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xz: Move sandboxing code to sandbox.c and improve Landlock sandbox.

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Landlock is now always used just like pledge(2) is: first in more
permissive mode and later (under certain common conditions) in
a strict mode that doesn't allow opening more files.

I put pledge(2) first in sandbox.c because it's the simplest API
to use and still somewhat fine-grained for basic applications.
So it's the simplest thing to understand for anyone reading sandbox.c.
This commit is contained in:
Lasse Collin 2024-02-17 23:07:35 +02:00
parent 7312dfbb02
commit 374868d81d
8 changed files with 357 additions and 213 deletions
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2
CMakeLists.txt
View file

@ -1393,6 +1393,8 @@ if(NOT MSVC OR MSVC_VERSION GREATER_EQUAL 1900)
src/xz/options.c src/xz/options.c
src/xz/options.h src/xz/options.h
src/xz/private.h src/xz/private.h
src/xz/sandbox.c
src/xz/sandbox.h
src/xz/signals.c src/xz/signals.c
src/xz/signals.h src/xz/signals.h
src/xz/suffix.c src/xz/suffix.c

2
src/xz/Makefile.am
View file

@ -21,6 +21,8 @@ xz_SOURCES = \
options.c \ options.c \
options.h \ options.h \
private.h \ private.h \
sandbox.c \
sandbox.h \
signals.c \ signals.c \
signals.h \ signals.h \
suffix.c \ suffix.c \

170
src/xz/file_io.c
View file

@ -28,15 +28,6 @@ static bool warn_fchown;
# include <utime.h> # include <utime.h>
#endif #endif
#ifdef HAVE_CAP_RIGHTS_LIMIT
# include <sys/capsicum.h>
#endif
#ifdef HAVE_LINUX_LANDLOCK_H
# include <linux/landlock.h>
# include <sys/syscall.h>
#endif
#include "tuklib_open_stdxxx.h" #include "tuklib_open_stdxxx.h"
#ifdef _MSC_VER #ifdef _MSC_VER
@ -92,11 +83,6 @@ typedef enum {
/// If true, try to create sparse files when decompressing. /// If true, try to create sparse files when decompressing.
static bool try_sparse = true; static bool try_sparse = true;
#ifdef ENABLE_SANDBOX
/// True if the conditions for sandboxing (described in main()) have been met.
static bool sandbox_allowed = false;
#endif
#ifndef TUKLIB_DOSLIKE #ifndef TUKLIB_DOSLIKE
/// File status flags of standard input. This is used by io_open_src() /// File status flags of standard input. This is used by io_open_src()
/// and io_close_src(). /// and io_close_src().
@ -181,159 +167,6 @@ io_no_sparse(void)
} }
#ifdef ENABLE_SANDBOX
extern void
io_allow_sandbox(void)
{
sandbox_allowed = true;
return;
}
/// Enables operating-system-specific sandbox if it is possible.
/// src_fd is the file descriptor of the input file.
static void
io_sandbox_enter(int src_fd)
{
if (!sandbox_allowed) {
// This message is more often annoying than useful so
// it's commented out. It can be useful when developing
// the sandboxing code.
//message(V_DEBUG, _("Sandbox is disabled due "
// "to incompatible command line arguments"));
return;
}
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);
#ifdef HAVE_CAP_RIGHTS_LIMIT
// 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;
#elif defined(HAVE_PLEDGE)
// pledge() was introduced in OpenBSD 5.9.
//
// main() unconditionally calls pledge() with fairly relaxed
// promises which work in all situations. Here we make the
// sandbox more strict.
if (pledge("stdio", ""))
goto error;
(void)src_fd;
#elif defined(HAVE_LINUX_LANDLOCK_H)
int landlock_abi = syscall(SYS_landlock_create_ruleset,
(void *)NULL, 0, LANDLOCK_CREATE_RULESET_VERSION);
if (landlock_abi > 0) {
// We support ABI versions 1-3.
if (landlock_abi > 3)
landlock_abi = 3;
// 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>.
const struct landlock_ruleset_attr attr = {
.handled_access_fs = (1ULL << (12 + landlock_abi)) - 1
};
const int ruleset_fd = syscall(SYS_landlock_create_ruleset,
&attr, sizeof(attr), 0U);
if (ruleset_fd < 0)
goto error;
// 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 at the beginning of this
// function 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
// main() so we don't do it here again.
if (syscall(SYS_landlock_restrict_self, ruleset_fd, 0U) != 0)
goto error;
}
(void)src_fd;
#else
# error ENABLE_SANDBOX is defined but no sandboxing method was found.
#endif
// This message is annoying in xz -lvv.
//message(V_DEBUG, _("Sandbox was successfully enabled"));
return;
error:
#ifdef HAVE_CAP_RIGHTS_LIMIT
// 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;
#endif
message_fatal(_("Failed to enable the sandbox"));
}
#endif // ENABLE_SANDBOX
#ifndef TUKLIB_DOSLIKE #ifndef TUKLIB_DOSLIKE
/// \brief Waits for input or output to become available or for a signal /// \brief Waits for input or output to become available or for a signal
/// ///
@ -889,7 +722,8 @@ io_open_src(const char *src_name)
#ifdef ENABLE_SANDBOX #ifdef ENABLE_SANDBOX
if (!error) if (!error)
io_sandbox_enter(pair.src_fd); sandbox_enable_strict_if_allowed(pair.src_fd,
user_abort_pipe[0], user_abort_pipe[1]);
#endif #endif
return error ? NULL : &pair; return error ? NULL : &pair;

