[PATCH net-next 5/5] 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.

Use skb_header_pointer() and skb_store_bits() to access the
full packet data, allowing any bit in the packet to be
corrupted regardless of how the skb is laid out.

Replaces 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, 12 insertions(+), 4 deletions(-)

diff --git a/net/sched/sch_netem.c b/net/sched/sch_netem.c
index 6dc1ba8e999b..9b5731a8cd15 100644
--- a/net/sched/sch_netem.c
+++ b/net/sched/sch_netem.c
@@ -525,10 +525,18 @@ static int netem_enqueue(struct sk_buff *skb, struct Qdisc *sch,
 			goto finish_segs;
 		}
 
-		if (skb_headlen(skb))
-			skb->data[get_random_u32_below(skb_headlen(skb))] ^=
-				1 << get_random_u32_below(8);
-		q->xstats.corrupted++;
+		if (skb->len > 0) {
+			unsigned int offset = get_random_u32_below(skb->len);
+			u8 *ptr, val;
+
+			/* handle multi-segment skb's */
+			ptr = skb_header_pointer(skb, offset, 1, &val);
+			if (ptr) {
+				val = *ptr ^ (1 << get_random_u32_below(8));
+				skb_store_bits(skb, offset, &val, 1);
+			}
+			q->xstats.corrupted++;
+		}
 	}
 
 	if (unlikely(q->t_len >= sch->limit)) {
-- 
2.53.0