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bubblewrap.c
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/* bubblewrap
* Copyright (C) 2016 Alexander Larsson
* SPDX-License-Identifier: LGPL-2.0-or-later
*
* This program is free software; you can redistribute it and/or
* modify it under the terms of the GNU Lesser General Public
* License as published by the Free Software Foundation; either
* version 2 of the License, or (at your option) any later version.
*
* This library is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
* Lesser General Public License for more details.
*
* You should have received a copy of the GNU Lesser General Public
* License along with this library. If not, see <http://www.gnu.org/licenses/>.
*
*/
#include "config.h"
#include <poll.h>
#include <sched.h>
#include <pwd.h>
#include <grp.h>
#include <ctype.h>
#include <time.h>
#include <sys/mount.h>
#include <sys/socket.h>
#include <sys/wait.h>
#include <sys/eventfd.h>
#include <sys/fsuid.h>
#include <sys/signalfd.h>
#include <sys/capability.h>
#include <sys/prctl.h>
#include <linux/sched.h>
#include <linux/seccomp.h>
#include <linux/filter.h>
#include <sys/syscall.h>
#include "utils.h"
#include "network.h"
#include "bind-mount.h"
#ifndef CLONE_NEWCGROUP
#define CLONE_NEWCGROUP 0x02000000 /* New cgroup namespace */
#endif
/* We limit the size of a tmpfs to half the architecture's address space,
* to avoid hitting arbitrary limits in the kernel.
* For example, on at least one x86_64 machine, the actual limit seems to be
* 2^64 - 2^12. */
#define MAX_TMPFS_BYTES ((size_t) (SIZE_MAX >> 1))
/* Globals to avoid having to use getuid(), since the uid/gid changes during runtime */
static uid_t real_uid;
static gid_t real_gid;
static uid_t overflow_uid;
static gid_t overflow_gid;
static bool is_privileged; /* See acquire_privs() */
static const char *argv0;
static const char *host_tty_dev;
static int proc_fd = -1;
static const char *opt_exec_label = NULL;
static const char *opt_file_label = NULL;
static bool opt_as_pid_1;
static const char *opt_argv0 = NULL;
static const char *opt_chdir_path = NULL;
static bool opt_assert_userns_disabled = false;
static bool opt_disable_userns = false;
static bool opt_unshare_user = false;
static bool opt_unshare_user_try = false;
static bool opt_unshare_pid = false;
static bool opt_unshare_ipc = false;
static bool opt_unshare_net = false;
static bool opt_unshare_uts = false;
static bool opt_unshare_cgroup = false;
static bool opt_unshare_cgroup_try = false;
static bool opt_needs_devpts = false;
static bool opt_new_session = false;
static bool opt_die_with_parent = false;
static uid_t opt_sandbox_uid = -1;
static gid_t opt_sandbox_gid = -1;
static int opt_sync_fd = -1;
static int opt_block_fd = -1;
static int opt_error_fd = -1;
static int opt_userns_block_fd = -1;
static int opt_info_fd = -1;
static int opt_json_status_fd = -1;
static int opt_seccomp_fd = -1;
static const char *opt_sandbox_hostname = NULL;
static char *opt_args_data = NULL; /* owned */
static int opt_userns_fd = -1;
static int opt_userns2_fd = -1;
static int opt_pidns_fd = -1;
static int opt_tmp_overlay_count = 0;
static int next_perms = -1;
static size_t next_size_arg = 0;
static int next_overlay_src_count = 0;
#define CAP_TO_MASK_0(x) (1L << ((x) & 31))
#define CAP_TO_MASK_1(x) CAP_TO_MASK_0(x - 32)
typedef struct _NsInfo NsInfo;
struct _NsInfo {
const char *name;
bool *do_unshare;
ino_t id;
};
static NsInfo ns_infos[] = {
{"cgroup", &opt_unshare_cgroup, 0},
{"ipc", &opt_unshare_ipc, 0},
{"mnt", NULL, 0},
{"net", &opt_unshare_net, 0},
{"pid", &opt_unshare_pid, 0},
