Re: xenstored file descriptor leak

[Jürgen Groß]

On 02.02.21 19:37, Manuel Bouyer wrote:

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).

Yeah, Linux seems to return POLLHUP additionally.

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.

No security issue, as sockets are restricted to dom0 and user root.

In case you are suspecting a security issue, please don't send such
mails to xen-devel in future, but to secur...@lists.xenproject.org.

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);
                }
        }

Yes, I think this is a good idea.

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 @@

        ignore_connection(conn);
+       /* we don't want to keep this connection alive */
+       talloc_free(conn);
  }

bad_client:

This is wrong for non-socket connections, as we want to keep the domain
in question to be known to xenstored.

For socket connections this should be okay, though.


Juergen

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