Re: [Qemu-devel] [PATCH v5 17/24] ssi-sd: fix buffer overrun on invalid state load

[Peter Maydell]

On 3 April 2014 17:52, Michael S. Tsirkin <address@hidden> wrote:
> CVE-2013-4537
>
> s->arglen is taken from wire and used as idx
> in ssi_sd_transfer().
>
> Validate it before access.
>
> Signed-off-by: Michael S. Tsirkin <address@hidden>
> ---
>  hw/sd/ssi-sd.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
>
> diff --git a/hw/sd/ssi-sd.c b/hw/sd/ssi-sd.c
> index 3273c8a..2fa2b2b 100644
> --- a/hw/sd/ssi-sd.c
> +++ b/hw/sd/ssi-sd.c
> @@ -230,6 +230,14 @@ static int ssi_sd_load(QEMUFile *f, void *opaque, int 
> version_id)
>      for (i = 0; i < 5; i++)
>          s->response[i] = qemu_get_be32(f);
>      s->arglen = qemu_get_be32(f);
> +    if (s->mode == SSI_SD_CMDARG &&
> +        (s->arglen < 0 || s->arglen > ARRAY_SIZE(s->cmdarg))) {
> +        return -EINVAL;
> +    }
> +    if (s->mode == SSI_SD_RESPONSE &&
> +        (s->response_pos < 0 || s->response_pos > ARRAY_SIZE(s->response))) {
> +        return -EINVAL;
> +    }
>      s->response_pos = qemu_get_be32(f);
>      s->stopping = qemu_get_be32(f);

Surely we should read s->response_pos off the wire before
validating it rather than after?

Also, your checks on arglen aren't sufficient. Consider
the case where the attacker:
 * sets mode to SSI_SD_RESPONSE
 * sets arglen to something huge
 * sets response_pos to 0
 * gets the guest to repeatedly provoke calls to ssi_sd_transfer

We'll happily read off the end of the s->response[] buffer,
because our "when do we stop returning response bytes" check
is "s->response_pos >= s->arglen".

thanks
-- PMM