/* user namespace info omitted because it
* is not (yet) valid when we obtain the
* namespace info (get un-shared later) */
{"uts", &opt_unshare_uts, 0},
{NULL, NULL, 0}
};
typedef enum {
SETUP_BIND_MOUNT,
SETUP_RO_BIND_MOUNT,
SETUP_DEV_BIND_MOUNT,
SETUP_OVERLAY_MOUNT,
SETUP_TMP_OVERLAY_MOUNT,
SETUP_RO_OVERLAY_MOUNT,
SETUP_OVERLAY_SRC,
SETUP_MOUNT_PROC,
SETUP_MOUNT_DEV,
SETUP_MOUNT_TMPFS,
SETUP_MOUNT_MQUEUE,
SETUP_MAKE_DIR,
SETUP_MAKE_FILE,
SETUP_MAKE_BIND_FILE,
SETUP_MAKE_RO_BIND_FILE,
SETUP_MAKE_SYMLINK,
SETUP_REMOUNT_RO_NO_RECURSIVE,
SETUP_SET_HOSTNAME,
SETUP_CHMOD,
} SetupOpType;
typedef enum {
NO_CREATE_DEST = (1 << 0),
ALLOW_NOTEXIST = (1 << 1),
} SetupOpFlag;
typedef struct _SetupOp SetupOp;
struct _SetupOp
{
SetupOpType type;
const char *source;
const char *dest;
int fd;
SetupOpFlag flags;
int perms;
size_t size; /* number of bytes, zero means unset/default */
SetupOp *next;
};
typedef struct _LockFile LockFile;
struct _LockFile
{
const char *path;
int fd;
LockFile *next;
};
enum {
PRIV_SEP_OP_DONE,
PRIV_SEP_OP_BIND_MOUNT,
PRIV_SEP_OP_OVERLAY_MOUNT,
PRIV_SEP_OP_PROC_MOUNT,
PRIV_SEP_OP_TMPFS_MOUNT,
PRIV_SEP_OP_DEVPTS_MOUNT,
PRIV_SEP_OP_MQUEUE_MOUNT,
PRIV_SEP_OP_REMOUNT_RO_NO_RECURSIVE,
PRIV_SEP_OP_SET_HOSTNAME,
};
typedef struct
{
uint32_t op;
uint32_t flags;
uint32_t perms;
size_t size_arg;
uint32_t arg1_offset;
uint32_t arg2_offset;
} PrivSepOp;
/*
* DEFINE_LINKED_LIST:
* @Type: A struct with a `Type *next` member
* @name: Used to form the names of variables and functions
*
* Define a global linked list of @Type structures, with pointers
* `NAMEs` to the head of the list and `last_NAME` to the tail of the
* list.
*
* A new zero-filled item can be allocated and appended to the list
* by calling `_NAME_append_new()`, which returns the new item.
*/
#define DEFINE_LINKED_LIST(Type, name) \
static Type *name ## s = NULL; \
static Type *last_ ## name = NULL; \
\
static inline Type * \
_ ## name ## _append_new (void) \
{ \
Type *self = xcalloc (1, sizeof (Type)); \
\
if (last_ ## name != NULL) \
last_ ## name ->next = self; \
else \
name ## s = self; \
\
last_ ## name = self; \
return self; \
}
DEFINE_LINKED_LIST (SetupOp, op)
static SetupOp *
setup_op_new (SetupOpType type)
{
SetupOp *op = _op_append_new ();
op->type = type;
op->fd = -1;
op->flags = 0;
return op;
}
DEFINE_LINKED_LIST (LockFile, lock_file)
static LockFile *
lock_file_new (const char *path)
{
LockFile *lock = _lock_file_append_new ();
lock->path = path;
return lock;
}
typedef struct _SeccompProgram SeccompProgram;
struct _SeccompProgram
{
struct sock_fprog program;
SeccompProgram *next;
};
DEFINE_LINKED_LIST (SeccompProgram, seccomp_program)
static SeccompProgram *
seccomp_program_new (int *fd)
{
SeccompProgram *self = _seccomp_program_append_new ();
cleanup_free char *data = NULL;
size_t len;
data = load_file_data (*fd, &len);
if (data == NULL)
die_with_error ("Can't read seccomp data");
close (*fd);
*fd = -1;
if (len % 8 != 0)
die ("Invalid seccomp data, must be multiple of 8");
self->program.len = len / 8;
self->program.filter = (struct sock_filter *) steal_pointer (&data);
return self;
}
static void
seccomp_programs_apply (void)
{
SeccompProgram *program;
for (program = seccomp_programs; program != NULL; program = program->next)
{
if (prctl (PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &program->program) != 0)
{
if (errno == EINVAL)
die ("Unable to set up system call filtering as requested: "
"prctl(PR_SET_SECCOMP) reported EINVAL. "
"(Hint: this requires a kernel configured with "
"CONFIG_SECCOMP and CONFIG_SECCOMP_FILTER.)");
die_with_error ("prctl(PR_SET_SECCOMP)");
}
}
}
static void
usage (int ecode, FILE *out)
