Re: [PATCH net v2] seg6: validate SRH length before reading fixed fields

[Andrea Mayer <andrea.mayer@uniroma2.it>]

On Tue, 23 Jun 2026 18:32:31 +0800
Nuoqi Gui <gnq25@mails.tsinghua.edu.cn> wrote:

> seg6_validate_srh() reads fixed SRH fields such as srh->type and
> srh->hdrlen before checking that the supplied length covers the fixed
> struct ipv6_sr_hdr fields.
> 
> The BPF SEG6 encap path reaches this with a BPF program-supplied pointer
> and length: bpf_lwt_push_encap() and the SEG6 local BPF END_B6 and
> END_B6_ENCAP actions call bpf_push_seg6_encap(), which forwards the
> length to seg6_validate_srh() with no minimum-size guard.  A 2-byte SEG6
> encap header can therefore make the validator read srh->type at offset 2
> beyond the caller-supplied buffer.
> 
> Reject lengths shorter than the fixed SRH at the top of
> seg6_validate_srh(), before any field is read.  This fixes the BPF helper
> path and keeps the common validator robust.
> 
> Fixes: fe94cc290f53 ("bpf: Add IPv6 Segment Routing helpers")
> Signed-off-by: Nuoqi Gui <gnq25@mails.tsinghua.edu.cn>
> ---
> Changes in v2:
> - Narrowed the commit message to the BPF encap callers that can supply a
>   too-short SRH length.
> - Dropped the unnecessary cast in the minimum SRH length check.
> - Link to v1: https://patch.msgid.link/20260620-f01-17-seg6-srh-len-v1-1-36cbb29c12f1@mails.tsinghua.edu.cn  
> 
> To: Andrea Mayer <andrea.mayer@uniroma2.it>
> To: "David S. Miller" <davem@davemloft.net>
> To: Eric Dumazet <edumazet@google.com>
> To: Jakub Kicinski <kuba@kernel.org>
> To: Paolo Abeni <pabeni@redhat.com>
> To: Simon Horman <horms@kernel.org>
> To: Mathieu Xhonneux <m.xhonneux@gmail.com>
> To: Daniel Borkmann <daniel@iogearbox.net>
> To: David Lebrun <dlebrun@google.com>
> Cc: netdev@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org
> Cc: bpf@vger.kernel.org
> ---
>  net/ipv6/seg6.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/net/ipv6/seg6.c b/net/ipv6/seg6.c
> index 1c3ad25700c4c..62a7eb7792026 100644
> --- a/net/ipv6/seg6.c
> +++ b/net/ipv6/seg6.c
> @@ -29,6 +29,9 @@ bool seg6_validate_srh(struct ipv6_sr_hdr *srh, int len, bool reduced)
>  	int max_last_entry;
>  	int trailing;
>  
> +	if (len < sizeof(*srh))
> +		return false;
> +

Thanks for the patch.

Looks good to me.

Reviewed-by: Andrea Mayer <andrea.mayer@uniroma2.it>

On a separate note: the AI review message seems correct. The reported
issue is a separate, pre-existing bug in the BPF SEG6 encap path, not
introduced by this patch.

Regards,
Andrea

>  	if (srh->type != IPV6_SRCRT_TYPE_4)
>  		return false;
>  
> 
> ---
> base-commit: 96e7f9122aae0ed000ee321f324b812a447906d9
> change-id: 20260619-f01-17-seg6-srh-len-a85f35427e0b
> 
> Best regards,
> --  
> Nuoqi Gui <gnq25@mails.tsinghua.edu.cn>
>