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Date: Wed, 19 Apr 2017 01:35:47 +0200
From: Mickaël Salaün <mic@...ikod.net>
To: Kees Cook <keescook@...omium.org>
Cc: LKML <linux-kernel@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        Andy Lutomirski <luto@...capital.net>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Casey Schaufler <casey@...aufler-ca.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        David Drysdale
 <drysdale@...gle.com>,
        "David S . Miller" <davem@...emloft.net>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        James Morris <james.l.morris@...cle.com>, Jann Horn <jann@...jh.net>,
        Jonathan Corbet <corbet@....net>,
        Matthew Garrett <mjg59@...f.ucam.org>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Paul Moore <paul@...l-moore.com>, Sargun Dhillon <sargun@...gun.me>,
        "Serge E . Hallyn" <serge@...lyn.com>, Shuah Khan <shuah@...nel.org>,
        Tejun Heo <tj@...nel.org>, Thomas Graf <tgraf@...g.ch>,
        Will Drewry <wad@...omium.org>,
        "kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>,
        Linux API <linux-api@...r.kernel.org>,
        linux-security-module <linux-security-module@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next v6 08/11] bpf: Add a Landlock sandbox example


On 19/04/2017 01:06, Kees Cook wrote:
> On Tue, Mar 28, 2017 at 4:46 PM, Mickaël Salaün <mic@...ikod.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@...ikod.net>
>> Cc: Alexei Starovoitov <ast@...nel.org>
>> Cc: Andy Lutomirski <luto@...capital.net>
>> Cc: Daniel Borkmann <daniel@...earbox.net>
>> Cc: David S. Miller <davem@...emloft.net>
>> Cc: James Morris <james.l.morris@...cle.com>
>> Cc: Kees Cook <keescook@...omium.org>
>> Cc: Serge E. Hallyn <serge@...lyn.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@...ikod.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@...ikod.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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