lkml.org 
[lkml]   [2020]   [Mar]   [5]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    From
    Date
    SubjectRe: [PATCH bpf-next v4 3/7] bpf: Introduce BPF_MODIFY_RETURN
    On Wed, Mar 4, 2020 at 2:20 PM KP Singh <kpsingh@chromium.org> wrote:
    >
    > From: KP Singh <kpsingh@google.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")
    > int BPF_PROG(func_name, int a, int b, int ret)
    > {
    > // 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@google.com>
    > Acked-by: Andrii Nakryiko <andriin@fb.com>

    IIUC you've switched from a model where the BPF program would be
    invoked after the original function logic
    and the BPF program is skipped if the original function logic returns
    non-zero to a model where the BPF program is invoked first and
    the original function logic is skipped if the BPF program returns
    non-zero. I'm not keen on that for userspace-loaded code attached
    to LSM hooks; it means that userspace BPF programs can run even if
    SELinux would have denied access and SELinux hooks get
    skipped entirely if the BPF program returns an error. I think Casey
    may have wrongly pointed you in this direction on the grounds
    it can already happen with the base DAC checking logic. But that's
    kernel DAC checking logic, not userspace-loaded code.
    And the existing checking on attachment is not sufficient for SELinux
    since CAP_MAC_ADMIN is not all powerful to SELinux.
    Be careful about designing your mechanisms around Smack because Smack
    is not the only LSM.

    \
     
     \ /
      Last update: 2020-03-05 14:51    [W:4.081 / U:0.260 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site