mac80211: fix invalid read in minstrel_sort_best_tp_rates()

At the last iteration of the loop, j may equal zero and thus
tp_list[j - 1] causes an invalid read.
Change the logic of the loop so that j - 1 is always >= 0.

Cc: stable@vger.kernel.org
Signed-off-by: Adrien Schildknecht <adrien+dev@schischi.me>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

1 files changed, 6 insertions, 5 deletions

diff --git a/net/mac80211/rc80211_minstrel.c b/net/mac80211/rc80211_minstrel.c
index 247552a7f6c2..3ece7d1034c8 100644
--- a/net/mac80211/rc80211_minstrel.c
+++ b/net/mac80211/rc80211_minstrel.c
@@ -92,14 +92,15 @@ int minstrel_get_tp_avg(struct minstrel_rate *mr, int prob_ewma)
 static inline void
 minstrel_sort_best_tp_rates(struct minstrel_sta_info *mi, int i, u8 *tp_list)
 {
-        int j = MAX_THR_RATES;
-        struct minstrel_rate_stats *tmp_mrs = &mi->r[j - 1].stats;
+        int j;
+        struct minstrel_rate_stats *tmp_mrs;
         struct minstrel_rate_stats *cur_mrs = &mi->r[i].stats;
 
-        while (j > 0 && (minstrel_get_tp_avg(&mi->r[i], cur_mrs->prob_ewma) >
-               minstrel_get_tp_avg(&mi->r[tp_list[j - 1]], tmp_mrs->prob_ewma))) {
-                j--;
+        for (j = MAX_THR_RATES; j > 0; --j) {
                 tmp_mrs = &mi->r[tp_list[j - 1]].stats;
+                if (minstrel_get_tp_avg(&mi->r[i], cur_mrs->prob_ewma) <=
+                    minstrel_get_tp_avg(&mi->r[tp_list[j - 1]], tmp_mrs->prob_ewma))
+                        break;
         }
 
         if (j < MAX_THR_RATES - 1)