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 @@ -2817,6 +2817,11 @@ 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) { @@ -2835,9 +2840,12 @@ 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("-");