[09/27] xen/xenbus: Reject replies with payload > XENSTORE_PAYLOAD_MAX.

Greg KH
2012-01-23T23:45:58Z

From: Ian Campbell <Ian.Campbell@citrix.com>

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>

---
drivers/xen/xenbus/xenbus_xs.c | 6 ++++++
include/xen/interface/io/xs_wire.h | 3 +++
2 files changed, 9 insertions(+)

--- 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);
--- 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 */