{
fprintf (out, "usage: %s [OPTIONS...] [--] COMMAND [ARGS...]\n\n", argv0 ? argv0 : "bwrap");
fprintf (out,
" --help Print this help\n"
" --version Print version\n"
" --args FD Parse NUL-separated args from FD\n"
" --argv0 VALUE Set argv[0] to the value VALUE before running the program\n"
" --level-prefix Prepend e.g. <3> to diagnostic messages\n"
" --unshare-all Unshare every namespace we support by default\n"
" --share-net Retain the network namespace (can only combine with --unshare-all)\n"
" --unshare-user Create new user namespace (may be automatically implied if not setuid)\n"
" --unshare-user-try Create new user namespace if possible else continue by skipping it\n"
" --unshare-ipc Create new ipc namespace\n"
" --unshare-pid Create new pid namespace\n"
" --unshare-net Create new network namespace\n"
" --unshare-uts Create new uts namespace\n"
" --unshare-cgroup Create new cgroup namespace\n"
" --unshare-cgroup-try Create new cgroup namespace if possible else continue by skipping it\n"
" --userns FD Use this user namespace (cannot combine with --unshare-user)\n"
" --userns2 FD After setup switch to this user namespace, only useful with --userns\n"
" --disable-userns Disable further use of user namespaces inside sandbox\n"
" --assert-userns-disabled Fail unless further use of user namespace inside sandbox is disabled\n"
" --pidns FD Use this pid namespace (as parent namespace if using --unshare-pid)\n"
" --uid UID Custom uid in the sandbox (requires --unshare-user or --userns)\n"
" --gid GID Custom gid in the sandbox (requires --unshare-user or --userns)\n"
" --hostname NAME Custom hostname in the sandbox (requires --unshare-uts)\n"
" --chdir DIR Change directory to DIR\n"
" --clearenv Unset all environment variables\n"
" --setenv VAR VALUE Set an environment variable\n"
" --unsetenv VAR Unset an environment variable\n"
" --lock-file DEST Take a lock on DEST while sandbox is running\n"
" --sync-fd FD Keep this fd open while sandbox is running\n"
" --bind SRC DEST Bind mount the host path SRC on DEST\n"
" --bind-try SRC DEST Equal to --bind but ignores non-existent SRC\n"
" --dev-bind SRC DEST Bind mount the host path SRC on DEST, allowing device access\n"
" --dev-bind-try SRC DEST Equal to --dev-bind but ignores non-existent SRC\n"
" --ro-bind SRC DEST Bind mount the host path SRC readonly on DEST\n"
" --ro-bind-try SRC DEST Equal to --ro-bind but ignores non-existent SRC\n"
" --bind-fd FD DEST Bind open directory or path fd on DEST\n"
" --ro-bind-fd FD DEST Bind open directory or path fd read-only on DEST\n"
" --remount-ro DEST Remount DEST as readonly; does not recursively remount\n"
" --overlay-src SRC Read files from SRC in the following overlay\n"
" --overlay RWSRC WORKDIR DEST Mount overlayfs on DEST, with RWSRC as the host path for writes and\n"
" WORKDIR an empty directory on the same filesystem as RWSRC\n"
" --tmp-overlay DEST Mount overlayfs on DEST, with writes going to an invisible tmpfs\n"
" --ro-overlay DEST Mount overlayfs read-only on DEST\n"
" --exec-label LABEL Exec label for the sandbox\n"
" --file-label LABEL File label for temporary sandbox content\n"
" --proc DEST Mount new procfs on DEST\n"
" --dev DEST Mount new dev on DEST\n"
" --tmpfs DEST Mount new tmpfs on DEST\n"
" --mqueue DEST Mount new mqueue on DEST\n"
" --dir DEST Create dir at DEST\n"
" --file FD DEST Copy from FD to destination DEST\n"
" --bind-data FD DEST Copy from FD to file which is bind-mounted on DEST\n"
" --ro-bind-data FD DEST Copy from FD to file which is readonly bind-mounted on DEST\n"
" --symlink SRC DEST Create symlink at DEST with target SRC\n"
