ima: Fix use after free in ima_read_modsig()

If we can't parse the PKCS7 in the appended modsig, we will free the modsig
structure and then access one of its members to determine the error value.

Fixes: 39b07096364a ("ima: Implement support for module-style appended signatures")
Reported-by: kbuild test robot <[e-mail]>
Reported-by: Julia Lawall <[e-mail]>
Reported-by: Dan Carpenter <[e-mail]>
Signed-off-by: Thiago Jung Bauermann <[e-mail]>
Reviewed-by: Gustavo A. R. Silva <[e-mail]>
Signed-off-by: Mimi Zohar <[e-mail]>

security/integrity/ima/ima_modsig.c

···
 
 	hdr->pkcs7_msg = pkcs7_parse_message(buf + buf_len, sig_len);
 	if (IS_ERR(hdr->pkcs7_msg)) {
+		rc = PTR_ERR(hdr->pkcs7_msg);
 		kfree(hdr);
-		return PTR_ERR(hdr->pkcs7_msg);
+		return rc;
 	}
 
 	memcpy(hdr->raw_pkcs7, buf + buf_len, sig_len);