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

[Simon Horman <horms@kernel.org>]

On Fri, Feb 16, 2024 at 12:36:57PM +0100, Florian Westphal wrote:
> 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>