" --seccomp FD Load and use seccomp rules from FD (not repeatable)\n"
" --add-seccomp-fd FD Load and use seccomp rules from FD (repeatable)\n"
" --block-fd FD Block on FD until some data to read is available\n"
" --userns-block-fd FD Block on FD until the user namespace is ready\n"
" --info-fd FD Write information about the running container to FD\n"
" --json-status-fd FD Write container status to FD as multiple JSON documents\n"
" --new-session Create a new terminal session\n"
" --die-with-parent Kills with SIGKILL child process (COMMAND) when bwrap or bwrap's parent dies.\n"
" --as-pid-1 Do not install a reaper process with PID=1\n"
" --cap-add CAP Add cap CAP when running as privileged user\n"
" --cap-drop CAP Drop cap CAP when running as privileged user\n"
" --perms OCTAL Set permissions of next argument (--bind-data, --file, etc.)\n"
" --size BYTES Set size of next argument (only for --tmpfs)\n"
" --chmod OCTAL PATH Change permissions of PATH (must already exist)\n"
);
exit (ecode);
}
/* If --die-with-parent was specified, use PDEATHSIG to ensure SIGKILL
* is sent to the current process when our parent dies.
*/
static void
handle_die_with_parent (void)
{
if (opt_die_with_parent && prctl (PR_SET_PDEATHSIG, SIGKILL, 0, 0, 0) != 0)
die_with_error ("prctl");
}
static void
block_sigchild (void)
{
sigset_t mask;
int status;
sigemptyset (&mask);
sigaddset (&mask, SIGCHLD);
if (sigprocmask (SIG_BLOCK, &mask, NULL) == -1)
die_with_error ("sigprocmask");
/* Reap any outstanding zombies that we may have inherited */
while (waitpid (-1, &status, WNOHANG) > 0)
;
}
static void
unblock_sigchild (void)
{
sigset_t mask;
sigemptyset (&mask);
sigaddset (&mask, SIGCHLD);
if (sigprocmask (SIG_UNBLOCK, &mask, NULL) == -1)
die_with_error ("sigprocmask");
}
/* Closes all fd:s except 0,1,2 and the passed in array of extra fds */
static int
close_extra_fds (void *data, int fd)
{
int *extra_fds = (int *) data;
int i;
for (i = 0; extra_fds[i] != -1; i++)
if (fd == extra_fds[i])
return 0;
if (fd <= 2)
return 0;
close (fd);
return 0;
}
static int
propagate_exit_status (int status)
{
if (WIFEXITED (status))
return WEXITSTATUS (status);
/* The process died of a signal, we can't really report that, but we
* can at least be bash-compatible. The bash manpage says:
* The return value of a simple command is its
* exit status, or 128+n if the command is
* terminated by signal n.
*/
if (WIFSIGNALED (status))
return 128 + WTERMSIG (status);
/* Weird? */
return 255;
}
static void
dump_info (int fd, const char *output, bool exit_on_error)
{
size_t len = strlen (output);
if (write_to_fd (fd, output, len))
{
if (exit_on_error)
die_with_error ("Write to info_fd");
}
}
static void
report_child_exit_status (int exitc, int setup_finished_fd)
{
ssize_t s;
char data[2];
cleanup_free char *output = NULL;
if (opt_json_status_fd == -1 || setup_finished_fd == -1)
return;
s = TEMP_FAILURE_RETRY (read (setup_finished_fd, data, sizeof data));
if (s == -1 && errno != EAGAIN)
die_with_error ("read eventfd");
if (s != 1) // Is 0 if pipe closed before exec, is 2 if closed after exec.
return;
output = xasprintf ("{ \"exit-code\": %i }\n", exitc);
dump_info (opt_json_status_fd, output, false);
close (opt_json_status_fd);
opt_json_status_fd = -1;
close (setup_finished_fd);
}
/* This stays around for as long as the initial process in the app does
* and when that exits it exits, propagating the exit status. We do this
* by having pid 1 in the sandbox detect this exit and tell the monitor
* the exit status via a eventfd. We also track the exit of the sandbox
* pid 1 via a signalfd for SIGCHLD, and exit with an error in this case.
