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

[Heinrich Schuchardt]

On 19.06.2014 01:50, Kees Cook wrote:
> On Wed, Jun 18, 2014 at 3:02 PM, Heinrich Schuchardt <xypron.glpk@xxxxxx> 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@xxxxxx>
> > ---
> >   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.

b[] and len are provided by the caller of dib9000_risc_apb_access_write.
dib9000_risc_apb_access_write cannot verify if the length of b[] matches 
len at all.

Currently dib9000_risc_apb_access_write cannot handle odd values of len. 
This holds even true if b[] has been padded with zero to an even length: 
For odd values of len the last byte is not passed to dib9000_mbx_send_attr.

What is left unclear is how odd values of len should be handled correctly:

Should the caller provide a b[] padded with 0 to the next even number of 
bytes,
or should dib9000_risc_apb_access_write take care not to read more then 
len bytes,
or should odd values of len cause an error EINVAL.

From what I read in the coding one source of the value of len is 
tuner_attach(), which is called from outside the dib9000 driver.

Heinrich

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

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