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

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Date:   Mon, 12 Jul 2021 11:38:43 -0700
From:   Brian Norris <briannorris@...omium.org>
To:     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 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.

> > 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.

> 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