linux-kernel - Re: [PATCH 1/1] media: dib9000: avoid out of bound access

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Date:	Wed, 18 Jun 2014 16:50:09 -0700
From:	Kees Cook <keescook@...omium.org>
To:	Heinrich Schuchardt <xypron.glpk@....de>
Cc:	Mauro Carvalho Chehab <m.chehab@...sung.com>,
	Michael Krufky <mkrufky@...uxtv.org>,
	linux-media@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/1] media: dib9000: avoid out of bound access

On Wed, Jun 18, 2014 at 3:02 PM, Heinrich Schuchardt <xypron.glpk@....de> wrote:
> The current test to avoid out of bound access to mb[] is insufficient.
> For len = 19 non-existent mb[10] will be accessed.
>
> A check in the for loop is insufficient to avoid out of bound access in
> dib9000_mbx_send_attr.
>
> Signed-off-by: Heinrich Schuchardt <xypron.glpk@....de>
> ---
>  drivers/media/dvb-frontends/dib9000.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/media/dvb-frontends/dib9000.c b/drivers/media/dvb-frontends/dib9000.c
> index e540cfb..6a71917 100644
> --- a/drivers/media/dvb-frontends/dib9000.c
> +++ b/drivers/media/dvb-frontends/dib9000.c
> @@ -1040,10 +1040,13 @@ static int dib9000_risc_apb_access_write(struct dib9000_state *state, u32 addres
>         if (address >= 1024 || !state->platform.risc.fw_is_running)
>                 return -EINVAL;
>
> +       if (len > 18)
> +               return -EINVAL;
> +
>         /* dprintk( "APB access thru wr fw %d %x", address, attribute); */
>
>         mb[0] = (unsigned short)address;
> -       for (i = 0; i < len && i < 20; i += 2)
> +       for (i = 0; i < len; i += 2)
>                 mb[1 + (i / 2)] = (b[i] << 8 | b[i + 1]);

Good catch on the mb[] access! However, I think there is still more to
fix since b[i + 1] can read past the end of b: Say b is defined as "u8
b[3]". Passing len 3 means the second loop, with i==2 will access b[2]
and b[3], the latter is out of range.

-Kees

>
>         dib9000_mbx_send_attr(state, OUT_MSG_BRIDGE_APB_W, mb, 1 + len / 2, attribute);
> --
> 2.0.0
>



-- 
Kees Cook
Chrome OS Security
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