Subject: Re: [PATCH 08/18] carl9170: prevent bounds-check bypass via speculative execution

From: Christian Lamparter <[email-masked]>
Date: Sat, 06 Jan 2018 15:23:40 +0100

On Saturday, January 6, 2018 2:10:37 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 <[email-masked]>
> Cc: Kalle Valo <[email-masked]>
> Cc: [email-masked]
> Cc: [email-masked]
> Signed-off-by: Elena Reshetova <[email-masked]>
> Signed-off-by: Dan Williams <[email-masked]>
> ---
>  drivers/net/wireless/ath/carl9170/main.c |    6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/net/wireless/ath/carl9170/main.c b/drivers/net/wireless/ath/carl9170/main.c
> index 988c8857d78c..0ff34cbe2b62 100644
> --- a/drivers/net/wireless/ath/carl9170/main.c
> +++ b/drivers/net/wireless/ath/carl9170/main.c
> @@ -41,6 +41,7 @@
>  #include <linux/module.h>
>  #include <linux/etherdevice.h>
>  #include <linux/random.h>
> +#include <linux/compiler.h>
>  #include <net/mac80211.h>
>  #include <net/cfg80211.h>
>  #include "hw.h"
> @@ -1384,11 +1385,12 @@ static int carl9170_op_conf_tx(struct ieee80211_hw *hw,
>  			       const struct ieee80211_tx_queue_params *param)
>  {
>  	struct ar9170 *ar = hw->priv;
> +	const u8 *elem;
>  	int ret;
>  
>  	mutex_lock(&ar->mutex);
> -	if (queue < ar->hw->queues) {
> -		memcpy(&ar->edcf[ar9170_qmap[queue]], param, sizeof(*param));
> +	if ((elem = nospec_array_ptr(ar9170_qmap, queue, ar->hw->queues))) {
> +		memcpy(&ar->edcf[*elem], param, sizeof(*param));
>  		ret = carl9170_set_qos(ar);
>  	} else {
>  		ret = -EINVAL;
> 
> 
About the "queue" in carl9170_op_conf_tx, p54_conf_tx and cw1200_conf_tx:

The only way a user can set this in any meaningful way would be via
a NL80211_CMD_SET_WIPHY netlink message. However, the value will get
vetted there by cfg80211's parse_txq_params [0]. This is long before
it reaches any of the *_op_conf_tx functions.

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.

[0] <https://github.com/torvalds/linux/blob/master/net/wireless/nl80211.c#L2070>
[1] <https://github.com/torvalds/linux/blob/master/net/mac80211/cfg.c#L2157>