[PATCH net] net: skbuff: add overflow debug check to pull/push helpers

[Florian Westphal <fw@strlen.de>]

syzbot managed to trigger following splat:
BUG: KASAN: use-after-free in __skb_flow_dissect+0x4a3b/0x5e50
Read of size 1 at addr ffff888208a4000e by task a.out/2313
[..]
  __skb_flow_dissect+0x4a3b/0x5e50
  __skb_get_hash+0xb4/0x400
  ip_tunnel_xmit+0x77e/0x26f0
  ipip_tunnel_xmit+0x298/0x410
  ..

Analysis shows that the skb has a valid ->head, but bogus ->data
pointer.

skb->data gets its bogus value via the neigh layer, which does:

1556    __skb_pull(skb, skb_network_offset(skb));

... and the skb was already dodgy at this point:

skb_network_offset(skb) returns a negative value due to an
earlier overflow of skb->network_header (u16).  __skb_pull thus
"adjusts" skb->data by a huge offset, pointing outside skb->head
area.

Allow debug builds to splat when we try to pull/push more than
INT_MAX bytes.

After this, the syzkaller reproducer yields a more precise splat
before the flow dissector attempts to read off skb->data memory:

WARNING: CPU: 5 PID: 2313 at include/linux/skbuff.h:2653 neigh_connected_output+0x28e/0x400
  ip_finish_output2+0xb25/0xed0
  iptunnel_xmit+0x4ff/0x870
  ipgre_xmit+0x78e/0xbb0

Signed-off-by: Florian Westphal <fw@strlen.de>
---
 A fix to prevent network_header overflow will follow shortly.

 include/linux/skbuff.h | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 2dde34c29203..fd9198fcc3c3 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -2642,6 +2642,8 @@ static inline void skb_put_u8(struct sk_buff *skb, u8 val)
 void *skb_push(struct sk_buff *skb, unsigned int len);
 static inline void *__skb_push(struct sk_buff *skb, unsigned int len)
 {
+	DEBUG_NET_WARN_ON_ONCE(len > INT_MAX);
+
 	skb->data -= len;
 	skb->len  += len;
 	return skb->data;
@@ -2650,6 +2652,8 @@ static inline void *__skb_push(struct sk_buff *skb, unsigned int len)
 void *skb_pull(struct sk_buff *skb, unsigned int len);
 static inline void *__skb_pull(struct sk_buff *skb, unsigned int len)
 {
+	DEBUG_NET_WARN_ON_ONCE(len > INT_MAX);
+
 	skb->len -= len;
 	if (unlikely(skb->len < skb->data_len)) {
 #if defined(CONFIG_DEBUG_NET)
@@ -2674,6 +2678,8 @@ void *__pskb_pull_tail(struct sk_buff *skb, int delta);
 static inline enum skb_drop_reason
 pskb_may_pull_reason(struct sk_buff *skb, unsigned int len)
 {
+	DEBUG_NET_WARN_ON_ONCE(len > INT_MAX);
+
 	if (likely(len <= skb_headlen(skb)))
 		return SKB_NOT_DROPPED_YET;
 
-- 
2.43.0