linux-kernel - Re: [PATCH v2] rtw88: Fix out-of-bounds write

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Date:   Sat, 17 Jul 2021 15:33:43 +0200
From:   Len Baker <len.baker@....com>
To:     Greg KH <gregkh@...uxfoundation.org>,
        Brian Norris <briannorris@...omium.org>
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>,
        Pkshih <pkshih@...ltek.com>, linux-wireless@...r.kernel.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org
Subject: Re: [PATCH v2] rtw88: Fix out-of-bounds write

On Fri, Jul 16, 2021 at 07:20:48PM +0200, Greg KH wrote:
> On Fri, Jul 16, 2021 at 05:53:11PM +0200, Len Baker wrote:
> > 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.
> >
> > However, this overflow never happens due to the rtw_pci_init_rx_ring is
> > only ever called with a fixed constant of RTK_MAX_RX_DESC_NUM. But it is
> > better to be defensive in this case and add a new check to avoid
> > overflows if this function is called in a future with a value greater
> > than 512.
>
> If this can never happen, then no, this is not needed.

Then, if this can never happen, the current check would not be necessary
either.

> Why would you check twice for the same thing?

Ok, it makes no sense to double check the "len" variable twice. So, I
propose to modify the current check as follows:

diff --git a/drivers/net/wireless/realtek/rtw88/pci.c b/drivers/net/wireless/realtek/rtw88/pci.c
index e7d17ab8f113..0fd140523868 100644
--- a/drivers/net/wireless/realtek/rtw88/pci.c
+++ b/drivers/net/wireless/realtek/rtw88/pci.c
@@ -268,8 +268,8 @@ static int rtw_pci_init_rx_ring(struct rtw_dev *rtwdev,
        int i, allocated;
        int ret = 0;

-       if (len > TRX_BD_IDX_MASK) {
-               rtw_err(rtwdev, "len %d exceeds maximum RX entries\n", len);
+       if (len > ARRAY_SIZE(rx_ring->buf)) {
+               rtw_err(rtwdev, "len %d exceeds maximum RX ring buffer\n", len);
                return -EINVAL;
        }

This way the overflow can never happen with the current call to
rtw_pci_init_rx_ring function or with a future call with a "len" parameter
greater than 512. What do you think?

If there are no objections I will send a v3 for review.

Another question: If this can never happen should I include the "Fixes" tag,
"Addresses-Coverity-ID" tag and Cc to stable?

Thanks,
Len

>
> thanks,
>
> greg k-h