FreeBSD: disable the use of hardware crypto offload drivers for now

First, the crypto request completion handler contains a bug in that it
fails to reset fs_done correctly after the request is completed.  This
is only a problem for asynchronous drivers.  Second, some hardware
drivers have input constraints which ZFS does not satisfy.  For
instance, ccp(4) apparently requires the AAD length for AES-GCM to be a
multiple of the cipher block size, and with qat(4) the AES-GCM AAD
length may not be longer than 240 bytes.  FreeBSD's generic crypto
framework doesn't have a mechanism to automatically fall back to a
software implementation if a hardware driver cannot process a request,
and ZFS does not tolerate such errors.

The plan is to implement such a fallback mechanism, but with FreeBSD
13.0 approaching we should simply disable the use hardware drivers for
now.

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Mark Johnston <markj@FreeBSD.org>
Closes #11612

module/os/freebsd/zfs/crypto_os.c

@@ -293,8 +293,19 @@ freebsd_crypt_newsession(freebsd_crypt_session_t *sessp,
 		error = ENOTSUP;
 		goto bad;
 	}
-	error = crypto_newsession(&sessp->fs_sid, &csp,
+
-	    CRYPTOCAP_F_HARDWARE | CRYPTOCAP_F_SOFTWARE);
+	/*
+	 * Disable the use of hardware drivers on FreeBSD 13 and later since
+	 * common crypto offload drivers impose constraints on AES-GCM AAD
+	 * lengths that make them unusable for ZFS, and we currently do not have
+	 * a mechanism to fall back to a software driver for requests not
+	 * handled by a hardware driver.
+	 *
+	 * On 12 we continue to permit the use of hardware drivers since
+	 * CPU-accelerated drivers such as aesni(4) register themselves as
+	 * hardware drivers.
+	 */
+	error = crypto_newsession(&sessp->fs_sid, &csp, CRYPTOCAP_F_SOFTWARE);
 	mtx_init(&sessp->fs_lock, "FreeBSD Cryptographic Session Lock",
 	    NULL, MTX_DEF);
 	crypt_sessions++;