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Date: Thu, 6 Jun 2019 15:33:44 +0200
From: Jesper Dangaard Brouer <brouer@...hat.com>
To: Toke Høiland-Jørgensen <toke@...hat.com>
Cc: David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
Daniel Borkmann <daniel@...earbox.net>,
Alexei Starovoitov <ast@...nel.org>, brouer@...hat.com
Subject: Re: [PATCH net-next v2 2/2] devmap: Allow map lookups from eBPF
On Thu, 06 Jun 2019 15:24:14 +0200
Toke Høiland-Jørgensen <toke@...hat.com> wrote:
> From: Toke Høiland-Jørgensen <toke@...hat.com>
>
> We don't currently allow lookups into a devmap from eBPF, because the map
> lookup returns a pointer directly to the dev->ifindex, which shouldn't be
> modifiable from eBPF.
>
> However, being able to do lookups in devmaps is useful to know (e.g.)
> whether forwarding to a specific interface is enabled. Currently, programs
> work around this by keeping a shadow map of another type which indicates
> whether a map index is valid.
>
> Since we now have a flag to make maps read-only from the eBPF side, we can
> simply lift the lookup restriction if we make sure this flag is always set.
Nice, I didn't know this was possible. I like it! :-)
> Signed-off-by: Toke Høiland-Jørgensen <toke@...hat.com>
> ---
> kernel/bpf/devmap.c | 5 +++++
> kernel/bpf/verifier.c | 7 ++-----
> 2 files changed, 7 insertions(+), 5 deletions(-)
>
> diff --git a/kernel/bpf/devmap.c b/kernel/bpf/devmap.c
> index 5ae7cce5ef16..0e6875a462ef 100644
> --- a/kernel/bpf/devmap.c
> +++ b/kernel/bpf/devmap.c
> @@ -99,6 +99,11 @@ static struct bpf_map *dev_map_alloc(union bpf_attr *attr)
> attr->value_size != 4 || attr->map_flags & ~DEV_CREATE_FLAG_MASK)
> return ERR_PTR(-EINVAL);
>
> + /* Lookup returns a pointer straight to dev->ifindex, so make sure the
> + * verifier prevents writes from the BPF side
> + */
> + attr->map_flags |= BPF_F_RDONLY_PROG;
> +
> dtab = kzalloc(sizeof(*dtab), GFP_USER);
> if (!dtab)
> return ERR_PTR(-ENOMEM);
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 5c2cb5bd84ce..7128a9821481 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -2893,12 +2893,9 @@ static int check_map_func_compatibility(struct bpf_verifier_env *env,
> if (func_id != BPF_FUNC_get_local_storage)
> goto error;
> break;
> - /* devmap returns a pointer to a live net_device ifindex that we cannot
> - * allow to be modified from bpf side. So do not allow lookup elements
> - * for now.
> - */
> case BPF_MAP_TYPE_DEVMAP:
> - if (func_id != BPF_FUNC_redirect_map)
> + if (func_id != BPF_FUNC_redirect_map &&
> + func_id != BPF_FUNC_map_lookup_elem)
> goto error;
> break;
> /* Restrict bpf side of cpumap and xskmap, open when use-cases
>
--
Best regards,
Jesper Dangaard Brouer
MSc.CS, Principal Kernel Engineer at Red Hat
LinkedIn: http://www.linkedin.com/in/brouer
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