
1.7.3.1

Re: [PATCH] MODSIGN: Discard previous signature when signing modules

[David Howells]

Michal Marek <mmarek@suse.cz> wrote:

> The format only supports one signature, so discard any previous
> signature before signing the module.

That's not totally true.  The format does not preclude multiple signatures.
You can just add another signature block on the end that signs everything
inside of that, including all previous signatures.  The alteration to the code
to check all of them would be very small, I think.

David

Re: [PATCH] MODSIGN: Discard previous signature when signing modules

[Michal Marek]

On 27.3.2013 00:34, David Howells wrote:
> Michal Marek <mmarek@suse.cz> wrote:
> 
>> The format only supports one signature, so discard any previous
>> signature before signing the module.
> 
> That's not totally true.  The format does not preclude multiple signatures.
> You can just add another signature block on the end that signs everything
> inside of that, including all previous signatures.  The alteration to the code
> to check all of them would be very small, I think.

The question is if this would be useful and desired, as opposed to
supporting a list of signatures that all sign the *same* message. My bet
is that such chain of signatures is more likely going to indicate error
than intention. But I don't care much, I haven't run into this myself, I
just thought that others migh do.

Michal