Re: [PATCH v4 10/10] nl80211: sanitize array index in parse_txq_params

[Johannes Berg]

On Thu, 2018-01-18 at 16:02 -0800, Dan Williams wrote:
> Wireless drivers rely on parse_txq_params to validate that
> txq_params->ac is less than NL80211_NUM_ACS by the time the low-level
> driver's ->conf_tx() handler is called. Use a new helper, 'array_idx',
> to sanitize txq_params->ac with respect to speculation. I.e. ensure that
> any speculation into ->conf_tx() handlers is done with a value of
> txq_params->ac that is within the bounds of [0, NL80211_NUM_ACS).

Sorry, I didn't realize you were waiting for me to review.

LGTM.

Reviewed-by: Johannes Berg <johannes@xxxxxxxxxxxxxxxx>


> Reported-by: Christian Lamparter <chunkeey@xxxxxxxxx>
> Reported-by: Elena Reshetova <elena.reshetova@xxxxxxxxx>
> Cc: Johannes Berg <johannes@xxxxxxxxxxxxxxxx>
> Cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
> Cc: linux-wireless@xxxxxxxxxxxxxxx
> Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
> ---
>  include/linux/nospec.h |   21 +++++++++++++++++++++
>  net/wireless/nl80211.c |   10 +++++++---
>  2 files changed, 28 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/nospec.h b/include/linux/nospec.h
> index f841c11f3f1f..8af35be1869e 100644
> --- a/include/linux/nospec.h
> +++ b/include/linux/nospec.h
> @@ -41,4 +41,25 @@ static inline unsigned long array_ptr_mask(unsigned long idx, unsigned long sz)
>  	__u._bit &= _mask;						\
>  	__u._ptr;							\
>  })
> +
> +/**
> + * array_idx - Generate a pointer to an array index, ensuring the
> + * pointer is bounded under speculation to NULL.
> + *
> + * @idx: the index of the element, must be less than LONG_MAX
> + * @sz: the number of elements in the array, must be less than LONG_MAX
> + *
> + * If @idx falls in the interval [0, @sz), returns &@idx otherwise
> + * returns NULL.
> + */
> +#define array_idx(idx, sz)						\
> +({									\
> +	union { typeof((idx)) *_ptr; unsigned long _bit; } __u;		\
> +	typeof(idx) *_i = &(idx);					\
> +	unsigned long _mask = array_ptr_mask(*_i, (sz));		\
> +									\
> +	__u._ptr = _i;							\
> +	__u._bit &= _mask;						\
> +	__u._ptr;							\
> +})
>  #endif /* __NOSPEC_H__ */
> diff --git a/net/wireless/nl80211.c b/net/wireless/nl80211.c
> index 2b3dbcd40e46..202cb1dc03ee 100644
> --- a/net/wireless/nl80211.c
> +++ b/net/wireless/nl80211.c
> @@ -16,6 +16,7 @@
>  #include <linux/nl80211.h>
>  #include <linux/rtnetlink.h>
>  #include <linux/netlink.h>
> +#include <linux/nospec.h>
>  #include <linux/etherdevice.h>
>  #include <net/net_namespace.h>
>  #include <net/genetlink.h>
> @@ -2056,20 +2057,23 @@ static const struct nla_policy txq_params_policy[NL80211_TXQ_ATTR_MAX + 1] = {
>  static int parse_txq_params(struct nlattr *tb[],
>  			    struct ieee80211_txq_params *txq_params)
>  {
> +	u8 ac, *idx;
> +
>  	if (!tb[NL80211_TXQ_ATTR_AC] || !tb[NL80211_TXQ_ATTR_TXOP] ||
>  	    !tb[NL80211_TXQ_ATTR_CWMIN] || !tb[NL80211_TXQ_ATTR_CWMAX] ||
>  	    !tb[NL80211_TXQ_ATTR_AIFS])
>  		return -EINVAL;
>  
> -	txq_params->ac = nla_get_u8(tb[NL80211_TXQ_ATTR_AC]);
> +	ac = nla_get_u8(tb[NL80211_TXQ_ATTR_AC]);
>  	txq_params->txop = nla_get_u16(tb[NL80211_TXQ_ATTR_TXOP]);
>  	txq_params->cwmin = nla_get_u16(tb[NL80211_TXQ_ATTR_CWMIN]);
>  	txq_params->cwmax = nla_get_u16(tb[NL80211_TXQ_ATTR_CWMAX]);
>  	txq_params->aifs = nla_get_u8(tb[NL80211_TXQ_ATTR_AIFS]);
>  
> -	if (txq_params->ac >= NL80211_NUM_ACS)
> +	idx = array_idx(ac, NL80211_NUM_ACS);
> +	if (!idx)
>  		return -EINVAL;
> -
> +	txq_params->ac = *idx;
>  	return 0;
>  }
>  
> 
>