linux-kernel - Re: [PATCH] rtw88: Fix out-of-bounds write

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Date:   Fri, 16 Jul 2021 17:08:32 +0200
From:   Len Baker <len.baker@....com>
To:     Brian Norris <briannorris@...omium.org>,
        Pkshih <pkshih@...ltek.com>
Cc:     Len Baker <len.baker@....com>,
        Yan-Hsuan Chuang <tony0620emma@...il.com>,
        Kalle Valo <kvalo@...eaurora.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Stanislaw Gruszka <sgruszka@...hat.com>,
        "linux-wireless@...r.kernel.org" <linux-wireless@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: Re: [PATCH] rtw88: Fix out-of-bounds write

On Mon, Jul 12, 2021 at 11:38:43AM -0700, Brian Norris wrote:
> On Sun, Jul 11, 2021 at 6:43 PM Pkshih <pkshih@...ltek.com> wrote:
> > > -----Original Message-----
> > > From: Len Baker [mailto:len.baker@....com]
> > >
> > > In the rtw_pci_init_rx_ring function the "if (len > TRX_BD_IDX_MASK)"
> > > statement guarantees that len is less than or equal to GENMASK(11, 0) or
> > > in other words that len is less than or equal to 4095. However the
> > > rx_ring->buf has a size of RTK_MAX_RX_DESC_NUM (defined as 512). This
> > > way it is possible an out-of-bounds write in the for statement due to
> > > the i variable can exceed the rx_ring->buff size.
> > >
> > > Fix it using the ARRAY_SIZE macro.
> > >
> > > Cc: stable@...r.kernel.org
> > > Addresses-Coverity-ID: 1461515 ("Out-of-bounds write")
>
> Coverity seems to be giving a false warning here. I presume it's
> taking the |len| comparison as proof that |len| might be as large as
> TRX_BD_IDX_MASK, but as noted below, that's not really true; the |len|
> comparison is really just dead code.

I agree.

> > > Fixes: e3037485c68ec ("rtw88: new Realtek 802.11ac driver")
> > > Signed-off-by: Len Baker <len.baker@....com>
>
> > To prevent the 'len' argument from exceeding the array size of rx_ring->buff, I
> > suggest to add another checking statement, like
> >
> >         if (len > ARRAY_SIZE(rx_ring->buf)) {
> >                 rtw_err(rtwdev, "len %d exceeds maximum RX ring buffer\n", len);
> >                 return -EINVAL;
> >         }
>
> That seems like a better idea, if we really need to patch anything.

I think it is reasonable to protect any potencial overflow (for example, if
this function is used in the future with a parameter greater than 512). It
is better to be defensive in this case :)

> > But, I wonder if this a false alarm because 'len' is equal to ARRAY_SIZE(rx_ring->buf)
> > for now.
>
> Or to the point: rtw_pci_init_rx_ring() is only ever called with a
> fixed constant -- RTK_MAX_RX_DESC_NUM (i.e., 512) -- so the alleged
> overflow cannot happen.
>
> Brian

I will send a v2 for review.

Thanks,
Len