[PATCH net-next v6 08/11] bpf: Add a Landlock sandbox example
Mickaël Salaün
mic at digikod.net
Tue Apr 18 23:35:47 UTC 2017
On 19/04/2017 01:06, Kees Cook wrote:
> On Tue, Mar 28, 2017 at 4:46 PM, Mickaël Salaün <mic at digikod.net> wrote:
>> 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
>>
>> 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
>>
>> Signed-off-by: Mickaël Salaün <mic at digikod.net>
>> Cc: Alexei Starovoitov <ast at kernel.org>
>> Cc: Andy Lutomirski <luto at amacapital.net>
>> Cc: Daniel Borkmann <daniel at iogearbox.net>
>> Cc: David S. Miller <davem at davemloft.net>
>> Cc: James Morris <james.l.morris at oracle.com>
>> Cc: Kees Cook <keescook at chromium.org>
>> Cc: Serge E. Hallyn <serge at hallyn.com>
>> ---
>> samples/bpf/Makefile | 4 ++
>> samples/bpf/bpf_load.c | 31 +++++++++++--
>> samples/bpf/landlock1_kern.c | 46 +++++++++++++++++++
>> samples/bpf/landlock1_user.c | 102 +++++++++++++++++++++++++++++++++++++++++++
>> 4 files changed, 179 insertions(+), 4 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 d42b495b0992..4743674a3fa3 100644
>> --- a/samples/bpf/Makefile
>> +++ b/samples/bpf/Makefile
>> @@ -36,6 +36,7 @@ hostprogs-y += lwt_len_hist
>> hostprogs-y += xdp_tx_iptunnel
>> hostprogs-y += test_map_in_map
>> hostprogs-y += per_socket_stats_example
>> +hostprogs-y += landlock1
>>
>> # Libbpf dependencies
>> LIBBPF := ../../tools/lib/bpf/bpf.o
>> @@ -76,6 +77,7 @@ lwt_len_hist-objs := bpf_load.o $(LIBBPF) lwt_len_hist_user.o
>> xdp_tx_iptunnel-objs := bpf_load.o $(LIBBPF) xdp_tx_iptunnel_user.o
>> test_map_in_map-objs := bpf_load.o $(LIBBPF) test_map_in_map_user.o
>> per_socket_stats_example-objs := $(LIBBPF) cookie_uid_helper_example.o
>> +landlock1-objs := bpf_load.o $(LIBBPF) landlock1_user.o
>>
>> # Tell kbuild to always build the programs
>> always := $(hostprogs-y)
>> @@ -111,6 +113,7 @@ always += lwt_len_hist_kern.o
>> always += xdp_tx_iptunnel_kern.o
>> always += test_map_in_map_kern.o
>> always += cookie_uid_helper_example.o
>> +always += landlock1_kern.o
>>
>> HOSTCFLAGS += -I$(objtree)/usr/include
>> HOSTCFLAGS += -I$(srctree)/tools/lib/
>> @@ -146,6 +149,7 @@ HOSTLOADLIBES_tc_l2_redirect += -l elf
>> HOSTLOADLIBES_lwt_len_hist += -l elf
>> HOSTLOADLIBES_xdp_tx_iptunnel += -lelf
>> HOSTLOADLIBES_test_map_in_map += -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 4a3460d7c01f..3713e5e2e998 100644
>> --- a/samples/bpf/bpf_load.c
>> +++ b/samples/bpf/bpf_load.c
>> @@ -29,6 +29,8 @@
>>
>> 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];
>> @@ -68,6 +70,7 @@ static int load_and_attach(const char *event, struct bpf_insn *prog, int size)
>> bool is_perf_event = strncmp(event, "perf_event", 10) == 0;
>> bool is_cgroup_skb = strncmp(event, "cgroup/skb", 10) == 0;
>> bool is_cgroup_sk = strncmp(event, "cgroup/sock", 11) == 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];
>> @@ -94,6 +97,13 @@ static int load_and_attach(const char *event, struct bpf_insn *prog, int size)
>> prog_type = BPF_PROG_TYPE_CGROUP_SKB;
>> } else if (is_cgroup_sk) {
>> prog_type = BPF_PROG_TYPE_CGROUP_SOCK;
>> + } else if (is_landlock) {
>> + prog_type = BPF_PROG_TYPE_LANDLOCK;
>> + if (!has_subtype) {
>> + printf("No subtype\n");
>> + return -1;
>> + }
>> + st = &subtype;
>> } else {
>> printf("Unknown event '%s'\n", event);
>> return -1;
>> @@ -108,7 +118,8 @@ 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) {
>> @@ -294,6 +305,7 @@ int load_bpf_file(char *path)
>> 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;
>> @@ -339,6 +351,16 @@ int load_bpf_file(char *path)
>> processed_sec[i] = true;
>> if (load_maps(data->d_buf, data->d_size))