* This is to catch e.g. problems during setup. */
static int
monitor_child (int event_fd, pid_t child_pid, int setup_finished_fd)
{
int res;
uint64_t val;
ssize_t s;
int signal_fd;
sigset_t mask;
struct pollfd fds[2];
int num_fds;
struct signalfd_siginfo fdsi;
int dont_close[] = {-1, -1, -1, -1};
unsigned int j = 0;
int exitc;
pid_t died_pid;
int died_status;
/* Close all extra fds in the monitoring process.
Any passed in fds have been passed on to the child anyway. */
if (event_fd != -1)
dont_close[j++] = event_fd;
if (opt_json_status_fd != -1)
dont_close[j++] = opt_json_status_fd;
if (setup_finished_fd != -1)
dont_close[j++] = setup_finished_fd;
assert (j < sizeof(dont_close)/sizeof(*dont_close));
fdwalk (proc_fd, close_extra_fds, dont_close);
sigemptyset (&mask);
sigaddset (&mask, SIGCHLD);
signal_fd = signalfd (-1, &mask, SFD_CLOEXEC | SFD_NONBLOCK);
if (signal_fd == -1)
die_with_error ("Can't create signalfd");
num_fds = 1;
fds[0].fd = signal_fd;
fds[0].events = POLLIN;
if (event_fd != -1)
{
fds[1].fd = event_fd;
fds[1].events = POLLIN;
num_fds++;
}
while (1)
{
fds[0].revents = fds[1].revents = 0;
res = poll (fds, num_fds, -1);
if (res == -1 && errno != EINTR)
die_with_error ("poll");
/* Always read from the eventfd first, if pid 2 died then pid 1 often
* dies too, and we could race, reporting that first and we'd lose
* the real exit status. */
if (event_fd != -1)
{
s = read (event_fd, &val, 8);
if (s == -1 && errno != EINTR && errno != EAGAIN)
die_with_error ("read eventfd");
else if (s == 8)
{
exitc = (int) val - 1;
report_child_exit_status (exitc, setup_finished_fd);
return exitc;
}
}
/* We need to read the signal_fd, or it will keep polling as read,
* however we ignore the details as we get them from waitpid
* below anyway */
s = read (signal_fd, &fdsi, sizeof (struct signalfd_siginfo));
if (s == -1 && errno != EINTR && errno != EAGAIN)
die_with_error ("read signalfd");
/* We may actually get several sigchld compressed into one
SIGCHLD, so we have to handle all of them. */
while ((died_pid = waitpid (-1, &died_status, WNOHANG)) > 0)
{
/* We may be getting sigchild from other children too. For instance if
someone created a child process, and then exec:ed bubblewrap. Ignore them */
if (died_pid == child_pid)
{
exitc = propagate_exit_status (died_status);
report_child_exit_status (exitc, setup_finished_fd);
return exitc;
}
}
}
die ("Should not be reached");
return 0;
}
/* This is pid 1 in the app sandbox. It is needed because we're using
* pid namespaces, and someone has to reap zombies in it. We also detect
* when the initial process (pid 2) dies and report its exit status to
* the monitor so that it can return it to the original spawner.
