From 219eee9c0d16f1b754a8b85275854ab17df0850a Mon Sep 17 00:00:00 2001 From: Florian Westphal Date: Fri, 16 Feb 2024 12:36:57 +0100 Subject: [PATCH] net: skbuff: add overflow debug check to pull/push helpers 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 Reviewed-by: Simon Horman Link: https://lore.kernel.org/r/20240216113700.23013-1-fw@strlen.de Signed-off-by: Paolo Abeni --- include/linux/skbuff.h | 6 ++++++ 1 file changed, 6 insertions(+) diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h index e3a2ed5d09ad..28c7cb7ce251 100644 --- a/include/linux/skbuff.h +++ b/include/linux/skbuff.h @@ -2684,6 +2684,8 @@ 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; @@ -2692,6 +2694,8 @@ 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) @@ -2716,6 +2720,8 @@ 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;