// SPDX-License-Identifier: 0BSD
///////////////////////////////////////////////////////////////////////////////
//
/// \file sandbox.c
/// \brief Sandbox support
//
// Author: Lasse Collin
//
///////////////////////////////////////////////////////////////////////////////
#include "private.h"
#ifndef ENABLE_SANDBOX
// Prevent an empty translation unit when no sandboxing is supported.
typedef int dummy;
#else
/// If the conditions for strict sandboxing (described in main())
/// have been met, sandbox_allow_strict() can be called to set this
/// variable to true.
static bool strict_sandbox_allowed = false;
extern void
sandbox_allow_strict(void)
{
strict_sandbox_allowed = true;
return;
}
// Strict sandboxing prevents opening any files. This *tries* to ensure
// that any auxiliary files that might be required are already open.
//
// Returns true if strict sandboxing is allowed, false otherwise.
static bool
prepare_for_strict_sandbox(void)
{
if (!strict_sandbox_allowed)
return false;
const char dummy_str[] = "x";
// Try to ensure that both libc and xz locale files have been
// loaded when NLS is enabled.
snprintf(NULL, 0, "%s%s", _(dummy_str), strerror(EINVAL));
// Try to ensure that iconv data files needed for handling multibyte
// characters have been loaded. This is needed at least with glibc.
tuklib_mbstr_width(dummy_str, NULL);
return true;
}
#endif
#if defined(HAVE_PLEDGE)
///////////////
// pledge(2) //
///////////////
#include <unistd.h>
extern void
sandbox_init(void)
{
if (pledge("stdio rpath wpath cpath fattr", "")) {
// gettext hasn't been initialized yet so
// there's no point to call it here.
message_fatal("Failed to enable the sandbox");
}
return;
}
extern void
sandbox_enable_strict_if_allowed(int src_fd lzma_attribute((__unused__)),
int pipe_event_fd lzma_attribute((__unused__)),
int pipe_write_fd lzma_attribute((__unused__)))
{
if (!prepare_for_strict_sandbox())
return;
if (pledge("stdio", ""))
message_fatal(_("Failed to enable the sandbox"));
return;
}
#elif defined(HAVE_LINUX_LANDLOCK_H)
//////////////
// Landlock //
//////////////
#include <linux/landlock.h>
#include <sys/syscall.h>
#include <sys/prctl.h>
// Highest Landlock ABI version supported by this file:
// - For ABI versions 1-3 we don't need anything from <linux/landlock.h>
// that isn't part of version 1.
// - For ABI version 4 we need the larger struct landlock_ruleset_attr
// with the handled_access_net member. That is bundled with the macros
// LANDLOCK_ACCESS_NET_BIND_TCP and LANDLOCK_ACCESS_NET_CONNECT_TCP.
#ifdef LANDLOCK_ACCESS_NET_BIND_TCP
# define LANDLOCK_ABI_MAX 4
#else
# define LANDLOCK_ABI_MAX 3
#endif
/// Landlock ABI version supported by the kernel
static int landlock_abi;
// The required_rights should have those bits set that must not be restricted.
// This function will then bitwise-and ~required_rights with a mask matching
// the Landlock ABI version, leaving only those bits set that are supported
// by the ABI and allowed to be restricted by the function argument.
static void
enable_landlock(uint64_t required_rights)
{
assert(landlock_abi <= LANDLOCK_ABI_MAX);
if (landlock_abi <= 0)
return;
// We want to set all supported flags in handled_access_fs.
// This way the ruleset will initially forbid access to all
// actions that the available Landlock ABI version supports.
// Exceptions can be added using landlock_add_rule(2) to
// allow certain actions on certain files or directories.
//
// The same flag values are used on all archs. ABI v2 and v3
// both add one new flag.
//
// First in ABI v1: LANDLOCK_ACCESS_FS_EXECUTE = 1ULL << 0
// Last in ABI v1: LANDLOCK_ACCESS_FS_MAKE_SYM = 1ULL << 12
// Last in ABI v2: LANDLOCK_ACCESS_FS_REFER = 1ULL << 13
// Last in ABI v3: LANDLOCK_ACCESS_FS_TRUNCATE = 1ULL << 14
//
// This makes it simple to set the mask based on the ABI
// version and we don't need to care which flags are #defined
// in the installed <linux/landlock.h> for ABI versions 1-3.
const struct landlock_ruleset_attr attr = {
.handled_access_fs = ~required_rights
& ((1ULL << (12 + my_min(3, landlock_abi))) - 1),
#if LANDLOCK_ABI_MAX >= 4
.handled_access_net = landlock_abi < 4 ? 0 :
(LANDLOCK_ACCESS_NET_BIND_TCP
| LANDLOCK_ACCESS_NET_CONNECT_TCP),
#endif
};
const int ruleset_fd = syscall(SYS_landlock_create_ruleset,
&attr, sizeof(attr), 0U);
if (ruleset_fd < 0)
message_fatal(_("Failed to enable the sandbox"));
// All files we need should have already been opened. Thus,
// we don't need to add any rules using landlock_add_rule(2)
// before activating the sandbox.
