From fd77cb62207431d6dff0f17315ad169b3df4d9ef Mon Sep 17 00:00:00 2001 From: Mikulas Patocka Date: Mon, 25 Apr 2022 08:53:29 -0400 Subject: [PATCH] dm crypt: make printing of the key constant-time 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 Tested-by: Milan Broz Signed-off-by: Mike Snitzer Signed-off-by: Greg Kroah-Hartman --- drivers/md/dm-crypt.c | 14 +++++++++++--- 1 file changed, 11 insertions(+), 3 deletions(-) diff --git a/drivers/md/dm-crypt.c b/drivers/md/dm-crypt.c index 154139b..f30fd38 100644 --- a/drivers/md/dm-crypt.c +++ b/drivers/md/dm-crypt.c @@ -3435,6 +3435,11 @@ static int crypt_map(struct dm_target *ti, struct bio *bio) 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) { @@ -3453,9 +3458,12 @@ static void crypt_status(struct dm_target *ti, status_type_t type, 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("-"); -- 2.7.4