Linux Virtualization - Jan 2012 - [PATCH] XEN: xenbus: integer overflow in process_msg()

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. This causes
kernel oops in the receiving guest and hangs its xenbus kernel thread.
The patch returns -EINVAL in that case.

Signed-off-by: Haogang Chen <haogangchen at gmail.com>
---
 drivers/xen/xenbus/xenbus_xs.c |    6 ++++++
 1 files changed, 6 insertions(+), 0 deletions(-)

diff --git a/drivers/xen/xenbus/xenbus_xs.c b/drivers/xen/xenbus/xenbus_xs.c
index ede860f..e32aefb 100644
--- a/drivers/xen/xenbus/xenbus_xs.c
+++ b/drivers/xen/xenbus/xenbus_xs.c
@@ -801,6 +801,12 @@ static int process_msg(void)
 		goto out;
 	}
 
+	if (msg->hdr.len == UINT_MAX) {
+		kfree(msg);
+		err = -EINVAL;
+		goto out;
+	}
+
 	body = kmalloc(msg->hdr.len + 1, GFP_NOIO | __GFP_HIGH);
 	if (body == NULL) {
 		kfree(msg);
-- 
1.7.5.4

On Tue, 2012-01-03 at 19:42 +0000, Haogang Chen wrote:
> 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.
> This causes
> kernel oops in the receiving guest and hangs its xenbus kernel thread.
> The patch returns -EINVAL in that case.
> 
> Signed-off-by: Haogang Chen <haogangchen at gmail.com>
Acked-by: Ian Campbell <ian.campbell at citrix.com>
> ---
>  drivers/xen/xenbus/xenbus_xs.c |    6 ++++++
>  1 files changed, 6 insertions(+), 0 deletions(-)
> 
> diff --git a/drivers/xen/xenbus/xenbus_xs.c
b/drivers/xen/xenbus/xenbus_xs.c
> index ede860f..e32aefb 100644
> --- a/drivers/xen/xenbus/xenbus_xs.c
> +++ b/drivers/xen/xenbus/xenbus_xs.c
> @@ -801,6 +801,12 @@ static int process_msg(void)
>  		goto out;
>  	}
>  
> +	if (msg->hdr.len == UINT_MAX) {
> +		kfree(msg);
> +		err = -EINVAL;
> +		goto out;
> +	}
> +
>  	body = kmalloc(msg->hdr.len + 1, GFP_NOIO | __GFP_HIGH);
>  	if (body == NULL) {
>  		kfree(msg);