net/sched: sch_netem: fix out-of-bounds access in packet corruption

In netem_enqueue(), the packet corruption logic uses
get_random_u32_below(skb_headlen(skb)) to select an index for
modifying skb->data. When an AF_PACKET TX_RING sends fully non-linear
packets over an IPIP tunnel, skb_headlen(skb) evaluates to 0.

Passing 0 to get_random_u32_below() takes the variable-ceil slow path
which returns an unconstrained 32-bit random integer. Using this
unconstrained value as an offset into skb->data results in an
out-of-bounds memory access.

Fix this by verifying skb_headlen(skb) is non-zero before attempting
to corrupt the linear data area. Fully non-linear packets will silently
bypass the corruption logic.

Fixes: c865e5d99e ("[PKT_SCHED] netem: packet corruption option")
Reported-by: Yifan Wu <yifanwucs@gmail.com>
Reported-by: Juefei Pu <tomapufckgml@gmail.com>
Signed-off-by: Yuan Tan <tanyuan98@outlook.com>
Signed-off-by: Xin Liu <bird@lzu.edu.cn>
Signed-off-by: Yuhang Zheng <z1652074432@gmail.com>
Signed-off-by: Yucheng Lu <kanolyc@gmail.com>
Reviewed-by: Stephen Hemminger <stephen@networkplumber.org>
Link: https://patch.msgid.link/45435c0935df877853a81e6d06205ac738ec65fa.1774941614.git.kanolyc@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/sched/sch_netem.c

@@ -519,8 +519,9 @@ static int netem_enqueue(struct sk_buff *skb, struct Qdisc *sch,
 			goto finish_segs;
 		}
 
-		skb->data[get_random_u32_below(skb_headlen(skb))] ^=
+		if (skb_headlen(skb))
-			1<<get_random_u32_below(8);
+			skb->data[get_random_u32_below(skb_headlen(skb))] ^=
+				1 << get_random_u32_below(8);
 	}
 
 	if (unlikely(q->t_len >= sch->limit)) {