xen/xenbus: Reject replies with payload > XENSTORE_PAYLOAD_MAX.

commit 9e7860cee18241633eddb36a4c34c7b61d8cecbc upstream.

Haogang Chen found out that:

 There is a potential integer overflow in process_msg() that could result
 in cross-domain attack.

 	body = kmalloc(msg->hdr.len + 1, GFP_NOIO | __GFP_HIGH);

 When a malicious guest passes 0xffffffff in msg->hdr.len, the subsequent
 call to xb_read() would write to a zero-length buffer.

 The other end of this connection is always the xenstore backend daemon
 so there is no guest (malicious or otherwise) which can do this. The
 xenstore daemon is a trusted component in the system.

 However this seem like a reasonable robustness improvement so we should
 have it.

And Ian when read the API docs found that:
        The payload length (len field of the header) is limited to 4096
        (XENSTORE_PAYLOAD_MAX) in both directions.  If a client exceeds the
        limit, its xenstored connection will be immediately killed by
        xenstored, which is usually catastrophic from the client's point of
        view.  Clients (particularly domains, which cannot just reconnect)
        should avoid this.

so this patch checks against that instead.

This also avoids a potential integer overflow pointed out by Haogang Chen.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Cc: Haogang Chen <haogangchen@gmail.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

diff --git a/drivers/xen/xenbus/xenbus_xs.c b/drivers/xen/xenbus/xenbus_xs.c
index 7b547f53f65ee9267f19dd493a6ab7321310ed4c..54226d2f2cfeed73912c2ce5019232656c500f71 100644 (file)
--- a/drivers/xen/xenbus/xenbus_xs.c
+++ b/drivers/xen/xenbus/xenbus_xs.c
@@ -766,6 +766,12 @@ static int process_msg(void)
                goto out;
        }
 
+       if (msg->hdr.len > XENSTORE_PAYLOAD_MAX) {
+               kfree(msg);
+               err = -EINVAL;
+               goto out;
+       }
+
        body = kmalloc(msg->hdr.len + 1, GFP_NOIO | __GFP_HIGH);
        if (body == NULL) {
                kfree(msg);

diff --git a/include/xen/interface/io/xs_wire.h b/include/xen/interface/io/xs_wire.h
index 99fcffb372d15fb2580adfc4759ff48ee9b570c9..454ee262923897a0f678fb37abd95f96b342b7b3 100644 (file)
--- a/include/xen/interface/io/xs_wire.h
+++ b/include/xen/interface/io/xs_wire.h
@@ -84,4 +84,7 @@ struct xenstore_domain_interface {
     XENSTORE_RING_IDX rsp_cons, rsp_prod;
 };
 
+/* Violating this is very bad.  See docs/misc/xenstore.txt. */
+#define XENSTORE_PAYLOAD_MAX 4096
+
 #endif /* _XS_WIRE_H */