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Subject[PATCH net-next v7 08/10] bpf: Add a Landlock sandbox example
Date
Add a basic sandbox tool to create a process isolated from some part of
the system. This sandbox create a read-only environment. It is only
allowed to write to a character device such as a TTY:

# :> X
# echo $?
0
# ./samples/bpf/landlock1 /bin/sh -i
Launching a new sandboxed process.
# :> Y
cannot create Y: Operation not permitted

Signed-off-by: Mickaël Salaün <mic@digikod.net>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: David S. Miller <davem@davemloft.net>
Cc: James Morris <james.l.morris@oracle.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Serge E. Hallyn <serge@hallyn.com>
---

Changes since v6:
* check return value of load_and_attach()
* allow to write on pipes
* rename BPF_PROG_TYPE_LANDLOCK to BPF_PROG_TYPE_LANDLOCK_RULE
* rename Landlock version to ABI to better reflect its purpose
* use const variable (suggested by Kees Cook)
* remove useless definitions (suggested by Kees Cook)
* add detailed explanations (suggested by Kees Cook)

Changes since v5:
* cosmetic fixes
* rebase

Changes since v4:
* write Landlock rule in C and compiled it with LLVM
* remove cgroup handling
* remove path handling: only handle a read-only environment
* remove errno return codes

Changes since v3:
* remove seccomp and origin field: completely free from seccomp programs
* handle more FS-related hooks
* handle inode hooks and directory traversal
* add faked but consistent view thanks to ENOENT
* add /lib64 in the example
* fix spelling
* rename some types and definitions (e.g. SECCOMP_ADD_LANDLOCK_RULE)

Changes since v2:
* use BPF_PROG_ATTACH for cgroup handling
---
samples/bpf/Makefile | 4 ++
samples/bpf/bpf_load.c | 28 ++++++++++--
samples/bpf/landlock1_kern.c | 100 +++++++++++++++++++++++++++++++++++++++++++
samples/bpf/landlock1_user.c | 100 +++++++++++++++++++++++++++++++++++++++++++
4 files changed, 229 insertions(+), 3 deletions(-)
create mode 100644 samples/bpf/landlock1_kern.c
create mode 100644 samples/bpf/landlock1_user.c

diff --git a/samples/bpf/Makefile b/samples/bpf/Makefile
index f1010fe759fe..08d5d728e3e0 100644
--- a/samples/bpf/Makefile
+++ b/samples/bpf/Makefile
@@ -40,6 +40,7 @@ hostprogs-y += load_sock_ops
hostprogs-y += xdp_redirect
hostprogs-y += xdp_redirect_map
hostprogs-y += syscall_tp
+hostprogs-y += landlock1

# Libbpf dependencies
LIBBPF := ../../tools/lib/bpf/bpf.o
@@ -84,6 +85,7 @@ per_socket_stats_example-objs := $(LIBBPF) cookie_uid_helper_example.o
xdp_redirect-objs := bpf_load.o $(LIBBPF) xdp_redirect_user.o
xdp_redirect_map-objs := bpf_load.o $(LIBBPF) xdp_redirect_map_user.o
syscall_tp-objs := bpf_load.o $(LIBBPF) syscall_tp_user.o
+landlock1-objs := bpf_load.o $(LIBBPF) landlock1_user.o

# Tell kbuild to always build the programs
always := $(hostprogs-y)
@@ -128,6 +130,7 @@ always += tcp_clamp_kern.o
always += xdp_redirect_kern.o
always += xdp_redirect_map_kern.o
always += syscall_tp_kern.o
+always += landlock1_kern.o

HOSTCFLAGS += -I$(objtree)/usr/include
HOSTCFLAGS += -I$(srctree)/tools/lib/
@@ -167,6 +170,7 @@ HOSTLOADLIBES_test_map_in_map += -lelf
HOSTLOADLIBES_xdp_redirect += -lelf
HOSTLOADLIBES_xdp_redirect_map += -lelf
HOSTLOADLIBES_syscall_tp += -lelf
+HOSTLOADLIBES_landlock1 += -lelf

