xenstored file descriptor leak

[Manuel Bouyer <bouyer@antioche.eu.org>]

Hello,
on NetBSD I'm tracking down an issue where xenstored never closes its
file descriptor to /var/run/xenstored/socket and instead loops at 100%
CPU on these descriptors.

xenstored loops because poll(2) returns a POLLIN event for these
descriptors but they are marked is_ignored = true. 

I'm seeing this with xen 4.15, 4.13 and has also been reported with 4.11
with latest security patches.
It seems to have started with patches for XSA-115 (A user reported this
for 4.11)

I've tracked it down to a difference in poll(2) implementation; it seems
that linux will return something that is not (POLLIN|POLLOUT) when a
socket if closed; while NetBSD returns POLLIN (this matches the NetBSD's
man page).

First I think there may be a security issue here, as, even on linux it should
be possible for a client to cause a socket to go to the is_ignored state,
while still sending data and cause xenstored to go to a 100% CPU loop.
I think this is needed anyway:

--- xenstored_core.c.orig	2021-02-02 18:06:33.389316841 +0100
+++ xenstored_core.c	2021-02-02 19:27:43.761877371 +0100
@@ -397,9 +397,12 @@
 			     !list_empty(&conn->out_list)))
 				*ptimeout = 0;
 		} else {
-			short events = POLLIN|POLLPRI;
-			if (!list_empty(&conn->out_list))
-				events |= POLLOUT;
+			short events = 0;
+			if (!conn->is_ignored) {
+				events |= POLLIN|POLLPRI;
+			        if (!list_empty(&conn->out_list))
+				        events |= POLLOUT;
+			}
 			conn->pollfd_idx = set_fd(conn->fd, events);
 		}
 	}

Now I wonder if, on NetBSD at last, a read error or short read shouldn't
cause the socket to be closed, as with:

@@ -1561,6 +1565,8 @@
 
 bad_client:
 	ignore_connection(conn);
+	/* we don't want to keep this connection alive */
+	talloc_free(conn);
 }
 
 static void handle_output(struct connection *conn)


what do you think ?

-- 
Manuel Bouyer <bouyer@antioche.eu.org>
     NetBSD: 26 ans d'experience feront toujours la difference
--