6
src/xz/file_io.h
View file

@ -99,12 +99,6 @@ extern void io_write_to_user_abort_pipe(void);
extern void io_no_sparse(void); extern void io_no_sparse(void);
#ifdef ENABLE_SANDBOX
/// \brief main() calls this if conditions for sandboxing have been met.
extern void io_allow_sandbox(void);
#endif
/// \brief Open the source file /// \brief Open the source file
extern file_pair *io_open_src(const char *src_name); extern file_pair *io_open_src(const char *src_name);

50
src/xz/main.c
View file

@ -12,12 +12,6 @@
#include "private.h" #include "private.h"
#include <ctype.h> #include <ctype.h>
// prctl(PR_SET_NO_NEW_PRIVS, ...) is required with Landlock but it can be
// activated even when conditions for strict sandboxing aren't met.
#ifdef HAVE_LINUX_LANDLOCK_H
# include <sys/prctl.h>
#endif
/// Exit status to use. This can be changed with set_exit_status(). /// Exit status to use. This can be changed with set_exit_status().
static enum exit_status_type exit_status = E_SUCCESS; static enum exit_status_type exit_status = E_SUCCESS;
@ -148,32 +142,6 @@ read_name(const args_info *args)
int int
main(int argc, char **argv) main(int argc, char **argv)
{ {
#ifdef HAVE_PLEDGE
// OpenBSD's pledge(2) sandbox
//
// Unconditionally enable sandboxing with fairly relaxed promises.
// This is still way better than having no sandbox at all. :-)
// More strict promises will be made later in file_io.c if possible.
if (pledge("stdio rpath wpath cpath fattr", "")) {
// Don't translate the string or use message_fatal() as
// those haven't been initialized yet.
fprintf(stderr, "%s: Failed to enable the sandbox\n", argv[0]);
return E_ERROR;
}
#endif
#ifdef HAVE_LINUX_LANDLOCK_H
// Prevent the process from gaining new privileges. This must be done
// before landlock_restrict_self(2) in file_io.c 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);
#endif
#if defined(_WIN32) && !defined(__CYGWIN__) #if defined(_WIN32) && !defined(__CYGWIN__)
InitializeCriticalSection(&exit_status_cs); InitializeCriticalSection(&exit_status_cs);
#endif #endif
@ -187,6 +155,20 @@ main(int argc, char **argv)
// even indirectly like locale and gettext initializations. // even indirectly like locale and gettext initializations.
io_init(); io_init();
#ifdef ENABLE_SANDBOX
// Enable such sandboxing that can always be enabled.
// This requires that progname has been set up.
// It's also good that io_init() has been called because it
// might need to do things that the initial sandbox won't allow.
// Otherwise this should be called as early as possible.
//
// NOTE: Calling this before tuklib_gettext_init() means that
// translated error message won't be available if sandbox
// initialization fails. However, sandbox_init() shouldn't
// fail and this order simply feels better.
sandbox_init();
#endif
// Set up the locale and message translations. // Set up the locale and message translations.
tuklib_gettext_init(PACKAGE, LOCALEDIR); tuklib_gettext_init(PACKAGE, LOCALEDIR);
@ -241,7 +223,7 @@ main(int argc, char **argv)
signals_init(); signals_init();
#ifdef ENABLE_SANDBOX #ifdef ENABLE_SANDBOX
// Set a flag that sandboxing is allowed if all these are true: // Set a flag that strict sandboxing is allowed if all these are true:
// - --files or --files0 wasn't used. // - --files or --files0 wasn't used.
// - There is exactly one input file or we are reading from stdin. // - There is exactly one input file or we are reading from stdin.
// - We won't create any files: output goes to stdout or --test // - We won't create any files: output goes to stdout or --test
@ -255,7 +237,7 @@ main(int argc, char **argv)
if (args.files_name == NULL && args.arg_count == 1 if (args.files_name == NULL && args.arg_count == 1
&& (opt_stdout || strcmp("-", args.arg_names[0]) == 0 && (opt_stdout || strcmp("-", args.arg_names[0]) == 0
|| opt_mode == MODE_LIST)) || opt_mode == MODE_LIST))
io_allow_sandbox(); sandbox_allow_strict();
#endif #endif
// coder_run() handles compression, decompression, and testing. // coder_run() handles compression, decompression, and testing.