# Allows pointing LLC/CLANG to a LLVM backend with bpf support, redefine on cmdline:
# make samples/bpf/ LLC=~/git/llvm/build/bin/llc CLANG=~/git/llvm/build/bin/clang
diff --git a/samples/bpf/bpf_load.c b/samples/bpf/bpf_load.c
index 01a506f768da..30fcddda8b81 100644
--- a/samples/bpf/bpf_load.c
+++ b/samples/bpf/bpf_load.c
@@ -31,6 +31,8 @@

static char license[128];
static int kern_version;
+static union bpf_prog_subtype subtype = {};
+static bool has_subtype;
static bool processed_sec[128];
char bpf_log_buf[BPF_LOG_BUF_SIZE];
int map_fd[MAX_MAPS];
@@ -66,6 +68,7 @@ static int load_and_attach(const char *event, struct bpf_insn *prog, int size)
bool is_cgroup_sk = strncmp(event, "cgroup/sock", 11) == 0;
bool is_sockops = strncmp(event, "sockops", 7) == 0;
bool is_sk_skb = strncmp(event, "sk_skb", 6) == 0;
+ bool is_landlock = strncmp(event, "landlock", 8) == 0;
size_t insns_cnt = size / sizeof(struct bpf_insn);
enum bpf_prog_type prog_type;
char buf[256];
@@ -96,6 +99,13 @@ static int load_and_attach(const char *event, struct bpf_insn *prog, int size)
prog_type = BPF_PROG_TYPE_SOCK_OPS;
} else if (is_sk_skb) {
prog_type = BPF_PROG_TYPE_SK_SKB;
+ } else if (is_landlock) {
+ prog_type = BPF_PROG_TYPE_LANDLOCK_RULE;
+ if (!has_subtype) {
+ printf("No subtype\n");
+ return -1;
+ }
+ st = &subtype;
} else {
printf("Unknown event '%s'\n", event);
return -1;
@@ -110,7 +120,8 @@ static int load_and_attach(const char *event, struct bpf_insn *prog, int size)

prog_fd[prog_cnt++] = fd;

- if (is_xdp || is_perf_event || is_cgroup_skb || is_cgroup_sk)
+ if (is_xdp || is_perf_event || is_cgroup_skb || is_cgroup_sk ||
+ is_landlock)
return 0;

if (is_socket || is_sockops || is_sk_skb) {
@@ -454,6 +465,7 @@ static int do_load_bpf_file(const char *path, fixup_map_cb fixup_map)
kern_version = 0;
memset(license, 0, sizeof(license));
memset(processed_sec, 0, sizeof(processed_sec));
+ has_subtype = false;

if (elf_version(EV_CURRENT) == EV_NONE)
return 1;
@@ -502,6 +514,16 @@ static int do_load_bpf_file(const char *path, fixup_map_cb fixup_map)
data_maps = data;
for (j = 0; j < MAX_MAPS; j++)
map_data[j].fd = -1;
+ } else if (strcmp(shname, "subtype") == 0) {
+ processed_sec[i] = true;
+ if (data->d_size != sizeof(union bpf_prog_subtype)) {
+ printf("invalid size of subtype section %zd\n",
+ data->d_size);
+ return 1;
+ }
+ memcpy(&subtype, data->d_buf,
+ sizeof(union bpf_prog_subtype));
+ has_subtype = true;
} else if (shdr.sh_type == SHT_SYMTAB) {
strtabidx = shdr.sh_link;
symbols = data;
@@ -562,7 +584,6 @@ static int do_load_bpf_file(const char *path, fixup_map_cb fixup_map)

/* load programs */
for (i = 1; i < ehdr.e_shnum; i++) {
-
if (processed_sec[i])
continue;

