Re: [RFC PATCH security-next 1/4] security: Hornet LSM

sergeh
2025-03-21T22:29:49Z

On Fri, Mar 21, 2025 at 09:45:03AM -0700, Blaise Boscaccy wrote:
> This adds the Hornet Linux Security Module which provides signature
> verification of eBPF programs.
>
> Hornet uses a similar signature verification scheme similar to that of

used 'similar' twice

> kernel modules. A pkcs#7 signature is appended to the end of an
> executable file. During an invocation of bpf_prog_load, the signature
> is fetched from the current task's executable file. That signature is
> used to verify the integrity of the bpf instructions and maps which
> where passed into the kernel. Additionally, Hornet implicitly trusts any

s/where/were

> programs which where loaded from inside kernel rather than userspace,

s/where/were

> which allows BPF_PRELOAD programs along with outputs for BPF_SYSCALL
> programs to run.
>
> Hornet allows users to continue to maintain an invariant that all code
> running inside of the kernel has been signed and works well with
> light-skeleton based loaders, or any statically generated program that
> doesn't require userspace instruction rewriting.
>
> Signed-off-by: Blaise Boscaccy <bboscaccy@linux.microsoft.com>
> ---
> Documentation/admin-guide/LSM/Hornet.rst | 51 +++++
> crypto/asymmetric_keys/pkcs7_verify.c | 10 +
> include/linux/kernel_read_file.h | 1 +
> include/linux/verification.h | 1 +
> include/uapi/linux/lsm.h | 1 +
> security/Kconfig | 3 +-
> security/Makefile | 1 +
> security/hornet/Kconfig | 11 ++
> security/hornet/Makefile | 4 +
> security/hornet/hornet_lsm.c | 239 +++++++++++++++++++++++
> 10 files changed, 321 insertions(+), 1 deletion(-)
> create mode 100644 Documentation/admin-guide/LSM/Hornet.rst
> create mode 100644 security/hornet/Kconfig
> create mode 100644 security/hornet/Makefile
> create mode 100644 security/hornet/hornet_lsm.c
>
> diff --git a/Documentation/admin-guide/LSM/Hornet.rst b/Documentation/admin-guide/LSM/Hornet.rst
> new file mode 100644
> index 0000000000000..fa112412638f1
> --- /dev/null
> +++ b/Documentation/admin-guide/LSM/Hornet.rst
> @@ -0,0 +1,51 @@
> +======
> +Hornet
> +======
> +
> +Hornet is a Linux Security Module that provides signature verification
> +for eBPF programs. This is selectable at build-time with
> +``CONFIG_SECURITY_HORNET``.
> +
> +Overview
> +========
> +
> +Hornet provides signature verification for eBPF programs by utilizing
> +the existing PKCS#7 infrastructure that's used for module signature
> +verification. Hornet works by creating a buffer containing the eBPF
> +program instructions along with its associated maps and checking a
> +signature against that buffer. The signature is appended to the end of
> +the lskel executable file and is extracted at runtime via
> +get_task_exe_file. Hornet works by hooking into the
> +security_bpf_prog_load hook. Load invocations that originate from the
> +kernel (bpf preload, results of bpf_syscall programs, etc.) are
> +allowed to run unconditionally. Calls that originate from userspace
> +require signature verification. If signature verification fails, the
> +program will fail to load.
> +
> +Instruction/Map Ordering
> +========================
> +
> +Hornet supports both sparse-array based maps via map discovery along
> +with the newly added fd_array_cnt API for continuous map arrays. The
> +buffer used for signature verification is assumed to be the