*
* When there are no other processes in the sandbox the wait will return
* ECHILD, and we then exit pid 1 to clean up the sandbox. */
static int
do_init (int event_fd, pid_t initial_pid)
{
int initial_exit_status = 1;
LockFile *lock;
for (lock = lock_files; lock != NULL; lock = lock->next)
{
int fd = TEMP_FAILURE_RETRY (open (lock->path, O_RDONLY | O_CLOEXEC));
if (fd == -1)
die_with_error ("Unable to open lock file %s", lock->path);
struct flock l = {
.l_type = F_RDLCK,
.l_whence = SEEK_SET,
.l_start = 0,
.l_len = 0
};
if (TEMP_FAILURE_RETRY (fcntl (fd, F_SETLK, &l)) < 0)
die_with_error ("Unable to lock file %s", lock->path);
/* Keep fd open to hang on to lock */
lock->fd = fd;
}
/* Optionally bind our lifecycle to that of the caller */
handle_die_with_parent ();
seccomp_programs_apply ();
while (true)
{
pid_t child;
int status;
child = TEMP_FAILURE_RETRY (wait (&status));
if (child == initial_pid)
{
initial_exit_status = propagate_exit_status (status);
if(event_fd != -1)
{
uint64_t val;
int res UNUSED;
val = initial_exit_status + 1;
res = TEMP_FAILURE_RETRY (write (event_fd, &val, 8));
/* Ignore res, if e.g. the parent died and closed event_fd
we don't want to error out here */
}
}
if (child == -1 && errno != EINTR)
{
if (errno != ECHILD)
die_with_error ("init wait()");
break;
}
}
/* Close FDs. */
for (lock = lock_files; lock != NULL; lock = lock->next)
{
if (lock->fd >= 0)
{
close (lock->fd);
lock->fd = -1;
}
}
return initial_exit_status;
}
#define CAP_TO_MASK_0(x) (1L << ((x) & 31))
#define CAP_TO_MASK_1(x) CAP_TO_MASK_0(x - 32)
/* Set if --cap-add or --cap-drop were used */
static bool opt_cap_add_or_drop_used;
/* The capability set we'll target, used if above is true */
static uint32_t requested_caps[2] = {0, 0};
/* low 32bit caps needed */
/* CAP_SYS_PTRACE is needed to dereference the symlinks in /proc/<pid>/ns/, see namespaces(7) */
#define REQUIRED_CAPS_0 (CAP_TO_MASK_0 (CAP_SYS_ADMIN) | CAP_TO_MASK_0 (CAP_SYS_CHROOT) | CAP_TO_MASK_0 (CAP_NET_ADMIN) | CAP_TO_MASK_0 (CAP_SETUID) | CAP_TO_MASK_0 (CAP_SETGID) | CAP_TO_MASK_0 (CAP_SYS_PTRACE))
/* high 32bit caps needed */
#define REQUIRED_CAPS_1 0
static void
set_required_caps (void)
{
struct __user_cap_header_struct hdr = { _LINUX_CAPABILITY_VERSION_3, 0 };
struct __user_cap_data_struct data[2] = { { 0 } };
/* Drop all non-require capabilities */
data[0].effective = REQUIRED_CAPS_0;
data[0].permitted = REQUIRED_CAPS_0;
data[0].inheritable = 0;
data[1].effective = REQUIRED_CAPS_1;
data[1].permitted = REQUIRED_CAPS_1;
data[1].inheritable = 0;
if (capset (&hdr, data) < 0)
die_with_error ("capset failed");
}
static void
drop_all_caps (bool keep_requested_caps)
{
struct __user_cap_header_struct hdr = { _LINUX_CAPABILITY_VERSION_3, 0 };
struct __user_cap_data_struct data[2] = { { 0 } };
if (keep_requested_caps)
{
/* Avoid calling capset() unless we need to; currently
* systemd-nspawn at least is known to install a seccomp
* policy denying capset() for dubious reasons.
* <https://github.com/projectatomic/bubblewrap/pull/122>
*/
if (!opt_cap_add_or_drop_used && real_uid == 0)
{
assert (!is_privileged);
return;
}
data[0].effective = requested_caps[0];
data[0].permitted = requested_caps[0];
data[0].inheritable = requested_caps[0];
data[1].effective = requested_caps[1];
data[1].permitted = requested_caps[1];
data[1].inheritable = requested_caps[1];
}
if (capset (&hdr, data) < 0)
{
/* While the above logic ensures we don't call capset() for the primary
* process unless configured to do so, we still try to drop privileges for
* the init process unconditionally. Since due to the systemd seccomp
* filter that will fail, let's just ignore it.
