linux-kernel - Re: [PATCH bpf-next 3/7] bpf: Introduce BPF_MODIFY

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Date:   Tue, 3 Mar 2020 14:37:58 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     KP Singh <kpsingh@...omium.org>
Cc:     open list <linux-kernel@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Paul Turner <pjt@...gle.com>,
        Florent Revest <revest@...omium.org>,
        Brendan Jackman <jackmanb@...omium.org>
Subject: Re: [PATCH bpf-next 3/7] bpf: Introduce BPF_MODIFY_RETURN

On Tue, Mar 3, 2020 at 6:12 AM KP Singh <kpsingh@...omium.org> wrote:
>
> From: KP Singh <kpsingh@...gle.com>
>
> When multiple programs are attached, each program receives the return
> value from the previous program on the stack and the last program
> provides the return value to the attached function.
>
> The fmod_ret bpf programs are run after the fentry programs and before
> the fexit programs. The original function is only called if all the
> fmod_ret programs return 0 to avoid any unintended side-effects. The
> success value, i.e. 0 is not currently configurable but can be made so
> where user-space can specify it at load time.
>
> For example:
>
> int func_to_be_attached(int a, int b)
> {  <--- do_fentry
>
> do_fmod_ret:
>    <update ret by calling fmod_ret>
>    if (ret != 0)
>         goto do_fexit;
>
> original_function:
>
>     <side_effects_happen_here>
>
> }  <--- do_fexit
>
> The fmod_ret program attached to this function can be defined as:
>
> SEC("fmod_ret/func_to_be_attached")
> BPF_PROG(func_name, int a, int b, int ret)

same as on cover letter, return type is missing

> {
>         // This will skip the original function logic.
>         return 1;
> }
>
> The first fmod_ret program is passed 0 in its return argument.
>
> Signed-off-by: KP Singh <kpsingh@...gle.com>
> ---
>  arch/x86/net/bpf_jit_comp.c    | 96 ++++++++++++++++++++++++++++++++--
>  include/linux/bpf.h            |  1 +
>  include/uapi/linux/bpf.h       |  1 +
>  kernel/bpf/btf.c               |  3 +-
>  kernel/bpf/syscall.c           |  1 +
>  kernel/bpf/trampoline.c        |  5 +-
>  kernel/bpf/verifier.c          |  1 +
>  tools/include/uapi/linux/bpf.h |  1 +
>  8 files changed, 103 insertions(+), 6 deletions(-)
>

[...]

>
> +       if (fmod_ret->nr_progs) {
> +               branches = kcalloc(fmod_ret->nr_progs, sizeof(u8 *),
> +                                  GFP_KERNEL);
> +               if (!branches)
> +                       return -ENOMEM;
> +               if (invoke_bpf_mod_ret(m, &prog, fmod_ret, stack_size,
> +                                      branches))

branches leaks here

> +                       return -EINVAL;
> +       }
> +
>         if (flags & BPF_TRAMP_F_CALL_ORIG) {
> -               if (fentry->nr_progs)
> +               if (fentry->nr_progs || fmod_ret->nr_progs)
>                         restore_regs(m, &prog, nr_args, stack_size);
>
>                 /* call original function */
> @@ -1573,6 +1649,14 @@ int arch_prepare_bpf_trampoline(void *image, void *image_end,

there is early return one line above here, you need to free branches
in that case to not leak memory

So I guess it's better to do goto cleanup approach at this point?

>                 emit_stx(&prog, BPF_DW, BPF_REG_FP, BPF_REG_0, -8);
>         }
>
> +       if (fmod_ret->nr_progs) {
> +               align16_branch_target(&prog);
> +               for (i = 0; i < fmod_ret->nr_progs; i++)
> +                       emit_cond_near_jump(&branches[i], prog, branches[i],
> +                                           X86_JNE);
> +               kfree(branches);
> +       }
> +

[...]