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From:   Andrii Nakryiko <andrii.nakryiko@gmail.com>
Date:   Mon, 23 Dec 2019 22:49:02 -0800
Message-ID: <CAEf4BzY4K-vgSFPjV=pn3quc5DT1+eGkJnZfSw4+b0fERzPVfw@mail.gmail.com>
Subject: Re: [PATCH bpf-next v1 12/13] bpf: lsm: Add selftests for BPF_PROG_TYPE_LSM
To:     KP Singh <kpsingh@chromium.org>
Cc:     open list <linux-kernel@vger.kernel.org>,
        bpf <bpf@vger.kernel.org>, linux-security-module@vger.kernel.org,
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On Fri, Dec 20, 2019 at 7:42 AM KP Singh <kpsingh@chromium.org> wrote:
>
> From: KP Singh <kpsingh@google.com>
>
> * Load a BPF program that audits mprotect calls
> * Attach the program to the "file_mprotect" LSM hook
> * Verify if the program is actually loading by reading
>   securityfs
> * Initialize the perf events buffer and poll for audit events
> * Do an mprotect on some memory allocated on the heap
> * Verify if the audit event was received
>
> Signed-off-by: KP Singh <kpsingh@google.com>
> ---
>  MAINTAINERS                                   |   2 +
>  .../bpf/prog_tests/lsm_mprotect_audit.c       | 129 ++++++++++++++++++
>  .../selftests/bpf/progs/lsm_mprotect_audit.c  |  58 ++++++++
>  3 files changed, 189 insertions(+)
>  create mode 100644 tools/testing/selftests/bpf/prog_tests/lsm_mprotect_audit.c
>  create mode 100644 tools/testing/selftests/bpf/progs/lsm_mprotect_audit.c
>

[...]

> +/*
> + * Define some of the structs used in the BPF program.
> + * Only the field names and their sizes need to be the
> + * same as the kernel type, the order is irrelevant.
> + */
> +struct mm_struct {
> +       unsigned long start_brk, brk, start_stack;
> +};
> +
> +struct vm_area_struct {
> +       unsigned long start_brk, brk, start_stack;
> +       unsigned long vm_start, vm_end;
> +       struct mm_struct *vm_mm;
> +       unsigned long vm_flags;
> +};
> +
> +BPF_TRACE_3("lsm/file_mprotect", mprotect_audit,
> +           struct vm_area_struct *, vma,
> +           unsigned long, reqprot, unsigned long, prot)
> +{
> +       struct mprotect_audit_log audit_log = {};
> +       int is_heap = 0;
> +
> +       __builtin_preserve_access_index(({

you don't need __builtin_preserve_access_index, if you mark
vm_area_struct and mm_struct with
__attribute__((preserve_access_index)

> +               is_heap = (vma->vm_start >= vma->vm_mm->start_brk &&
> +                                    vma->vm_end <= vma->vm_mm->brk);
> +       }));
> +
> +       audit_log.magic = MPROTECT_AUDIT_MAGIC;
> +       audit_log.is_heap = is_heap;
> +       bpf_lsm_event_output(&perf_buf_map, BPF_F_CURRENT_CPU, &audit_log,
> +                            sizeof(audit_log));

You test would be much simpler if you use global variables to pass
data back to userspace, instead of using perf buffer.

Also please see fentry_fexit.c test for example of using BPF skeleton
to shorten and simpify userspace part of test.

> +       return 0;
> +}
> --
> 2.20.1
>