netdev - Re: [PATCH bpf-next 2/3] selftests/bpf: Avoid running unprivileged tests with alignment requirements

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Date:   Tue, 17 Nov 2020 17:43:20 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Björn Töpel <bjorn.topel@...il.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        xi.wang@...il.com, luke.r.nels@...il.com,
        linux-riscv@...ts.infradead.org
Subject: Re: [PATCH bpf-next 2/3] selftests/bpf: Avoid running unprivileged
 tests with alignment requirements

On Tue, Nov 17, 2020 at 12:29 AM Björn Töpel <bjorn.topel@...il.com> wrote:
>
> Some architectures have strict alignment requirements. In that case,
> the BPF verifier detects if a program has unaligned accesses and
> rejects them. A user can pass BPF_F_ANY_ALIGNMENT to a program to
> override this check. That, however, will only work when a privileged
> user loads a program. A unprivileged user loading a program with this
> flag will be rejected prior entering the verifier.

I'd include this paragraph as a code comment right next to the check below.

>
> Hence, it does not make sense to load unprivileged programs without
> strict alignment when testing the verifier. This patch avoids exactly
> that.
>
> Signed-off-by: Björn Töpel <bjorn.topel@...il.com>
> ---
>  tools/testing/selftests/bpf/test_verifier.c | 12 +++++++++---
>  1 file changed, 9 insertions(+), 3 deletions(-)
>
> diff --git a/tools/testing/selftests/bpf/test_verifier.c b/tools/testing/selftests/bpf/test_verifier.c
> index 9be395d9dc64..2075f6a98813 100644
> --- a/tools/testing/selftests/bpf/test_verifier.c
> +++ b/tools/testing/selftests/bpf/test_verifier.c
> @@ -1152,9 +1152,15 @@ static void get_unpriv_disabled()
>
>  static bool test_as_unpriv(struct bpf_test *test)
>  {
> -       return !test->prog_type ||
> -              test->prog_type == BPF_PROG_TYPE_SOCKET_FILTER ||
> -              test->prog_type == BPF_PROG_TYPE_CGROUP_SKB;
> +       bool req_aligned = false;
> +
> +#ifndef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
> +       req_aligned = test->flags & F_NEEDS_EFFICIENT_UNALIGNED_ACCESS;
> +#endif
> +       return (!test->prog_type ||
> +               test->prog_type == BPF_PROG_TYPE_SOCKET_FILTER ||
> +               test->prog_type == BPF_PROG_TYPE_CGROUP_SKB) &&
> +               !req_aligned;

It's a bit convoluted. This seems a bit more straightforward:

#ifndef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
    if (test->flags & F_NEEDS_EFFICIENT_UNALIGNED_ACCESS)
        return false;
#endif
/* the rest of logic untouched */

?






>  }
>
>  static int do_test(bool unpriv, unsigned int from, unsigned int to)
> --
> 2.27.0
>