> +instructions followed by all maps used, ordered by their index in
> +fd_array.
> +
> +Tooling
> +=======
> +
> +Some tooling is provided to aid with the development of signed eBPF lskels.
> +
> +extract-skel.sh
> +---------------
> +
> +This simple shell script extracts the instructions and map data used
> +by the light skeleton from the autogenerated header file created by
> +bpftool.
> +
> +sign-ebpf
> +---------
> +
> +sign-ebpf works similarly to the sign-file script with one key
> +difference: it takes a separate input binary used for signature
> +verification and will append the signature to a different output file.
> diff --git a/crypto/asymmetric_keys/pkcs7_verify.c b/crypto/asymmetric_keys/pkcs7_verify.c
> index f0d4ff3c20a83..1a5fbb3612188 100644
> --- a/crypto/asymmetric_keys/pkcs7_verify.c
> +++ b/crypto/asymmetric_keys/pkcs7_verify.c
> @@ -428,6 +428,16 @@ int pkcs7_verify(struct pkcs7_message *pkcs7,
> }
> /* Authattr presence checked in parser */
> break;
> + case VERIFYING_EBPF_SIGNATURE:
> + if (pkcs7->data_type != OID_data) {
> + pr_warn("Invalid ebpf sig (not pkcs7-data)\n");
> + return -EKEYREJECTED;
> + }
> + if (pkcs7->have_authattrs) {
> + pr_warn("Invalid ebpf sig (has authattrs)\n");
> + return -EKEYREJECTED;
> + }
> + break;
> case VERIFYING_UNSPECIFIED_SIGNATURE:
> if (pkcs7->data_type != OID_data) {
> pr_warn("Invalid unspecified sig (not pkcs7-data)\n");
> diff --git a/include/linux/kernel_read_file.h b/include/linux/kernel_read_file.h
> index 90451e2e12bd1..7ed9337be5423 100644
> --- a/include/linux/kernel_read_file.h
> +++ b/include/linux/kernel_read_file.h
> @@ -14,6 +14,7 @@
> id(KEXEC_INITRAMFS, kexec-initramfs) \
> id(POLICY, security-policy) \
> id(X509_CERTIFICATE, x509-certificate) \
> + id(EBPF, ebpf) \
> id(MAX_ID, )
>
> #define __fid_enumify(ENUM, dummy) READING_ ## ENUM,
> diff --git a/include/linux/verification.h b/include/linux/verification.h
> index 4f3022d081c31..812be8ad5f744 100644
> --- a/include/linux/verification.h
> +++ b/include/linux/verification.h
> @@ -35,6 +35,7 @@ enum key_being_used_for {
> VERIFYING_KEXEC_PE_SIGNATURE,
> VERIFYING_KEY_SIGNATURE,
> VERIFYING_KEY_SELF_SIGNATURE,
> + VERIFYING_EBPF_SIGNATURE,
> VERIFYING_UNSPECIFIED_SIGNATURE,
> NR__KEY_BEING_USED_FOR
> };
> diff --git a/include/uapi/linux/lsm.h b/include/uapi/linux/lsm.h
> index 938593dfd5daf..2ff9bcdd551e2 100644
> --- a/include/uapi/linux/lsm.h
> +++ b/include/uapi/linux/lsm.h
> @@ -65,6 +65,7 @@ struct lsm_ctx {
> #define LSM_ID_IMA 111
> #define LSM_ID_EVM 112
> #define LSM_ID_IPE 113
> +#define LSM_ID_HORNET 114
>
> /*
> * LSM_ATTR_XXX definitions identify different LSM attributes
> diff --git a/security/Kconfig b/security/Kconfig
> index f10dbf15c2947..0030f0224c7ab 100644
> --- a/security/Kconfig
> +++ b/security/Kconfig
> @@ -230,6 +230,7 @@ source "security/safesetid/Kconfig"
> source "security/lockdown/Kconfig"
> source "security/landlock/Kconfig"
> source "security/ipe/Kconfig"
> +source "security/hornet/Kconfig"
>
> source "security/integrity/Kconfig"
>
> @@ -273,7 +274,7 @@ config LSM
> default "landlock,lockdown,yama,loadpin,safesetid,apparmor,selinux,smack,tomoyo,ipe,bpf" if DEFAULT_SECURITY_APPARMOR
> default "landlock,lockdown,yama,loadpin,safesetid,tomoyo,ipe,bpf" if DEFAULT_SECURITY_TOMOYO
> default "landlock,lockdown,yama,loadpin,safesetid,ipe,bpf" if DEFAULT_SECURITY_DAC
> - default "landlock,lockdown,yama,loadpin,safesetid,selinux,smack,tomoyo,apparmor,ipe,bpf"
