dm crypt: fix benbi IV constructor crash if used in authenticated mode

If benbi IV is used in AEAD construction, for example:
  cryptsetup luksFormat <device> --cipher twofish-xts-benbi --key-size 512 --integrity=hmac-sha256
the constructor uses wrong skcipher function and crashes:

 BUG: kernel NULL pointer dereference, address: 00000014
 ...
 EIP: crypt_iv_benbi_ctr+0x15/0x70 [dm_crypt]
 Call Trace:
  ? crypt_subkey_size+0x20/0x20 [dm_crypt]
  crypt_ctr+0x567/0xfc0 [dm_crypt]
  dm_table_add_target+0x15f/0x340 [dm_mod]

Fix this by properly using crypt_aead_blocksize() in this case.

Fixes: ef43aa3806 ("dm crypt: add cryptographic data integrity protection (authenticated encryption)")
Cc: stable@vger.kernel.org # v4.12+
Link: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=941051
Reported-by: Jerad Simpson <jbsimpson@gmail.com>
Signed-off-by: Milan Broz <gmazyland@gmail.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-crypt.c

@@ -343,8 +343,14 @@ static int crypt_iv_essiv_gen(struct crypt_config *cc, u8 *iv,
 static int crypt_iv_benbi_ctr(struct crypt_config *cc, struct dm_target *ti,
 			      const char *opts)
 {
-	unsigned bs = crypto_skcipher_blocksize(any_tfm(cc));
-	int log = ilog2(bs);
+	unsigned bs;
+	int log;
+
+	if (test_bit(CRYPT_MODE_INTEGRITY_AEAD, &cc->cipher_flags))
+		bs = crypto_aead_blocksize(any_tfm_aead(cc));
+	else
+		bs = crypto_skcipher_blocksize(any_tfm(cc));
+	log = ilog2(bs);
 
 	/* we need to calculate how far we must shift the sector count
 	 * to get the cipher block count, we use this shift in _gen */