[dm-devel] [PATCH] dm-crypt: make printing of the key constant-time

[Mikulas Patocka]

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

This patch changes it to use "%c" instead. We introduce a function
hex2asc. hex2asc converts a number in the range 0 ... 15 to an ascii
character and it is coded in such a way that it contains no branches and
no memory accesses.

Signed-off-by: Mikulas Patocka <mpatocka at redhat.com>
Cc; stable at vger.kernel.org

---
 drivers/md/dm-crypt.c |    7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

Index: linux-2.6/drivers/md/dm-crypt.c
===================================================================
--- linux-2.6.orig/drivers/md/dm-crypt.c	2022-04-24 19:44:14.000000000 +0200
+++ linux-2.6/drivers/md/dm-crypt.c	2022-04-24 19:54:13.000000000 +0200
@@ -3439,6 +3439,11 @@ static int crypt_map(struct dm_target *t
 	return DM_MAPIO_SUBMITTED;
 }
 
+static char hex2asc(unsigned char c)
+{
+	return c + '0' + ((9 - c) >> 4 & 0x27);
+}
+
 static void crypt_status(struct dm_target *ti, status_type_t type,
 			 unsigned status_flags, char *result, unsigned maxlen)
 {
@@ -3459,7 +3464,7 @@ static void crypt_status(struct dm_targe
 				DMEMIT(":%u:%s", cc->key_size, cc->key_string);
 			else
 				for (i = 0; i < cc->key_size; i++)
-					DMEMIT("%02x", cc->key[i]);
+					DMEMIT("%c%c", hex2asc(cc->key[i] >> 4), hex2asc(cc->key[i] & 0xf));
 		} else
 			DMEMIT("-");