[PATCH 5.17 50/75] dm crypt: make printing of the key constant-time

Greg Kroah-Hartman
2022-06-03T18:11:47Z

From: Mikulas Patocka <mpatocka@redhat.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@vger.kernel.org
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Tested-by: Milan Broz <gmazyland@gmail.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.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
@@ -3449,6 +3449,11 @@ 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)
{
@@ -3467,9 +3472,12 @@ 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("-");