> + default "landlock,lockdown,yama,loadpin,safesetid,selinux,smack,tomoyo,apparmor,ipe,hornet,bpf"
> help
> A comma-separated list of LSMs, in initialization order.
> Any LSMs left off this list, except for those with order
> diff --git a/security/Makefile b/security/Makefile
> index 22ff4c8bd8cec..e24bccd951f88 100644
> --- a/security/Makefile
> +++ b/security/Makefile
> @@ -26,6 +26,7 @@ obj-$(CONFIG_CGROUPS) += device_cgroup.o
> obj-$(CONFIG_BPF_LSM) += bpf/
> obj-$(CONFIG_SECURITY_LANDLOCK) += landlock/
> obj-$(CONFIG_SECURITY_IPE) += ipe/
> +obj-$(CONFIG_SECURITY_HORNET) += hornet/
>
> # Object integrity file lists
> obj-$(CONFIG_INTEGRITY) += integrity/
> diff --git a/security/hornet/Kconfig b/security/hornet/Kconfig
> new file mode 100644
> index 0000000000000..19406aa237ac6
> --- /dev/null
> +++ b/security/hornet/Kconfig
> @@ -0,0 +1,11 @@
> +# SPDX-License-Identifier: GPL-2.0-only
> +config SECURITY_HORNET
> + bool "Hornet support"
> + depends on SECURITY
> + default n
> + help
> + This selects Hornet.
> + Further information can be found in
> + Documentation/admin-guide/LSM/Hornet.rst.
> +
> + If you are unsure how to answer this question, answer N.
> diff --git a/security/hornet/Makefile b/security/hornet/Makefile
> new file mode 100644
> index 0000000000000..79f4657b215fa
> --- /dev/null
> +++ b/security/hornet/Makefile
> @@ -0,0 +1,4 @@
> +# SPDX-License-Identifier: GPL-2.0-only
> +obj-$(CONFIG_SECURITY_HORNET) := hornet.o
> +
> +hornet-y := hornet_lsm.o
> diff --git a/security/hornet/hornet_lsm.c b/security/hornet/hornet_lsm.c
> new file mode 100644
> index 0000000000000..3616c68b76fbc
> --- /dev/null
> +++ b/security/hornet/hornet_lsm.c
> @@ -0,0 +1,239 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/*
> + * Hornet Linux Security Module
> + *
> + * Author: Blaise Boscaccy <bboscaccy@linux.microsoft.com>
> + *
> + * Copyright (C) 2025 Microsoft Corporation
> + */
> +
> +#include <linux/lsm_hooks.h>
> +#include <uapi/linux/lsm.h>
> +#include <linux/bpf.h>
> +#include <linux/verification.h>
> +#include <crypto/public_key.h>
> +#include <linux/module_signature.h>
> +#include <crypto/pkcs7.h>
> +#include <linux/bpf_verifier.h>
> +#include <linux/sort.h>
> +
> +#define EBPF_SIG_STRING "~eBPF signature appended~\n"
> +
> +struct hornet_maps {
> + u32 used_idx[MAX_USED_MAPS];
> + u32 used_map_cnt;
> + bpfptr_t fd_array;
> +};
> +
> +static int cmp_idx(const void *a, const void *b)
> +{
> + return *(const u32 *)a - *(const u32 *)b;
> +}
> +
> +static int add_used_map(struct hornet_maps *maps, int idx)
> +{
> + int i;
> +
> + for (i = 0; i < maps->used_map_cnt; i++)
> + if (maps->used_idx[i] == idx)
> + return i;
> +
> + if (maps->used_map_cnt >= MAX_USED_MAPS)
> + return -E2BIG;
> +
> + maps->used_idx[maps->used_map_cnt] = idx;
> + return maps->used_map_cnt++;
> +}
> +
> +static int hornet_find_maps(struct bpf_prog *prog, struct hornet_maps *maps)
> +{
> + struct bpf_insn *insn = prog->insnsi;
> + int insn_cnt = prog->len;
> + int i;
> + int err;
> +
> + for (i = 0; i < insn_cnt; i++, insn++) {
> + if (insn[0].code == (BPF_LD | BPF_IMM | BPF_DW)) {
> + switch (insn[0].src_reg) {
> + case BPF_PSEUDO_MAP_IDX_VALUE:
> + case BPF_PSEUDO_MAP_IDX:
> + err = add_used_map(maps, insn[0].imm);
> + if (err < 0)
> + return err;
> + break;
> + default:
> + break;
> + }
> + }
> + }
> + /* Sort the spare-array indices. This should match the map ordering used during
