[PATCH v4.1 10/10] nl80211: sanitize array index in parse_txq_params

[Dan Williams <dan.j.williams@xxxxxxxxx>]

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).

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 dd3aa05fab87..b8a9222e34d1 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 d396cb61a280..e3b3527c16d4 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;
 }