[PATCH v4] wifi: mac80211: fix integer overflow in hwmp_route_info_get()

[Gavrilov Ilia]

Since the new_metric and last_hop_metric variables can reach
the MAX_METRIC(0xffffffff) value, an integer overflow may occur
when multiplying them by 10/9. It can lead to incorrect behavior.

Found by InfoTeCS on behalf of Linux Verification Center
(linuxtesting.org) with SVACE.

Fixes: a8d418d9ac25 ("mac80211: mesh: only switch path when new metric is at least 10% better")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Ilia Gavrilov <Ilia.Gavrilov@xxxxxxxxxxx>
---
v2:
- Remove 64-bit arithmetic according to https://lore.kernel.org/all/a6bd38c58f2f7685eac53844f2336432503c328e.camel@xxxxxxxxxxxxxxxx/
- Replace multiplication by 10/9 with a function that compares metrics by adding 10% without integer overflow
v3:
- Fix a typo (persent->percent)
v4:
- Simplify the is_metric_better() function
- Remove 'inline', add a comment
 net/mac80211/mesh_hwmp.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/net/mac80211/mesh_hwmp.c b/net/mac80211/mesh_hwmp.c
index 4e9546e998b6..c94a9c7ca960 100644
--- a/net/mac80211/mesh_hwmp.c
+++ b/net/mac80211/mesh_hwmp.c
@@ -367,6 +367,12 @@ u32 airtime_link_metric_get(struct ieee80211_local *local,
 	return (u32)result;
 }
 
+/* Check that the first metric is at least 10% better than the second one */
+static bool is_metric_better(u32 x, u32 y)
+{
+	return (x < y) && (x < (y - x / 10));
+}
+
 /**
  * hwmp_route_info_get - Update routing info to originator and transmitter
  *
@@ -458,8 +464,8 @@ static u32 hwmp_route_info_get(struct ieee80211_sub_if_data *sdata,
 				    (mpath->sn == orig_sn &&
 				     (rcu_access_pointer(mpath->next_hop) !=
 						      sta ?
-					      mult_frac(new_metric, 10, 9) :
-					      new_metric) >= mpath->metric)) {
+					      !is_metric_better(new_metric, mpath->metric) :
+					      new_metric >= mpath->metric))) {
 					process = false;
 					fresh_info = false;
 				}
@@ -533,8 +539,8 @@ static u32 hwmp_route_info_get(struct ieee80211_sub_if_data *sdata,
 			if ((mpath->flags & MESH_PATH_FIXED) ||
 			    ((mpath->flags & MESH_PATH_ACTIVE) &&
 			     ((rcu_access_pointer(mpath->next_hop) != sta ?
-				       mult_frac(last_hop_metric, 10, 9) :
-				       last_hop_metric) > mpath->metric)))
+				      !is_metric_better(last_hop_metric, mpath->metric) :
+				       last_hop_metric > mpath->metric))))
 				fresh_info = false;
 		} else {
 			mpath = mesh_path_add(sdata, ta);
-- 
2.39.5