//
// NOTE: It's possible that the hack prepare_for_strict_sandbox()
// isn't be good enough. It tries to get translations and
// libc-specific files loaded but if it's not good enough
// then perhaps a Landlock rule to allow reading from /usr
// and/or the xz installation prefix would be needed.
//
// prctl(PR_SET_NO_NEW_PRIVS, ...) was already called in
// sandbox_init() so we don't do it here again.
if (syscall(SYS_landlock_restrict_self, ruleset_fd, 0U) != 0)
message_fatal(_("Failed to enable the sandbox"));
return;
}
extern void
sandbox_init(void)
{
// Prevent the process from gaining new privileges. This must be done
// before landlock_restrict_self(2) but since we will never need new
// privileges, this call can be done here already.
//
// This is supported since Linux 3.5. Ignore the return value to
// keep compatibility with old kernels. landlock_restrict_self(2)
// will fail if the no_new_privs attribute isn't set, thus if prctl()
// fails here the error will still be detected when it matters.
(void)prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0);
// Get the highest Landlock ABI version supported by the kernel.
landlock_abi = syscall(SYS_landlock_create_ruleset,
(void *)NULL, 0, LANDLOCK_CREATE_RULESET_VERSION);
// The kernel might support a newer ABI than this file.
if (landlock_abi > LANDLOCK_ABI_MAX)
landlock_abi = LANDLOCK_ABI_MAX;
// These are all in ABI version 1 already. We don't need truncate
// rights because files are created with open() using O_EXCL and
// without O_TRUNC.
const uint64_t required_rights
= LANDLOCK_ACCESS_FS_WRITE_FILE
| LANDLOCK_ACCESS_FS_READ_FILE
| LANDLOCK_ACCESS_FS_REMOVE_FILE
| LANDLOCK_ACCESS_FS_MAKE_REG;
enable_landlock(required_rights);
return;
}
extern void
sandbox_enable_strict_if_allowed(int src_fd lzma_attribute((__unused__)),
int pipe_event_fd lzma_attribute((__unused__)),
int pipe_write_fd lzma_attribute((__unused__)))
{
if (!prepare_for_strict_sandbox())
return;
// Allow all restrictions that the kernel supports with the
// highest Landlock ABI version that the kernel or xz supports.
enable_landlock(0);
return;
}
#elif defined(HAVE_CAP_RIGHTS_LIMIT)
//////////////
// Capsicum //
//////////////
#include <sys/capsicum.h>
extern void
sandbox_init(void)
{
// Nothing to do.
return;
}
extern void
sandbox_enable_strict_if_allowed(
int src_fd, int pipe_event_fd, int pipe_write_fd)
{
if (!prepare_for_strict_sandbox())
return;
// Capsicum needs FreeBSD 10.2 or later.
cap_rights_t rights;
if (cap_enter())
goto error;
if (cap_rights_limit(src_fd, cap_rights_init(&rights,
CAP_EVENT, CAP_FCNTL, CAP_LOOKUP, CAP_READ, CAP_SEEK)))
goto error;
// If not reading from stdin, remove all capabilities from it.
if (src_fd != STDIN_FILENO && cap_rights_limit(
STDIN_FILENO, cap_rights_clear(&rights)))
goto error;
if (cap_rights_limit(STDOUT_FILENO, cap_rights_init(&rights,
CAP_EVENT, CAP_FCNTL, CAP_FSTAT, CAP_LOOKUP,
CAP_WRITE, CAP_SEEK)))
goto error;
if (cap_rights_limit(STDERR_FILENO, cap_rights_init(&rights,
CAP_WRITE)))
goto error;
if (cap_rights_limit(user_abort_pipe[0], cap_rights_init(&rights,
CAP_EVENT)))
goto error;
if (cap_rights_limit(user_abort_pipe[1], cap_rights_init(&rights,
CAP_WRITE)))
goto error;
return;
error:
// If a kernel is configured without capability mode support or
// used in an emulator that does not implement the capability
// system calls, then the Capsicum system calls will fail and set
// errno to ENOSYS. In that case xz will silently run without
// the sandbox.
if (errno == ENOSYS)
return;
message_fatal(_("Failed to enable the sandbox"));
}
#endif
