[PATCH AUTOSEL 5.4 18/25] bpf: Don't redirect packets with invalid pkt_len

[Sasha Levin]

From: Zhengchao Shao <shaozhengchao@xxxxxxxxxx>

[ Upstream commit fd1894224407c484f652ad456e1ce423e89bb3eb ]

Syzbot found an issue [1]: fq_codel_drop() try to drop a flow whitout any
skbs, that is, the flow->head is null.
The root cause, as the [2] says, is because that bpf_prog_test_run_skb()
run a bpf prog which redirects empty skbs.
So we should determine whether the length of the packet modified by bpf
prog or others like bpf_prog_test is valid before forwarding it directly.

LINK: [1] https://syzkaller.appspot.com/bug?id=0b84da80c2917757915afa89f7738a9d16ec96c5
LINK: [2] https://www.spinics.net/lists/netdev/msg777503.html

Reported-by: syzbot+7a12909485b94426aceb@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Zhengchao Shao <shaozhengchao@xxxxxxxxxx>
Reviewed-by: Stanislav Fomichev <sdf@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20220715115559.139691-1-shaozhengchao@xxxxxxxxxx
Signed-off-by: Alexei Starovoitov <ast@xxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 include/linux/skbuff.h | 8 ++++++++
 net/bpf/test_run.c     | 3 +++
 net/core/dev.c         | 1 +
 3 files changed, 12 insertions(+)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index b04b5bd43f54..680f71ecdc08 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -2201,6 +2201,14 @@ static inline void skb_set_tail_pointer(struct sk_buff *skb, const int offset)
 
 #endif /* NET_SKBUFF_DATA_USES_OFFSET */
 
+static inline void skb_assert_len(struct sk_buff *skb)
+{
+#ifdef CONFIG_DEBUG_NET
+	if (WARN_ONCE(!skb->len, "%s\n", __func__))
+		DO_ONCE_LITE(skb_dump, KERN_ERR, skb, false);
+#endif /* CONFIG_DEBUG_NET */
+}
+
 /*
  *	Add data to an sk_buff
  */
diff --git a/net/bpf/test_run.c b/net/bpf/test_run.c
index 1153bbcdff72..5e6428cbd758 100644
--- a/net/bpf/test_run.c
+++ b/net/bpf/test_run.c
@@ -200,6 +200,9 @@ static int convert___skb_to_skb(struct sk_buff *skb, struct __sk_buff *__skb)
 {
 	struct qdisc_skb_cb *cb = (struct qdisc_skb_cb *)skb->cb;
 
+	if (!skb->len)
+		return -EINVAL;
+
 	if (!__skb)
 		return 0;
 
diff --git a/net/core/dev.c b/net/core/dev.c
index a03036456221..60cea97132f2 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -3712,6 +3712,7 @@ static int __dev_queue_xmit(struct sk_buff *skb, struct net_device *sb_dev)
 	bool again = false;
 
 	skb_reset_mac_header(skb);
+	skb_assert_len(skb);
 
 	if (unlikely(skb_shinfo(skb)->tx_flags & SKBTX_SCHED_TSTAMP))
 		__skb_tstamp_tx(skb, NULL, skb->sk, SCM_TSTAMP_SCHED);
-- 
2.35.1