netdev - Re: [PATCH v2 15/19] carl9170: prevent bounds-check bypass via speculative execution

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Message-ID: <CAPcyv4gne66ic3E-Y+pLi4QZdVeeeVmvayVPA3Jt1gyUqpaF9g@mail.gmail.com>
Date:   Fri, 12 Jan 2018 10:39:50 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Christian Lamparter <chunkeey@...il.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arch@...r.kernel.org, kernel-hardening@...ts.openwall.com,
        Netdev <netdev@...r.kernel.org>,
        Linux Wireless List <linux-wireless@...r.kernel.org>,
        Elena Reshetova <elena.reshetova@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Kalle Valo <kvalo@...eaurora.org>,
        Alan Cox <alan@...ux.intel.com>
Subject: Re: [PATCH v2 15/19] carl9170: prevent bounds-check bypass via
 speculative execution

On Fri, Jan 12, 2018 at 6:42 AM, Christian Lamparter <chunkeey@...il.com> wrote:
> On Friday, January 12, 2018 1:47:46 AM CET Dan Williams wrote:
>> Static analysis reports that 'queue' may be a user controlled value that
>> is used as a data dependency to read from the 'ar9170_qmap' array. In
>> order to avoid potential leaks of kernel memory values, block
>> speculative execution of the instruction stream that could issue reads
>> based on an invalid result of 'ar9170_qmap[queue]'. In this case the
>> value of 'ar9170_qmap[queue]' is immediately reused as an index to the
>> 'ar->edcf' array.
>>
>> Based on an original patch by Elena Reshetova.
>>
>> Cc: Christian Lamparter <chunkeey@...glemail.com>
>> Cc: Kalle Valo <kvalo@...eaurora.org>
>> Cc: linux-wireless@...r.kernel.org
>> Cc: netdev@...r.kernel.org
>> Signed-off-by: Elena Reshetova <elena.reshetova@...el.com>
>> Signed-off-by: Dan Williams <dan.j.williams@...el.com>
>> ---
> This patch (and p54, cw1200) look like the same patch?!
> Can you tell me what happend to:
>
> On Saturday, January 6, 2018 5:34:03 PM CET Dan Williams wrote:
>> On Sat, Jan 6, 2018 at 6:23 AM, Christian Lamparter <chunkeey@...il.com> wrote:
>> > And Furthermore a invalid queue (param->ac) would cause a crash in
>> > this line in mac80211 before it even reaches the driver [1]:
>> > |       sdata->tx_conf[params->ac] = p;
>> > |                   ^^^^^^^^
>> > |       if (drv_conf_tx(local, sdata, >>>> params->ac <<<<, &p)) {
>> > |        ^^ (this is a wrapper for the *_op_conf_tx)
>> >
>> > I don't think these chin-up exercises are needed.
>>
>> Quite the contrary, you've identified a better place in the call stack
>> to sanitize the input and disable speculation. Then we can kill the
>> whole class of the wireless driver reports at once it seems.
> <https://www.spinics.net/lists/netdev/msg476353.html>

I didn't see where ac is being validated against the driver specific
'queues' value in that earlier patch.

>
> Anyway, I think there's an easy way to solve this: remove the
> "if (queue < ar->hw->queues)" check altogether. It's no longer needed
> anymore as the "queue" value is validated long before the driver code
> gets called.

Can you point me to where that validation happens?

> And from my understanding, this will fix the "In this case
> the value of 'ar9170_qmap[queue]' is immediately reused as an index to
> the 'ar->edcf' array." gripe your tool complains about.
>
> This is here's a quick test-case for carl9170.:
> ---
> diff --git a/drivers/net/wireless/ath/carl9170/main.c b/drivers/net/wireless/ath/carl9170/main.c
> index 988c8857d78c..2d3afb15bb62 100644
> --- a/drivers/net/wireless/ath/carl9170/main.c
> +++ b/drivers/net/wireless/ath/carl9170/main.c
> @@ -1387,13 +1387,8 @@ static int carl9170_op_conf_tx(struct ieee80211_hw *hw,
>         int ret;
>
>         mutex_lock(&ar->mutex);
> -       if (queue < ar->hw->queues) {
> -               memcpy(&ar->edcf[ar9170_qmap[queue]], param, sizeof(*param));
> -               ret = carl9170_set_qos(ar);
> -       } else {
> -               ret = -EINVAL;
> -       }
> -
> +       memcpy(&ar->edcf[ar9170_qmap[queue]], param, sizeof(*param));
> +       ret = carl9170_set_qos(ar);
>         mutex_unlock(&ar->mutex);
>         return ret;
>  }
> ---
> What does your tool say about this?

If you take away the 'if' then I don't the tool will report on this.

> (If necessary, the "queue" value could be even sanitized with a
> queue %= ARRAY_SIZE(ar9170_qmap); before the mutex_lock.)

That is what array_ptr() is doing in a more sophisticated way.