Re: [PATCH bpf-next] bpf: allow readonly direct path access for skfilter

[Alexei Starovoitov]

On Tue, Nov 23, 2021 at 12:56 PM Maciej Żenczykowski
<zenczykowski@xxxxxxxxx> wrote:
>
> From: Maciej Żenczykowski <maze@xxxxxxxxxx>
>
> skfilter bpf programs can read the packet directly via llvm.bpf.load.byte/
> /half/word which are 8/16/32-bit primitive bpf instructions and thus
> behave basically as well as DPA reads.  But there is no 64-bit equivalent,
> due to the support for the equivalent 64-bit bpf opcode never having been
> added (unclear why, there was a patch posted).
> DPA uses a slightly different mechanism, so doesn't suffer this limitation.
>
> Using 64-bit reads, 128-bit ipv6 address comparisons can be done in just
> 2 steps, instead of the 4 steps needed with llvm.bpf.word.

llvm.bpf.word is a pseudo instruction.
It's actually a function call for classic bpf.
See bpf_gen_ld_abs.
We used to have ugly special cases for them in JITs,
but then got rid of it.
Don't use them if performance is a requirement.

> This should hopefully allow simpler (less instructions, and possibly less
> logic and maybe even less jumps) programs.  Less jumps may also mean vastly
> faster bpf verifier times (it can be exponential in the number of jumps...).
>
> This can be particularly important when trying to do something like scan
> a netlink message for a pattern (2000 iteration loop) to decide whether
> a message should be dropped, or delivered to userspace (thus waking it up).
>
> I'm requiring CAP_NET_ADMIN because I'm not sure of the security
> implications...
>
> Tested: only build tested
> Signed-off-by: Maciej Żenczykowski <maze@xxxxxxxxxx>
> ---
>  kernel/bpf/verifier.c | 5 +++++
>  1 file changed, 5 insertions(+)
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 331b170d9fcc..0c2e25fb9844 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -3258,6 +3258,11 @@ static bool may_access_direct_pkt_data(struct bpf_verifier_env *env,
>         enum bpf_prog_type prog_type = resolve_prog_type(env->prog);
>
>         switch (prog_type) {
> +       case BPF_PROG_TYPE_SOCKET_FILTER:
> +               if (meta || !capable(CAP_NET_ADMIN))
> +                       return false;

probably needs CAP_BPF too.

Other than that I think it's fine.