*/
if (errno == EPERM && real_uid == 0 && !is_privileged)
return;
else
die_with_error ("capset failed");
}
}
static bool
has_caps (void)
{
struct __user_cap_header_struct hdr = { _LINUX_CAPABILITY_VERSION_3, 0 };
struct __user_cap_data_struct data[2] = { { 0 } };
if (capget (&hdr, data) < 0)
die_with_error ("capget failed");
return data[0].permitted != 0 || data[1].permitted != 0;
}
/* Most of the code here is used both to add caps to the ambient capabilities
* and drop caps from the bounding set. Handle both cases here and add
* drop_cap_bounding_set/set_ambient_capabilities wrappers to facilitate its usage.
*/
static void
prctl_caps (uint32_t *caps, bool do_cap_bounding, bool do_set_ambient)
{
unsigned long cap;
/* We ignore both EINVAL and EPERM, as we are actually relying
* on PR_SET_NO_NEW_PRIVS to ensure the right capabilities are
* available. EPERM in particular can happen with old, buggy
* kernels. See:
* https://github.com/projectatomic/bubblewrap/pull/175#issuecomment-278051373
* https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/security/commoncap.c?id=160da84dbb39443fdade7151bc63a88f8e953077
*/
for (cap = 0; cap <= CAP_LAST_CAP; cap++)
{
bool keep = false;
if (cap < 32)
{
if (CAP_TO_MASK_0 (cap) & caps[0])
keep = true;
}
else
{
if (CAP_TO_MASK_1 (cap) & caps[1])
keep = true;
}
if (keep && do_set_ambient)
{
#ifdef PR_CAP_AMBIENT
int res = prctl (PR_CAP_AMBIENT, PR_CAP_AMBIENT_RAISE, cap, 0, 0);
if (res == -1 && !(errno == EINVAL || errno == EPERM))
die_with_error ("Adding ambient capability %ld", cap);
#else
/* We ignore the EINVAL that results from not having PR_CAP_AMBIENT
* in the current kernel at runtime, so also ignore not having it
* in the current kernel headers at compile-time */
#endif
}
if (!keep && do_cap_bounding)
{
int res = prctl (PR_CAPBSET_DROP, cap, 0, 0, 0);
if (res == -1 && !(errno == EINVAL || errno == EPERM))
die_with_error ("Dropping capability %ld from bounds", cap);
}
}
}
static void
drop_cap_bounding_set (bool drop_all)
{
if (!drop_all)
prctl_caps (requested_caps, true, false);
else
{
uint32_t no_caps[2] = {0, 0};
prctl_caps (no_caps, true, false);
}
}
static void
set_ambient_capabilities (void)
{
if (is_privileged)
return;
prctl_caps (requested_caps, false, true);
}
/* This acquires the privileges that the bwrap will need it to work.
* If bwrap is not setuid, then this does nothing, and it relies on
* unprivileged user namespaces to be used. This case is
* "is_privileged = false".
*
* If bwrap is setuid, then we do things in phases.
* The first part is run as euid 0, but with fsuid as the real user.
* The second part, inside the child, is run as the real user but with
* capabilities.
* And finally we drop all capabilities.
* The reason for the above dance is to avoid having the setup phase
* being able to read files the user can't, while at the same time
* working around various kernel issues. See below for details.
*/
static void
acquire_privs (void)
{
uid_t euid, new_fsuid;
euid = geteuid ();
/* Are we setuid ? */
if (real_uid != euid)
{
if (euid != 0)
die ("Unexpected setuid user %d, should be 0", euid);
is_privileged = true;
/* We want to keep running as euid=0 until at the clone()
* operation because doing so will make the user namespace be
* owned by root, which makes it not ptrace:able by the user as
* it otherwise would be. After that we will run fully as the
* user, which is necessary e.g. to be able to read from a fuse
* mount from the user.
*
* However, we don't want to accidentally mis-use euid=0 for
* escalated filesystem access before the clone(), so we set
* fsuid to the uid.
*/
if (setfsuid (real_uid) < 0)
die_with_error ("Unable to set fsuid");
/* setfsuid can't properly report errors, check that it worked (as per manpage) */
new_fsuid = setfsuid (-1);
if (new_fsuid != real_uid)
die ("Unable to set fsuid (was %d)", (int)new_fsuid);
/* We never need capabilities after execve(), so lets drop everything from the bounding set */
drop_cap_bounding_set (true);
/* Keep only the required capabilities for setup */
set_required_caps ();
}
else if (real_uid != 0 && has_caps ())
{
/* We have some capabilities in the non-setuid case, which should not happen.
