Subject: Re: [PATCH v1 4/5] bpf: add guard rails for new DECAP flags

From: Willem de Bruijn <[email-masked]>
Date: Tue, 17 Mar 2026 09:30:20 -0400

Nick Hudson wrote:
> Add checks to require shrink-only decap, reject conflicting decap flag combinations, and verify removed length is sufficient for claimed header decapsulation.
> 
> Co-developed-by: Max Tottenham <[email-masked]>
> Signed-off-by: Max Tottenham <[email-masked]>
> Co-developed-by: Anna Glasgall <[email-masked]>
> Signed-off-by: Anna Glasgall <[email-masked]>
> Signed-off-by: Nick Hudson <[email-masked]>

This patch probably should come before 3, as 3 enables the features
without the guard rails in place.

> ---
>  net/core/filter.c | 45 ++++++++++++++++++++++++++++++++++++---------
>  1 file changed, 36 insertions(+), 9 deletions(-)
> 
> diff --git a/net/core/filter.c b/net/core/filter.c
> index ac7e1068fe4c..437e0da34f84 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -56,6 +56,7 @@
>  #include <net/sock_reuseport.h>
>  #include <net/busy_poll.h>
>  #include <net/tcp.h>
> +#include <net/gre.h>
>  #include <net/xfrm.h>
>  #include <net/udp.h>
>  #include <linux/bpf_trace.h>
> @@ -3745,20 +3746,46 @@ BPF_CALL_4(bpf_skb_adjust_room, struct sk_buff *, skb, s32, len_diff,
>  		return -ENOTSUPP;
>  	}
>  
> -	if (flags & BPF_F_ADJ_ROOM_DECAP_L3_MASK) {
> +	if (flags & BPF_F_ADJ_ROOM_DECAP_MASK) {
> +		u32 len_decap_min = 0;
> +
>  		if (!shrink)
>  			return -EINVAL;
>  
> -		switch (flags & BPF_F_ADJ_ROOM_DECAP_L3_MASK) {
> -		case BPF_F_ADJ_ROOM_DECAP_L3_IPV4:
> +		if ((flags & BPF_F_ADJ_ROOM_DECAP_L3_MASK) ==
> +		    BPF_F_ADJ_ROOM_DECAP_L3_MASK)
> +			return -EINVAL;
> +
> +		if ((flags & BPF_F_ADJ_ROOM_DECAP_L4_MASK) ==
> +		    BPF_F_ADJ_ROOM_DECAP_L4_MASK)
> +			return -EINVAL;
> +
> +		if ((flags & BPF_F_ADJ_ROOM_DECAP_IPXIP_MASK) ==
> +		    BPF_F_ADJ_ROOM_DECAP_IPXIP_MASK)
> +			return -EINVAL;
> +

Are these equality tests shorthand based on knowledge that each only
have two options, so equality implies more than one option set? That
is not obvious/self documenting. Please add a brief comment.

> +		if ((flags & BPF_F_ADJ_ROOM_DECAP_L4_MASK) &&
> +		    (flags & BPF_F_ADJ_ROOM_DECAP_IPXIP_MASK))
> +			return -EINVAL;
> +
> +		if (mode == BPF_ADJ_ROOM_MAC)
> +			len_decap_min += proto == htons(ETH_P_IP) ?
> +					 sizeof(struct iphdr) : sizeof(struct ipv6hdr);

MAC is not a GSO related decap, can be used for insertion/deletion of
L2.5 headers. This should be dropped.

> +
> +		if (flags & BPF_F_ADJ_ROOM_DECAP_L4_UDP)
> +			len_decap_min += sizeof(struct udphdr);
> +
> +		if (flags & BPF_F_ADJ_ROOM_DECAP_L4_GRE)
> +			len_decap_min += sizeof(struct gre_base_hdr);
> +
> +		if (len_diff_abs < len_decap_min)
> +			return -EINVAL;
> +
> +		if (flags & BPF_F_ADJ_ROOM_DECAP_L3_IPV4)
>  			len_min = sizeof(struct iphdr);
> -			break;
> -		case BPF_F_ADJ_ROOM_DECAP_L3_IPV6:
> +
> +		if (flags & BPF_F_ADJ_ROOM_DECAP_L3_IPV6)
>  			len_min = sizeof(struct ipv6hdr);
> -			break;
> -		default:
> -			return -EINVAL;
> -		}
>  	}
>  
>  	len_cur = skb->len - skb_network_offset(skb);
> -- 
> 2.34.1
>