>> return 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) {
>> symbols = data;
>> }
>> @@ -376,14 +398,14 @@ int load_bpf_file(char *path)
>> memcmp(shname_prog, "xdp", 3) == 0 ||
>> memcmp(shname_prog, "perf_event", 10) == 0 ||
>> memcmp(shname_prog, "socket", 6) == 0 ||
>> - memcmp(shname_prog, "cgroup/", 7) == 0)
>> + memcmp(shname_prog, "cgroup/", 7) == 0 ||
>> + memcmp(shname_prog, "landlock", 8) == 0)
>> load_and_attach(shname_prog, insns, data_prog->d_size);
>> }
>> }
>>
>> /* load programs that don't use maps */
>> for (i = 1; i < ehdr.e_shnum; i++) {
>> -
>> if (processed_sec[i])
>> continue;
>>
>> @@ -396,7 +418,8 @@ int load_bpf_file(char *path)
>> memcmp(shname, "xdp", 3) == 0 ||
>> memcmp(shname, "perf_event", 10) == 0 ||
>> memcmp(shname, "socket", 6) == 0 ||
>> - memcmp(shname, "cgroup/", 7) == 0)
>> + memcmp(shname, "cgroup/", 7) == 0 ||
>> + memcmp(shname, "landlock", 8) == 0)
>> load_and_attach(shname, data->d_buf, data->d_size);
>> }
>>
>> diff --git a/samples/bpf/landlock1_kern.c b/samples/bpf/landlock1_kern.c
>> new file mode 100644
>> index 000000000000..b8a9b0ca84c9
>> --- /dev/null
>> +++ b/samples/bpf/landlock1_kern.c
>> @@ -0,0 +1,46 @@
>> +/*
>> + * Landlock rule - partial read-only filesystem
>> + *
>> + * Copyright © 2017 Mickaël Salaün <mic at 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.
>> + */
>> +
>> +#define KBUILD_MODNAME "foo"
>> +#include <uapi/linux/bpf.h>
>> +#include <uapi/linux/stat.h> /* S_ISCHR() */
>> +#include "bpf_helpers.h"
>> +
>> +SEC("landlock1")
>> +static int landlock_fs_prog1(struct landlock_context *ctx)
>
> Since this is in samples, I think this needs a lot more comments to
> describe each pieces, how it fits together, etc. This is where people
> are going to come to learn how to use landlock, so making it as clear
> as possible is important. This is especially true for each step of the
> rule logic. (e.g. some will wonder why is S_ISCHR excluded, etc.)
Right, I already extended a bit this example with a S_ISFIFO() and some
comments. There is also the Documentation/.../user.rst which should help
understand how it works.
>
>> +{
>> + char fmt_error[] = "landlock1: error: get_mode:%lld\n";
>> + char fmt_name[] = "landlock1: syscall:%d\n";
>> + long long ret;
>> +
>> + if (!(ctx->arg2 & LANDLOCK_ACTION_FS_WRITE))
>> + return 0;
>> + ret = bpf_handle_fs_get_mode((void *)ctx->arg1);
>> + if (ret < 0) {
>> + bpf_trace_printk(fmt_error, sizeof(fmt_error), ret);
>> + return 1;
>> + }
>> + if (S_ISCHR(ret))
>> + return 0;
>> + bpf_trace_printk(fmt_name, sizeof(fmt_name), ctx->syscall_nr);
>> + return 1;
>> +}
>> +
>> +SEC("subtype")
>> +static union bpf_prog_subtype _subtype = {
>
> Can this be const?
Yes
>
>> + .landlock_rule = {
>> + .version = 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..6f79eb0ee6db
>> --- /dev/null
>> +++ b/samples/bpf/landlock1_user.c
>> @@ -0,0 +1,102 @@
>> +/*
>> + * Landlock sandbox - partial read-only filesystem
>> + *
>> + * Copyright © 2017 Mickaël Salaün <mic at 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
>> +
>> +#define ARRAY_SIZE(a) (sizeof(a) / sizeof(a[0]))
>> +#define MAX_ERRNO 4095
>
> Is MAX_ERRNO actually needed?
Not anymore.
>
>> +
>> +
>> +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;
>> +
>> + /* set up the test sandbox */
>> + if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) {
>> + perror("prctl(no_new_priv)");
>> + return 1;
>> + }
>> + if (seccomp(SECCOMP_APPEND_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;
>> +}
>
> I like this example. It's a very powerful rule to for a program to
> enforce on itself. :)
>
> -Kees
>
Thanks!
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