linux-kernel - [PATCH 5.10 41/53] dm crypt: make printing of the key constant-time

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-Id: <20220603173819.915827409@linuxfoundation.org>
Date:   Fri,  3 Jun 2022 19:43:26 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Mikulas Patocka <mpatocka@...hat.com>,
        Milan Broz <gmazyland@...il.com>,
        Mike Snitzer <snitzer@...nel.org>
Subject: [PATCH 5.10 41/53] dm crypt: make printing of the key constant-time

From: Mikulas Patocka <mpatocka@...hat.com>

commit 567dd8f34560fa221a6343729474536aa7ede4fd upstream.

The device mapper dm-crypt target is using scnprintf("%02x", cc->key[i]) to
report the current key to userspace. However, this is not a constant-time
operation and it may leak information about the key via timing, via cache
access patterns or via the branch predictor.

Change dm-crypt's key printing to use "%c" instead of "%02x". Also
introduce hex2asc() that carefully avoids any branching or memory
accesses when converting a number in the range 0 ... 15 to an ascii
character.

Cc: stable@...r.kernel.org
Signed-off-by: Mikulas Patocka <mpatocka@...hat.com>
Tested-by: Milan Broz <gmazyland@...il.com>
Signed-off-by: Mike Snitzer <snitzer@...nel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 drivers/md/dm-crypt.c |   14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

--- a/drivers/md/dm-crypt.c
+++ b/drivers/md/dm-crypt.c
@@ -3404,6 +3404,11 @@ static int crypt_map(struct dm_target *t
 	return DM_MAPIO_SUBMITTED;
 }
 
+static char hex2asc(unsigned char c)
+{
+	return c + '0' + ((unsigned)(9 - c) >> 4 & 0x27);
+}
+
 static void crypt_status(struct dm_target *ti, status_type_t type,
 			 unsigned status_flags, char *result, unsigned maxlen)
 {
@@ -3422,9 +3427,12 @@ static void crypt_status(struct dm_targe
 		if (cc->key_size > 0) {
 			if (cc->key_string)
 				DMEMIT(":%u:%s", cc->key_size, cc->key_string);
-			else
-				for (i = 0; i < cc->key_size; i++)
-					DMEMIT("%02x", cc->key[i]);
+			else {
+				for (i = 0; i < cc->key_size; i++) {
+					DMEMIT("%c%c", hex2asc(cc->key[i] >> 4),
+					       hex2asc(cc->key[i] & 0xf));
+				}
+			}
 		} else
 			DMEMIT("-");