ima: avoid appraise error for hash calc interrupt

The integrity_kernel_read() call in ima_calc_file_hash_tfm() can return
a value of 0 before all bytes of the file are read. A value of 0 would
normally indicate an EOF. This has been observed if a user process is
causing a file appraisal and is terminated with a SIGTERM signal. The
most common occurrence of seeing the problem is if a shutdown or systemd
reload is initiated while files are being appraised.

The problem is similar to commit <f5e1040196db> (ima: always return
negative code for error) that fixed the problem in
ima_calc_file_hash_atfm().

Suggested-by: Mimi Zohar <zohar@linux.ibm.com>
Signed-off-by: Patrick Callaghan <patrickc@linux.ibm.com>
Reviewed-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>

security/integrity/ima/ima_crypto.c

@@ -362,8 +362,10 @@ static int ima_calc_file_hash_tfm(struct file *file,
 			rc = rbuf_len;
 			break;
 		}
-		if (rbuf_len == 0)
+		if (rbuf_len == 0) {	/* unexpected EOF */
+			rc = -EINVAL;
 			break;
+		}
 		offset += rbuf_len;
 
 		rc = crypto_shash_update(shash, rbuf, rbuf_len);