@@ -577,7 +598,8 @@ static int do_load_bpf_file(const char *path, fixup_map_cb fixup_map)
memcmp(shname, "socket", 6) == 0 ||
memcmp(shname, "cgroup/", 7) == 0 ||
memcmp(shname, "sockops", 7) == 0 ||
- memcmp(shname, "sk_skb", 6) == 0) {
+ memcmp(shname, "sk_skb", 6) == 0 ||
+ memcmp(shname, "landlock", 8) == 0) {
ret = load_and_attach(shname, data->d_buf,
data->d_size);
if (ret != 0)
diff --git a/samples/bpf/landlock1_kern.c b/samples/bpf/landlock1_kern.c
new file mode 100644
index 000000000000..4ed80d8eee4e
--- /dev/null
+++ b/samples/bpf/landlock1_kern.c
@@ -0,0 +1,100 @@
+/*
+ * Landlock rule - partial read-only filesystem
+ *
+ * Copyright © 2017 Mickaël Salaün <mic@digikod.net>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2, as
+ * published by the Free Software Foundation.
+ */
+
+/*
+ * This file contains a function that will be compiled to eBPF bytecode thanks
+ * to LLVM/Clang.
+ *
+ * Each SEC() means that the following function or variable will be part of a
+ * custom ELF section. This sections are then processed by the userspace part
+ * (see landlock1_user.c) to extract eBPF bytecode and take into account
+ * variables describing the eBPF program subtype or its license.
+ */
+
+#include <uapi/linux/bpf.h>
+#include <uapi/linux/stat.h> /* S_ISCHR() */
+#include "bpf_helpers.h"
+
+/*
+ * The function landlock_fs_prog1() is a simple Landlock rule enforced on a set
+ * of processes. This rule will be run for each file-system operations and will
+ * then forbid any write on a file-descriptor except if this file-descriptor
+ * point to a pipe. Hence, it will not be possible to create new files nor to
+ * modify a regular file.
+ *
+ * The argument ctx contains the context of the rule when it is run, which
+ * enable to check which action on which file is requested. This context can
+ * change for each run of the rule.
+ */
+SEC("landlock1")
+static int landlock_fs_prog1(struct landlock_context *ctx)
+{
+ char fmt_error_mode[] = "landlock1: error: get_mode:%lld\n";
+ char fmt_error_access[] = "landlock1: error: access denied\n";
+ long long ret;
+
+ /*
+ * The argument ctx->arg2 contains bitflags of actions for which the
+ * rule is run. The flag LANDLOCK_ACTION_FS_WRITE means that a write
+ * is requested by one of the userspace processes restricted by this
+ * rule. The following test allows any actions which does not include a
+ * write.
+ */
+ if (!(ctx->arg2 & LANDLOCK_ACTION_FS_WRITE))
+ return 0;
+
+ /*
+ * The argument ctx->arg1 is a file handle for which the process want
+ * to access. The function bpf_handle_fs_get_mode() return the mode of
+ * a file (e.g. S_IFBLK, S_IFDIR, S_IFREG...). If there is an error,
+ * for example if the argument is not a file handle, then an
+ * -errno value is returned. Otherwise the caller get the file mode as
+ * with stat(2).
+ */
+ ret = bpf_handle_fs_get_mode((void *)ctx->arg1);
+ if (ret < 0) {
+
+ /*
+ * The bpf_trace_printk() function enable to write in the
+ * kernel eBPF debug log, accessible through
+ * /sys/kernel/debug/tracing/trace_pipe . To be allowed to call
+ * this function, a Landlock rule must have the
+ * LANDLOCK_SUBTYPE_ABILITY_DEBUG ability, which is only
