Re: [Xen-devel] [PATCH] XEN: xenbus: integer overflow in process_msg()

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

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@xxxxxxxxx>

Acked-by: Ian Campbell <ian.campbell@xxxxxxxxxx>

> ---
>  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);



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