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dm crypt: make printing of the key constant-time

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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>
This commit is contained in:
Mikulas Patocka 2022-04-25 08:53:29 -04:00 committed by Mike Snitzer
parent d3f2a14b89
commit 567dd8f345
1 changed files with 11 additions and 3 deletions
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14
drivers/md/dm-crypt.c
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@ -3439,6 +3439,11 @@ 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)
{
@ -3457,9 +3462,12 @@ 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("-");