From: Florian Westphal fw@strlen.de
[ Upstream commit 219eee9c0d16f1b754a8b85275854ab17df0850a ]
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 Reviewed-by: Simon Horman horms@kernel.org Link: https://lore.kernel.org/r/20240216113700.23013-1-fw@strlen.de Signed-off-by: Paolo Abeni pabeni@redhat.com Signed-off-by: Sasha Levin sashal@kernel.org --- include/linux/skbuff.h | 6 ++++++ 1 file changed, 6 insertions(+)
diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h index ddfe86deb4e7f..2063253b2a6bd 100644 --- a/include/linux/skbuff.h +++ b/include/linux/skbuff.h @@ -2632,6 +2632,8 @@ 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; @@ -2640,6 +2642,8 @@ 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) @@ -2664,6 +2668,8 @@ 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;