On Wed, Mar 25, 2020 at 04:26:21PM +0100, KP Singh wrote: > From: KP Singh <kpsingh@xxxxxxxxxx> > > # v5 -> v6 > > https://lwn.net/Articles/815826/ Random question: why the switch to lwn.net from lore URLs? The lore URLs have been suggested to be the canonical way to refer to kernel development discussion threads. -Kees > > * Updated LSM_HOOK macro to define a default value and cleaned up the > BPF LSM hook declarations. > * Added Yonghong's Acks and Kees' Reviewed-by tags. > * Simplification of the selftest code. > * Rebase and fixes suggested by Andrii and Yonghong and some other minor > fixes noticed in internal review. > > # v4 -> v5 > > https://lwn.net/Articles/813057/ > > * Removed static keys and special casing of BPF calls from the LSM > framework. > * Initialized the BPF callbacks (nops) as proper LSM hooks. > * Updated to using the newly introduced BPF_TRAMP_MODIFY_RETURN > trampolines in https://lkml.org/lkml/2020/3/4/877 > * Addressed Andrii's feedback and rebased. > > # v3 -> v4 > > * Moved away from allocating a separate security_hook_heads and adding a > new special case for arch_prepare_bpf_trampoline to using BPF fexit > trampolines called from the right place in the LSM hook and toggled by > static keys based on the discussion in: > > https://lore.kernel.org/bpf/CAG48ez25mW+_oCxgCtbiGMX07g_ph79UOJa07h=o_6B6+Q-u5g@xxxxxxxxxxxxxx/ > > * Since the code does not deal with security_hook_heads anymore, it goes > from "being a BPF LSM" to "BPF program attachment to LSM hooks". > * Added a new test case which ensures that the BPF programs' return value > is reflected by the LSM hook. > > # v2 -> v3 does not change the overall design and has some minor fixes: > > * LSM_ORDER_LAST is introduced to represent the behaviour of the BPF LSM > * Fixed the inadvertent clobbering of the LSM Hook error codes > * Added GPL license requirement to the commit log > * The lsm_hook_idx is now the more conventional 0-based index > * Some changes were split into a separate patch ("Load btf_vmlinux only > once per object") > > https://lore.kernel.org/bpf/20200117212825.11755-1-kpsingh@xxxxxxxxxxxx/ > > * Addressed Andrii's feedback on the BTF implementation > * Documentation update for using generated vmlinux.h to simplify > programs > * Rebase > > # Changes since v1 > > https://lore.kernel.org/bpf/20191220154208.15895-1-kpsingh@xxxxxxxxxxxx > > * 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 allows BPF programs to be attached to LSM hooks 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 patchset 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. > Loading and attachment of BPF programs requires CAP_SYS_ADMIN. > > The new LSM registers nop functions (bpf_lsm_<hook_name>) as LSM hook > callbacks. Their purpose is to provide a definite point where BPF > programs can be attached as BPF_TRAMP_MODIFY_RETURN trampoline programs > for hooks that return an int, and BPF_TRAMP_FEXIT trampoline programs > for void LSM hooks. > > 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)); > > // 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, int ret) > { > 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 design presented in the RFC. > > (https://lore.kernel.org/bpf/20190910115527.5235-1-kpsingh@xxxxxxxxxxxx/). > > # Usage Examples > > A simple example and some documentation is included in the patchset. > In order to better illustrate the capabilities of the framework some > more advanced prototype (not-ready for review) code has also been > published separately: > > * Logging execution events (including environment variables and > arguments) > https://github.com/sinkap/linux-krsi/blob/patch/v1/examples/samples/bpf/lsm_audit_env.c > > * Detecting deletion of running executables: > https://github.com/sinkap/linux-krsi/blob/patch/v1/examples/samples/bpf/lsm_detect_exec_unlink.c > > * Detection of writes to /proc/<pid>/mem: > https://github.com/sinkap/linux-krsi/blob/patch/v1/examples/samples/bpf/lsm_audit_env.c > > We have updated Google's internal telemetry infrastructure and have > started deploying this LSM on our Linux Workstations. This gives us more > confidence in the real-world applications of such a system. > > > KP Singh (8): > bpf: Introduce BPF_PROG_TYPE_LSM > security: Refactor declaration of LSM hooks > bpf: lsm: provide attachment points for BPF LSM programs > bpf: lsm: Implement attach, detach and execution > bpf: lsm: Initialize the BPF LSM hooks > tools/libbpf: Add support for BPF_PROG_TYPE_LSM > bpf: lsm: Add selftests for BPF_PROG_TYPE_LSM > bpf: lsm: Add Documentation > > Documentation/bpf/bpf_lsm.rst | 150 +++++ > Documentation/bpf/index.rst | 1 + > MAINTAINERS | 1 + > include/linux/bpf.h | 3 + > include/linux/bpf_lsm.h | 32 + > include/linux/bpf_types.h | 4 + > include/linux/lsm_hook_defs.h | 378 +++++++++++ > include/linux/lsm_hooks.h | 627 +----------------- > include/uapi/linux/bpf.h | 2 + > init/Kconfig | 10 + > kernel/bpf/Makefile | 1 + > kernel/bpf/bpf_lsm.c | 60 ++ > kernel/bpf/btf.c | 9 +- > kernel/bpf/syscall.c | 56 +- > kernel/bpf/trampoline.c | 17 +- > kernel/bpf/verifier.c | 19 +- > kernel/trace/bpf_trace.c | 12 +- > security/Kconfig | 10 +- > security/Makefile | 2 + > security/bpf/Makefile | 5 + > security/bpf/hooks.c | 26 + > security/security.c | 432 ++++++------ > tools/include/uapi/linux/bpf.h | 2 + > tools/lib/bpf/bpf.c | 3 +- > tools/lib/bpf/libbpf.c | 39 +- > tools/lib/bpf/libbpf.h | 4 + > tools/lib/bpf/libbpf.map | 3 + > tools/lib/bpf/libbpf_probes.c | 1 + > tools/testing/selftests/bpf/config | 2 + > tools/testing/selftests/bpf/lsm_helpers.h | 19 + > .../selftests/bpf/prog_tests/lsm_test.c | 112 ++++ > .../selftests/bpf/progs/lsm_int_hook.c | 54 ++ > .../selftests/bpf/progs/lsm_void_hook.c | 41 ++ > 33 files changed, 1277 insertions(+), 860 deletions(-) > create mode 100644 Documentation/bpf/bpf_lsm.rst > create mode 100644 include/linux/bpf_lsm.h > create mode 100644 include/linux/lsm_hook_defs.h > create mode 100644 kernel/bpf/bpf_lsm.c > create mode 100644 security/bpf/Makefile > create mode 100644 security/bpf/hooks.c > create mode 100644 tools/testing/selftests/bpf/lsm_helpers.h > create mode 100644 tools/testing/selftests/bpf/prog_tests/lsm_test.c > create mode 100644 tools/testing/selftests/bpf/progs/lsm_int_hook.c > create mode 100644 tools/testing/selftests/bpf/progs/lsm_void_hook.c > > -- > 2.20.1 > -- Kees Cook
