netdev - Re: [BUG] mwifiex: possible null-pointer dereference in mwifiex_dnld_cmd_to

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Date:   Mon, 2 Aug 2021 13:44:41 -0700
From:   Brian Norris <briannorris@...omium.org>
To:     Li Tuo <islituo@...il.com>
Cc:     amit karwar <amitkarwar@...il.com>,
        Ganapathi Bhat <ganapathi017@...il.com>,
        Sharvari Harisangam <sharvari.harisangam@....com>,
        Xinming Hu <huxinming820@...il.com>,
        Kalle Valo <kvalo@...eaurora.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        linux-wireless <linux-wireless@...r.kernel.org>,
        "<netdev@...r.kernel.org>" <netdev@...r.kernel.org>,
        Linux Kernel <linux-kernel@...r.kernel.org>,
        baijiaju1990@...il.com
Subject: Re: [BUG] mwifiex: possible null-pointer dereference in mwifiex_dnld_cmd_to_fw()

Hi,

On Fri, Jul 30, 2021 at 9:13 PM Li Tuo <islituo@...il.com> wrote:
> Our static analysis tool finds a possible null-pointer dereference in
> the mwifiex driver in Linux 5.14.0-rc3:

Wouldn't be the first time a static analysis tool tripped up over
excessively redundant "safety" checks :)

For example:
https://lore.kernel.org/linux-wireless/20210731163546.10753-1-len.baker@gmx.com/T/#u

> The variable cmd_node->cmd_skb->data is assigned to the variable
> host_cmd, and host_cmd is checked in:
> 190:    if (host_cmd == NULL || host_cmd->size == 0)
>
> This indicates that host_cmd can be NULL.
> If so, the function mwifiex_recycle_cmd_node() will be called with the
> argument cmd_node:
> 196:    mwifiex_recycle_cmd_node(adapter, cmd_node);
>
> In this called function, the variable cmd_node->cmd_skb->data is
> assigned to the variable host_cmd, too.
> Thus the variable host_cmd in the function mwifiex_recycle_cmd_node()
> can be also NULL.
> However, it is dereferenced when calling le16_to_cpu():
> 144:    le16_to_cpu(host_cmd->command)
>
> I am not quite sure whether this possible null-pointer dereference is
> real and how to fix it if it is real.
> Any feedback would be appreciated, thanks!

I doubt it's real; the NULL check is probably excessive. I don't think
there's any case in which such skb's will have no ->data. If you're
interested, you could test and submit a "fix" to drop the excess
check.

Brian