> + * signature generation
> + */
> + sort(maps->used_idx, maps->used_map_cnt, sizeof(*maps->used_idx),
> + cmp_idx, NULL);
> +
> + return 0;
> +}
> +
> +static int hornet_populate_fd_array(struct hornet_maps *maps, u32 fd_array_cnt)
> +{
> + int i;
> +
> + if (fd_array_cnt > MAX_USED_MAPS)
> + return -E2BIG;
> +
> + for (i = 0; i < fd_array_cnt; i++)
> + maps->used_idx[i] = i;
> +
> + maps->used_map_cnt = fd_array_cnt;
> + return 0;
> +}
> +
> +/* kern_sys_bpf is declared as an EXPORT_SYMBOL in kernel/bpf/syscall.c, however no definition is
> + * provided in any bpf header files. If/when this function has a proper definition provided
> + * somewhere this declaration should be removed
> + */
> +int kern_sys_bpf(int cmd, union bpf_attr *attr, unsigned int size);
> +
> +static int hornet_verify_lskel(struct bpf_prog *prog, struct hornet_maps *maps,
> + void *sig, size_t sig_len)
> +{
> + int fd;
> + u32 i;
> + void *buf;
> + void *new;
> + size_t buf_sz;
> + struct bpf_map *map;
> + int err = 0;
> + int key = 0;
> + union bpf_attr attr = {0};
> +
> + buf = kmalloc_array(prog->len, sizeof(struct bpf_insn), GFP_KERNEL);
> + if (!buf)
> + return -ENOMEM;
> + buf_sz = prog->len * sizeof(struct bpf_insn);
> + memcpy(buf, prog->insnsi, buf_sz);
> +
> + for (i = 0; i < maps->used_map_cnt; i++) {
> + err = copy_from_bpfptr_offset(&fd, maps->fd_array,
> + maps->used_idx[i] * sizeof(fd),
> + sizeof(fd));
> + if (err < 0)
> + continue;
> + if (fd < 1)
> + continue;
> +
> + map = bpf_map_get(fd);
> + if (IS_ERR(map))
> + continue;
> +
> + /* don't allow userspace to change map data used for signature verification */
> + if (!map->frozen) {
> + attr.map_fd = fd;
> + err = kern_sys_bpf(BPF_MAP_FREEZE, &attr, sizeof(attr));
> + if (err < 0)
> + goto out;
> + }
> +
> + new = krealloc(buf, buf_sz + map->value_size, GFP_KERNEL);
> + if (!new) {
> + err = -ENOMEM;
> + goto out;
> + }
> + buf = new;
> + new = map->ops->map_lookup_elem(map, &key);
> + if (!new) {
> + err = -ENOENT;
> + goto out;
> + }
> + memcpy(buf + buf_sz, new, map->value_size);
> + buf_sz += map->value_size;
> + }
> +
> + err = verify_pkcs7_signature(buf, buf_sz, sig, sig_len,
> + VERIFY_USE_SECONDARY_KEYRING,
> + VERIFYING_EBPF_SIGNATURE,
> + NULL, NULL);
> +out:
> + kfree(buf);
> + return err;
> +}
> +
> +static int hornet_check_binary(struct bpf_prog *prog, union bpf_attr *attr,
> + struct hornet_maps *maps)
> +{
> + struct file *file = get_task_exe_file(current);
> + const unsigned long markerlen = sizeof(EBPF_SIG_STRING) - 1;
> + void *buf = NULL;
> + size_t sz = 0, sig_len, prog_len, buf_sz;
> + int err = 0;
> + struct module_signature sig;
> +
> + buf_sz = kernel_read_file(file, 0, &buf, INT_MAX, &sz, READING_EBPF);
> + fput(file);
> + if (!buf_sz)
> + return -1;
> +
> + prog_len = buf_sz;
> +
> + if (prog_len > markerlen &&
> + memcmp(buf + prog_len - markerlen, EBPF_SIG_STRING, markerlen) == 0)
> + prog_len -= markerlen;
> +
> + memcpy(&sig, buf + (prog_len - sizeof(sig)), sizeof(sig));
> + sig_len = be32_to_cpu(sig.sig_len);
> + prog_len -= sig_len + sizeof(sig);
> +
> + err = mod_check_sig(&sig, prog->len * sizeof(struct bpf_insn), "ebpf");
> + if (err)
> + return err;
> + return hornet_verify_lskel(prog, maps, buf + prog_len, sig_len);
> +}
> +
> +static int hornet_check_signature(struct bpf_prog *prog, union bpf_attr *attr,
> + struct bpf_token *token, bool is_kernel)