6
src/xz/private.h
View file

@ -51,11 +51,6 @@
# define STDERR_FILENO (fileno(stderr)) # define STDERR_FILENO (fileno(stderr))
#endif #endif
#if defined(HAVE_CAP_RIGHTS_LIMIT) || defined(HAVE_PLEDGE) \
|| defined(HAVE_LINUX_LANDLOCK_H)
# define ENABLE_SANDBOX 1
#endif
// Handling SIGTSTP keeps time-keeping for progress indicator correct // Handling SIGTSTP keeps time-keeping for progress indicator correct
// if xz is stopped. It requires use of clock_gettime() as that is // if xz is stopped. It requires use of clock_gettime() as that is
// async-signal safe in POSIX. Require also SIGALRM support since // async-signal safe in POSIX. Require also SIGALRM support since
@ -75,6 +70,7 @@
#include "hardware.h" #include "hardware.h"
#include "file_io.h" #include "file_io.h"
#include "options.h" #include "options.h"
#include "sandbox.h"
#include "signals.h" #include "signals.h"
#include "suffix.h" #include "suffix.h"
#include "util.h" #include "util.h"

295
src/xz/sandbox.c Normal file
View file

@ -0,0 +1,295 @@
// 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
#define LANDLOCK_ABI_MAX 3
/// 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>.
const struct landlock_ruleset_attr attr = {
.handled_access_fs = ((1ULL << (12 + landlock_abi)) - 1)
& ~required_rights,
};
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

39
src/xz/sandbox.h Normal file
View file

@ -0,0 +1,39 @@
// SPDX-License-Identifier: 0BSD
///////////////////////////////////////////////////////////////////////////////
//
/// \file sandbox.h
/// \brief Sandbox support
//
// Author: Lasse Collin
//
///////////////////////////////////////////////////////////////////////////////
#if defined(HAVE_PLEDGE) || defined(HAVE_LINUX_LANDLOCK_H) \
|| defined(HAVE_CAP_RIGHTS_LIMIT)
# define ENABLE_SANDBOX 1
#endif
/// \brief Enables early sandboxing that can always be enabled
///
/// This requires that tuklib_progname() and io_init() have been called.
extern void sandbox_init(void);
/// \brief Tell sandboxing code that strict sandboxing can be used
///
/// This function only sets a flag which will be read by
/// sandbox_enable_strict_if_allowed().
extern void sandbox_allow_strict(void);
/// \brief Enable sandboxing that allows reading from one file
///
/// This does nothing if sandbox_allow_strict() hasn't been called.
///
/// \param src_fd File descriptor open for reading
/// \param pipe_event_fd user_abort_pipe[0] from file_io.c
/// \param pipe_write_fd user_abort_pipe[1] from file_io.c
extern void sandbox_enable_strict_if_allowed(
int src_fd, int pipe_event_fd, int pipe_write_fd);