Probably caused by the binary being setcap instead of setuid which we
don't support anymore */
die ("Unexpected capabilities but not setuid, old file caps config?");
}
else if (real_uid == 0)
{
/* If our uid is 0, default to inheriting all caps; the caller
* can drop them via --cap-drop. This is used by at least rpm-ostree.
* Note this needs to happen before the argument parsing of --cap-drop.
*/
struct __user_cap_header_struct hdr = { _LINUX_CAPABILITY_VERSION_3, 0 };
struct __user_cap_data_struct data[2] = { { 0 } };
if (capget (&hdr, data) < 0)
die_with_error ("capget (for uid == 0) failed");
requested_caps[0] = data[0].effective;
requested_caps[1] = data[1].effective;
}
/* Else, we try unprivileged user namespaces */
}
/* This is called once we're inside the namespace */
static void
switch_to_user_with_privs (void)
{
/* If we're in a new user namespace, we got back the bounding set, clear it again */
if (opt_unshare_user || opt_userns_fd != -1)
drop_cap_bounding_set (false);
/* If we switched to a new user namespace it may allow other uids/gids, so switch to the target one */
if (opt_userns_fd != -1)
{
if (opt_sandbox_uid != real_uid && setuid (opt_sandbox_uid) < 0)
die_with_error ("unable to switch to uid %d", opt_sandbox_uid);
if (opt_sandbox_gid != real_gid && setgid (opt_sandbox_gid) < 0)
die_with_error ("unable to switch to gid %d", opt_sandbox_gid);
}
if (!is_privileged)
return;
/* Tell kernel not clear capabilities when later dropping root uid */
if (prctl (PR_SET_KEEPCAPS, 1, 0, 0, 0) < 0)
die_with_error ("prctl(PR_SET_KEEPCAPS) failed");
if (setuid (opt_sandbox_uid) < 0)
die_with_error ("unable to drop root uid");
/* Regain effective required capabilities from permitted */
set_required_caps ();
}
/* Call setuid() and use capset() to adjust capabilities */
static void
drop_privs (bool keep_requested_caps,
bool already_changed_uid)
{
assert (!keep_requested_caps || !is_privileged);
/* Drop root uid */
if (is_privileged && !already_changed_uid &&
setuid (opt_sandbox_uid) < 0)
die_with_error ("unable to drop root uid");
drop_all_caps (keep_requested_caps);
/* We don't have any privs now, so mark us dumpable which makes /proc/self be owned by the user instead of root */
if (prctl (PR_SET_DUMPABLE, 1, 0, 0, 0) != 0)
die_with_error ("can't set dumpable");
}
static void
write_uid_gid_map (uid_t sandbox_uid,
uid_t parent_uid,
uid_t sandbox_gid,
uid_t parent_gid,
pid_t pid,
bool deny_groups,
bool map_root)
{
cleanup_free char *uid_map = NULL;
cleanup_free char *gid_map = NULL;
cleanup_free char *dir = NULL;
cleanup_fd int dir_fd = -1;
uid_t old_fsuid = (uid_t)-1;
if (pid == -1)
dir = xstrdup ("self");
else
dir = xasprintf ("%d", pid);
dir_fd = openat (proc_fd, dir, O_PATH);
if (dir_fd < 0)
die_with_error ("open /proc/%s failed", dir);
if (map_root && parent_uid != 0 && sandbox_uid != 0)
uid_map = xasprintf ("0 %d 1\n"
"%d %d 1\n", overflow_uid, sandbox_uid, parent_uid);
else
uid_map = xasprintf ("%d %d 1\n", sandbox_uid, parent_uid);
if (map_root && parent_gid != 0 && sandbox_gid != 0)
gid_map = xasprintf ("0 %d 1\n"
"%d %d 1\n", overflow_gid, sandbox_gid, parent_gid);
else
gid_map = xasprintf ("%d %d 1\n", sandbox_gid, parent_gid);
/* We have to be root to be allowed to write to the uid map
* for setuid apps, so temporary set fsuid to 0 */
if (is_privileged)
old_fsuid = setfsuid (0);
if (write_file_at (dir_fd, "uid_map", uid_map) != 0)
die_with_error ("setting up uid map");