It's a little confusing that you are passing is_kernel in here, when the
only caller will always pass in true. Is there a good reason not to
drop the arg here and pass 'true' in to make_bpfptr(). Of course, then
people will ask why not define an IS_KERNEL to true as passing true to
second argument is cryptic... Maybe you just can't win here :)

> +{
> + struct hornet_maps maps = {0};
> + int err;
> +
> + /* support both sparse arrays and explicit continuous arrays of map fds */
> + if (attr->fd_array_cnt)
> + err = hornet_populate_fd_array(&maps, attr->fd_array_cnt);
> + else
> + err = hornet_find_maps(prog, &maps);
> +
> + if (err < 0)
> + return err;
> +
> + maps.fd_array = make_bpfptr(attr->fd_array, is_kernel);
> + return hornet_check_binary(prog, attr, &maps);
> +}
> +
> +static int hornet_bpf_prog_load(struct bpf_prog *prog, union bpf_attr *attr,
> + struct bpf_token *token, bool is_kernel)
> +{
> + if (is_kernel)
> + return 0;
> + return hornet_check_signature(prog, attr, token, is_kernel);
> +}
> +
> +static struct security_hook_list hornet_hooks[] __ro_after_init = {
> + LSM_HOOK_INIT(bpf_prog_load, hornet_bpf_prog_load),
> +};
> +
> +static const struct lsm_id hornet_lsmid = {
> + .name = "hornet",
> + .id = LSM_ID_HORNET,
> +};
> +
> +static int __init hornet_init(void)
> +{
> + pr_info("Hornet: eBPF signature verification enabled\n");
> + security_add_hooks(hornet_hooks, ARRAY_SIZE(hornet_hooks), &hornet_lsmid);
> + return 0;
> +}
> +
> +DEFINE_LSM(hornet) = {
> + .name = "hornet",
> + .init = hornet_init,
> +};
> --
> 2.48.1
>