Re: [PATCH 03/10] b43: Remove uninitialized_var() usage

[Nick Desaulniers <ndesaulniers@xxxxxxxxxx>]

On Thu, Jun 4, 2020 at 1:18 PM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>
> On Thu, Jun 04, 2020 at 01:08:44PM -0700, Nick Desaulniers wrote:
> > On Wed, Jun 3, 2020 at 4:32 PM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> > >
> > > Using uninitialized_var() is dangerous as it papers over real bugs[1]
> > > (or can in the future), and suppresses unrelated compiler warnings (e.g.
> > > "unused variable"). If the compiler thinks it is uninitialized, either
> > > simply initialize the variable or make compiler changes. As a precursor
> > > to removing[2] this[3] macro[4], just initialize this variable to NULL,
> > > and make the (unreachable!) code do a conditional test.
> > >
> > > [1] https://lore.kernel.org/lkml/20200603174714.192027-1-glider@xxxxxxxxxx/
> > > [2] https://lore.kernel.org/lkml/CA+55aFw+Vbj0i=1TGqCR5vQkCzWJ0QxK6CernOU6eedsudAixw@xxxxxxxxxxxxxx/
> > > [3] https://lore.kernel.org/lkml/CA+55aFwgbgqhbp1fkxvRKEpzyR5J8n1vKT1VZdz9knmPuXhOeg@xxxxxxxxxxxxxx/
> > > [4] https://lore.kernel.org/lkml/CA+55aFz2500WfbKXAx8s67wrm9=yVJu65TpLgN_ybYNv0VEOKA@xxxxxxxxxxxxxx/
> > >
> > > Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> > > ---
> > >  drivers/net/wireless/broadcom/b43/phy_n.c | 10 +++++++---
> > >  1 file changed, 7 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/drivers/net/wireless/broadcom/b43/phy_n.c b/drivers/net/wireless/broadcom/b43/phy_n.c
> > > index d3c001fa8eb4..88cdcea10d61 100644
> > > --- a/drivers/net/wireless/broadcom/b43/phy_n.c
> > > +++ b/drivers/net/wireless/broadcom/b43/phy_n.c
> > > @@ -4222,7 +4222,7 @@ static void b43_nphy_tx_gain_table_upload(struct b43_wldev *dev)
> >
> > The TODOs and `#if 0` in this function are concerning.  It looks like
> > `rf_pwr_offset_table` is only used when `phy->rev` is >=7 && < 19.
> >
> > Further, the loop has a case for `phy->rev >= 19` but we would have
> > returned earlier if that was the case.

oh, and there's an early return for `phy->rev < 3` I just noticed.

>
> Yeah, that's why I put the "(unreachable!)" note in the commit log. ;)

I don't think that note is correct.

>
> >
> > >         u32 rfpwr_offset;
> > >         u8 pga_gain, pad_gain;
> > >         int i;
> > > -       const s16 *uninitialized_var(rf_pwr_offset_table);
> > > +       const s16 *rf_pwr_offset_table = NULL;
> > >
> > >         table = b43_nphy_get_tx_gain_table(dev);
> > >         if (!table)
> > > @@ -4256,9 +4256,13 @@ static void b43_nphy_tx_gain_table_upload(struct b43_wldev *dev)
> > >                         pga_gain = (table[i] >> 24) & 0xf;
> > >                         pad_gain = (table[i] >> 19) & 0x1f;
> > >                         if (b43_current_band(dev->wl) == NL80211_BAND_2GHZ)
> > > -                               rfpwr_offset = rf_pwr_offset_table[pad_gain];
> > > +                               rfpwr_offset = rf_pwr_offset_table
> > > +                                               ? rf_pwr_offset_table[pad_gain]
> > > +                                               : 0;
> > >                         else
> > > -                               rfpwr_offset = rf_pwr_offset_table[pga_gain];
> > > +                               rfpwr_offset = rf_pwr_offset_table
> > > +                                               ? rf_pwr_offset_table[pga_gain]
> > > +                                               : 0;
> >
> >
> > The code is trying to check `phy->rev >= 7 && phy->rev < 19` once
> > before the loop, then set `rf_pwr_offset_table`, so having another
> > conditional on `rf_pwr_offset_table` in the loop is unnecessary. I'm
> > ok with initializing it to `NULL`, but I'm not sure the conditional
> > check is necessary.  Do you get a compiler warning otherwise?
>
> I mean, sort of the best thing to do is just remove nearly everything
> here since it's actually unreachable. But it is commented as "when

This code is reachable. Consider `phy->rev >= 7 && phy->rev < 19`.  If
`rf_pwr_offset_table` was NULL, it would have returned early on L4246,
so the checks added in this patch are unnecessary.  Forgive me if
there's some other control flow I'm not considering.

> supported ..." etc, so I figured I'd leave it. As part of that I didn't
> want to leave any chance of a NULL deref, so I added the explicit tests
> just for robustness.
>
> *shrug*
-- 
Thanks,
~Nick Desaulniers