Re: [PATCH bpf v2 1/2] bpf, xdp: clean head/meta when expanding it

[Alexei Starovoitov]

On Sun, Mar 30, 2025 at 8:27 PM Jiayuan Chen <jiayuan.chen@xxxxxxxxx> wrote:
>
> The device allocates an skb, it additionally allocates a prepad size
> (usually equal to NET_SKB_PAD or XDP_PACKET_HEADROOM) but leaves it
> uninitialized.
>
> The bpf_xdp_adjust_head function moves skb->data forward, which allows
> users to access data belonging to other programs, posing a security risk.
>
> Reported-by: syzbot+0e6ddb1ef80986bdfe64@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://lore.kernel.org/all/00000000000067f65105edbd295d@xxxxxxxxxx/T/
> Signed-off-by: Jiayuan Chen <jiayuan.chen@xxxxxxxxx>
> ---
>  include/uapi/linux/bpf.h       | 8 +++++---
>  net/core/filter.c              | 5 ++++-
>  tools/include/uapi/linux/bpf.h | 6 ++++--
>  3 files changed, 13 insertions(+), 6 deletions(-)
>
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index defa5bb881f4..be01a848cbbf 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -2760,8 +2760,9 @@ union bpf_attr {
>   *
>   * long bpf_xdp_adjust_head(struct xdp_buff *xdp_md, int delta)
>   *     Description
> - *             Adjust (move) *xdp_md*\ **->data** by *delta* bytes. Note that
> - *             it is possible to use a negative value for *delta*. This helper
> + *             Adjust (move) *xdp_md*\ **->data** by *delta* bytes. Note that
> + *             it is possible to use a negative value for *delta*. If *delta*
> + *             is negative, the new header will be memset to zero. This helper
>   *             can be used to prepare the packet for pushing or popping
>   *             headers.
>   *
> @@ -2989,7 +2990,8 @@ union bpf_attr {
>   * long bpf_xdp_adjust_meta(struct xdp_buff *xdp_md, int delta)
>   *     Description
>   *             Adjust the address pointed by *xdp_md*\ **->data_meta** by
> - *             *delta* (which can be positive or negative). Note that this
> + *             *delta* (which can be positive or negative). If *delta* is
> + *             negative, the new meta will be memset to zero. Note that this
>   *             operation modifies the address stored in *xdp_md*\ **->data**,
>   *             so the latter must be loaded only after the helper has been
>   *             called.
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 46ae8eb7a03c..5f01d373b719 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -3947,6 +3947,8 @@ BPF_CALL_2(bpf_xdp_adjust_head, struct xdp_buff *, xdp, int, offset)
>         if (metalen)
>                 memmove(xdp->data_meta + offset,
>                         xdp->data_meta, metalen);
> +       if (offset < 0)
> +               memset(data, 0, -offset);
>         xdp->data_meta += offset;
>         xdp->data = data;
>
> @@ -4239,7 +4241,8 @@ BPF_CALL_2(bpf_xdp_adjust_meta, struct xdp_buff *, xdp, int, offset)
>                 return -EINVAL;
>         if (unlikely(xdp_metalen_invalid(metalen)))
>                 return -EACCES;
> -
> +       if (offset < 0)
> +               memset(meta, 0, -offset);

Let's make everyone pay a performance penalty to silence
KMSAN warning?

I don't think it's a good trade off.

Soft nack.