+ * allowed for CAP_SYS_ADMIN.
+ */
+ bpf_trace_printk(fmt_error_mode, sizeof(fmt_error_mode), ret);
+ return 1;
+ }
+
+ /*
+ * This check allows the action on the file if it is a directory or a
+ * pipe. Otherwise, a message is printed to the eBPF log.
+ */
+ if (S_ISCHR(ret) || S_ISFIFO(ret))
+ return 0;
+ bpf_trace_printk(fmt_error_access, sizeof(fmt_error_access));
+ return 1;
+}
+
+/*
+ * This subtype enable to set the ABI, which ensure that the eBPF context and
+ * program behavior will be compatible with this Landlock rule.
+ */
+SEC("subtype")
+static const union bpf_prog_subtype _subtype = {
+ .landlock_rule = {
+ .abi = 1,
+ .event = LANDLOCK_SUBTYPE_EVENT_FS,
+ .ability = LANDLOCK_SUBTYPE_ABILITY_DEBUG,
+ }
+};
+
+SEC("license")
+static const char _license[] = "GPL";
diff --git a/samples/bpf/landlock1_user.c b/samples/bpf/landlock1_user.c
new file mode 100644
index 000000000000..27435dc4acb1
--- /dev/null
+++ b/samples/bpf/landlock1_user.c
@@ -0,0 +1,100 @@
+/*
+ * Landlock sandbox - partial read-only filesystem
+ *
+ * Copyright © 2017 Mickaël Salaün <mic@digikod.net>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2, as
+ * published by the Free Software Foundation.
+ */
+
+#include "bpf_load.h"
+#include "libbpf.h"
+
+#define _GNU_SOURCE
+#include <errno.h>
+#include <fcntl.h> /* open() */
+#include <linux/bpf.h>
+#include <linux/filter.h>
+#include <linux/prctl.h>
+#include <linux/seccomp.h>
+#include <stddef.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/prctl.h>
+#include <sys/syscall.h>
+#include <unistd.h>
+
+#ifndef seccomp
+static int seccomp(unsigned int op, unsigned int flags, void *args)
+{
+ errno = 0;
+ return syscall(__NR_seccomp, op, flags, args);
+}
+#endif
+
+
+struct landlock_rule {
+ enum landlock_subtype_event event;
+ struct bpf_insn *bpf;
+ size_t size;
+};
+
+static int apply_sandbox(int prog_fd)
+{
+ int ret = 0;
+
+ /* safer to set no_new_privs */
+ if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) {
+ perror("prctl(no_new_priv)");
+ return 1;
+ }
+ /* set up the test sandbox */
+ if (seccomp(SECCOMP_PREPEND_LANDLOCK_RULE, 0, &prog_fd)) {
+ perror("seccomp(set_hook)");
+ ret = 1;
+ }
+ close(prog_fd);
+
+ return ret;
+}
+
+int main(int argc, char * const argv[], char * const *envp)
+{
+ char filename[256];
+ char *cmd_path;
+ char * const *cmd_argv;
+
+ if (argc < 2) {
+ fprintf(stderr, "usage: %s <cmd> [args]...\n\n", argv[0]);
+ fprintf(stderr, "Launch a command in a read-only environment "
+ "(except for character devices).\n");
+ fprintf(stderr, "Display debug with: "
+ "cat /sys/kernel/debug/tracing/trace_pipe &\n");
+ return 1;
+ }
+
+ snprintf(filename, sizeof(filename), "%s_kern.o", argv[0]);
+ if (load_bpf_file(filename)) {
+ printf("%s", bpf_log_buf);
+ return 1;
+ }
+ if (!prog_fd[0]) {
+ if (errno) {
+ printf("load_bpf_file: %s\n", strerror(errno));
+ } else {
+ printf("load_bpf_file: Error\n");
+ }
+ return 1;
+ }
+
+ if (apply_sandbox(prog_fd[0]))
+ return 1;
+ cmd_path = argv[1];
+ cmd_argv = argv + 1;
+ fprintf(stderr, "Launching a new sandboxed process.\n");
+ execve(cmd_path, cmd_argv, envp);
+ perror("execve");
+ return 1;
+}
--
2.14.1
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