Re: [PATCH bpf v2 1/4] bpf: Fix TOCTOU issue in lwt

[Alexei Starovoitov]

On Fri, May 29, 2026 at 8:14 AM Leon Hwang <leon.hwang@xxxxxxxxx> wrote:
>
> Sashiko pointed out [1]:
> The hdr pointer passed to bpf_lwt_push_ip_encap() can point to concurrently
> mutable memory such as a BPF map value.
>
> So, the memory of hdr pointer can be updated after skb_postpush_rcsum().
>
> To fix it, memcpy() the hdr to a local buffer, which will be used for the
> following checks and updates.
>
> [1] https://lore.kernel.org/bpf/20260525150010.CDEBA1F000E9@xxxxxxxxxxxxxxx/
>
> Fixes: 52f278774e79 ("bpf: implement BPF_LWT_ENCAP_IP mode in bpf_lwt_push_encap")
> Signed-off-by: Leon Hwang <leon.hwang@xxxxxxxxx>
> ---
>  net/core/lwt_bpf.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/net/core/lwt_bpf.c b/net/core/lwt_bpf.c
> index f71ef82a5f3d..8009e427851f 100644
> --- a/net/core/lwt_bpf.c
> +++ b/net/core/lwt_bpf.c
> @@ -599,6 +599,7 @@ static int handle_gso_encap(struct sk_buff *skb, bool ipv4, int encap_len)
>
>  int bpf_lwt_push_ip_encap(struct sk_buff *skb, void *hdr, u32 len, bool ingress)
>  {
> +       u8 buff[LWT_BPF_MAX_HEADROOM];

extra 256 bytes of stack to partially close a hypothetical issue
is not worth it.
Ignore such AI complaints.
Not every "bug" needs a fix.
If a malicious bpf user wants to crash the kernel they will
find a way to do so. Especially with agents.
We cannot realistically close all of the holes.
Right now the priority is to fix the issues that normal
users can hit and not bots.

pw-bot: cr