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

[Dan Williams]

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 <chunk...@gmail.com>
Reported-by: Elena Reshetova <elena.reshet...@intel.com>
Cc: Johannes Berg <johan...@sipsolutions.net>
Cc: "David S. Miller" <da...@davemloft.net>
Cc: linux-wireless@vger.kernel.org
Signed-off-by: Dan Williams <dan.j.willi...@intel.com>
---
 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;
 }