net/9p: insulate the client against an invalid error code sent by a 9p server

A looney tunes server sending an invalid error code (which is !IS_ERR_VALUE)
can result in a client oops. So fix it by adding a check and converting unknown
or invalid error codes to -ESERVERFAULT.
Signed-off-by: Abhishek Kulkarni <adkulkar@umail.iu.edu>
Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>

net/9p/client.c

@@ -411,14 +411,9 @@ static int p9_check_errors(struct p9_client *c, struct p9_req_t *req)
 		if (c->dotu)
 			err = -ecode;
 
-		if (!err) {
+		if (!err || !IS_ERR_VALUE(err))
 			err = p9_errstr2errno(ename, strlen(ename));
 
-			/* string match failed */
-			if (!err)
-				err = -ESERVERFAULT;
-		}
-
 		P9_DPRINTK(P9_DEBUG_9P, "<<< RERROR (%d) %s\n", -ecode, ename);
 
 		kfree(ename);

net/9p/error.c

@@ -239,7 +239,7 @@ int p9_errstr2errno(char *errstr, int len)
 		errstr[len] = 0;
 		printk(KERN_ERR "%s: server reported unknown error %s\n",
 			__func__, errstr);
-		errno = 1;
+		errno = ESERVERFAULT;
 	}
 
 	return -errno;