[PATCH net-next v3 4/5] net/sched: netem: handle multi-segment skb in corruption

[Stephen Hemminger <stephen@networkplumber.org>]

The packet corruption code only flipped bits in the linear
header portion of the skb, skipping corruption when
skb_headlen() was zero.

Linearize the whole skb if necessary before corruption.
That step also makes calling skb_unshare() unnecessary.

Extends d64cb81dcbd5 ("net/sched: sch_netem: fix out-of-bounds access
in packet corruption") with a more general solution.

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
---
 net/sched/sch_netem.c | 16 ++++++----------
 1 file changed, 6 insertions(+), 10 deletions(-)

diff --git a/net/sched/sch_netem.c b/net/sched/sch_netem.c
index 53bdfa77ee2d..020dbfb6e4a7 100644
--- a/net/sched/sch_netem.c
+++ b/net/sched/sch_netem.c
@@ -508,21 +508,17 @@ static int netem_enqueue(struct sk_buff *skb, struct Qdisc *sch,
 			qdisc_skb_cb(skb)->pkt_len = skb->len;
 		}
 
-		skb = skb_unshare(skb, GFP_ATOMIC);
-		if (unlikely(!skb)) {
-			qdisc_qstats_drop(sch);
-			goto finish_segs;
-		}
-		if (skb->ip_summed == CHECKSUM_PARTIAL &&
-		    skb_checksum_help(skb)) {
+		if (skb_linearize(skb) ||
+		    (skb->ip_summed == CHECKSUM_PARTIAL && skb_checksum_help(skb))) {
 			qdisc_drop(skb, sch, to_free);
 			skb = NULL;
 			goto finish_segs;
 		}
 
-		if (skb_headlen(skb))
-			skb->data[get_random_u32_below(skb_headlen(skb))] ^=
-				1 << get_random_u32_below(8);
+		if (skb->len) {
+			u32 offset = get_random_u32_below(skb->len);
+			skb->data[offset] ^= 1 << get_random_u32_below(8);
+		}
 	}
 
 	if (unlikely(sch->q.qlen >= sch->limit)) {
-- 
2.53.0