Re: [PATCH bpf-next v2 00/10] MAC and Audit policy using eBPF (KRSI)

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Wed, Jan 15, 2020 at 9:15 AM KP Singh <kpsingh@chromium.org> wrote:
>
> From: KP Singh <kpsingh@google.com>
>
> # Changes since v1 (https://lore.kernel.org/bpf/20191220154208.15895-1-kpsingh@chromium.org/):
>
> * Eliminate the requirement to maintain LSM hooks separately in
>   security/bpf/hooks.h Use BPF trampolines to dynamically allocate
>   security hooks
> * Drop the use of securityfs as bpftool provides the required
>   introspection capabilities.  Update the tests to use the bpf_skeleton
>   and global variables
> * Use O_CLOEXEC anonymous fds to represent BPF attachment in line with
>   the other BPF programs with the possibility to use bpf program pinning
>   in the future to provide "permanent attachment".
> * Drop the logic based on prog names for handling re-attachment.
> * Drop bpf_lsm_event_output from this series and send it as a separate
>   patch.
>
> # Motivation
>
> Google does analysis of rich runtime security data to detect and thwart
> threats in real-time. Currently, this is done in custom kernel modules
> but we would like to replace this with something that's upstream and
> useful to others.
>
> The current kernel infrastructure for providing telemetry (Audit, Perf
> etc.) is disjoint from access enforcement (i.e. LSMs).  Augmenting the
> information provided by audit requires kernel changes to audit, its
> policy language and user-space components. Furthermore, building a MAC
> policy based on the newly added telemetry data requires changes to
> various LSMs and their respective policy languages.
>
> This patchset proposes a new stackable and privileged LSM which allows
> the LSM hooks to be implemented using eBPF. This facilitates a unified
> and dynamic (not requiring re-compilation of the kernel) audit and MAC
> policy.
>
> # Why an LSM?
>
> Linux Security Modules target security behaviours rather than the
> kernel's API. For example, it's easy to miss out a newly added system
> call for executing processes (eg. execve, execveat etc.) but the LSM
> framework ensures that all process executions trigger the relevant hooks
> irrespective of how the process was executed.
>
> Allowing users to implement LSM hooks at runtime also benefits the LSM
> eco-system by enabling a quick feedback loop from the security community
> about the kind of behaviours that the LSM Framework should be targeting.
>
> # How does it work?
>
> The LSM introduces a new eBPF (https://docs.cilium.io/en/v1.6/bpf/)
> program type BPF_PROG_TYPE_LSM which can only be attached to LSM hooks.
> Attachment requires CAP_SYS_ADMIN for loading eBPF programs and
> CAP_MAC_ADMIN for modifying MAC policies.
>
> The eBPF programs are attached to a separate security_hook_heads
> maintained by the BPF LSM for mutable hooks and executed after all the
> statically defined hooks (i.e. the ones declared by SELinux, AppArmor,
> Smack etc). This also ensures that statically defined LSM hooks retain
> the behaviour of "being read-only after init", i.e. __lsm_ro_after_init.
>
> Upon attachment, a security hook is dynamically allocated with
> arch_bpf_prepare_trampoline which generates code to handle the
> conversion from the signature of the hook to the BPF context and allows
> the JIT'ed BPF program to be called as a C function with the same
> arguments as the LSM hooks. If any of the attached eBPF programs returns
> an error (like ENOPERM), the behaviour represented by the hook is
> denied.
>
> Audit logs can be written using a format chosen by the eBPF program to
> the perf events buffer or to global eBPF variables or maps and can be
> further processed in user-space.
>
> # BTF Based Design
>
> The current design uses BTF
> (https://facebookmicrosites.github.io/bpf/blog/2018/11/14/btf-enhancement.html,
> https://lwn.net/Articles/803258/) which allows verifiable read-only
> structure accesses by field names rather than fixed offsets. This allows
> accessing the hook parameters using a dynamically created context which
> provides a certain degree of ABI stability:
>
>
> // Only declare the structure and fields intended to be used
> // in the program
> struct vm_area_struct {
>         unsigned long vm_start;
> } __attribute__((preserve_access_index));
>

It would be nice to also mention that you don't even have to
"re-define" these structs if you use vmlinux.h generated with `bpftool
btf dump file <path-to-vm-linux-or-/sys/kernel/btf/vmlinux> format c`.
Its output will contain all types of the kernel, including internal
ones not exposed through any public headers. And it will also
automatically have __attribute__((preserve_access_index)) applied to
all structs/unions. It can be pre-generated and checked in somewhere
along the application or generated on the fly, if environment and use
case allows.

> // Declare the eBPF program mprotect_audit which attaches to
> // to the file_mprotect LSM hook and accepts three arguments.
> SEC("lsm/file_mprotect")
> int BPF_PROG(mprotect_audit, struct vm_area_struct *vma,
>              unsigned long reqprot, unsigned long prot)
> {
>         unsigned long vm_start = vma->vm_start;
>
>         return 0;
> }
>
> By relocating field offsets, BTF makes a large portion of kernel data
> structures readily accessible across kernel versions without requiring a
> large corpus of BPF helper functions and requiring recompilation with
> every kernel version. The BTF type information is also used by the BPF
> verifier to validate memory accesses within the BPF program and also
> prevents arbitrary writes to the kernel memory.
>
> The limitations of BTF compatibility are described in BPF Co-Re
> (http://vger.kernel.org/bpfconf2019_talks/bpf-core.pdf, i.e. field
> renames, #defines and changes to the signature of LSM hooks).
>
> This design imposes that the MAC policy (eBPF programs) be updated when
> the inspected kernel structures change outside of BTF compatibility
> guarantees. In practice, this is only required when a structure field
> used by a current policy is removed (or renamed) or when the used LSM
> hooks change. We expect the maintenance cost of these changes to be
> acceptable as compared to the previous design
> (https://lore.kernel.org/bpf/20190910115527.5235-1-kpsingh@chromium.org/).
>

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