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xz: Support basic sandboxing with Linux Landlock (ABI versions 1-3).

It is enabled only when decompressing one file to stdout,
similar to how Capsicum is used.

Landlock was added in Linux 5.13.
This commit is contained in:
Lasse Collin 2023-10-09 22:07:52 +03:00
parent 3a1e9fd031
commit 8276c7f41c
5 changed files with 98 additions and 5 deletions

View file

@ -1246,7 +1246,8 @@ if(NOT MSVC OR MSVC_VERSION GREATER_EQUAL 1900)
# OFF Disable sandboxing.
# capsicum Require Capsicum (FreeBSD >= 10.2) and fail if not found.
# pledge Require pledge(2) (OpenBSD >= 5.9) and fail if not found.
set(SUPPORTED_SANDBOX_METHODS ON OFF capsicum pledge)
# landlock Require Landlock (Linux >= 5.13) and fail if not found.
set(SUPPORTED_SANDBOX_METHODS ON OFF capsicum pledge landlock)
set(ENABLE_SANDBOX ON CACHE STRING "Sandboxing method to use in 'xz'")
@ -1285,6 +1286,15 @@ if(NOT MSVC OR MSVC_VERSION GREATER_EQUAL 1900)
endif()
endif()
# Sandboxing: Landlock
if(NOT SANDBOX_FOUND AND ENABLE_SANDBOX MATCHES "^ON$|^landlock$")
check_include_file(linux/landlock.h HAVE_LINUX_LANDLOCK_H)
if(HAVE_LINUX_LANDLOCK_H)
target_compile_definitions(xz PRIVATE HAVE_LINUX_LANDLOCK_H)
set(SANDBOX_FOUND ON)
endif()
endif()
if(NOT SANDBOX_FOUND AND NOT ENABLE_SANDBOX MATCHES "^ON$|^OFF$")
message(SEND_ERROR "ENABLE_SANDBOX=${ENABLE_SANDBOX} was used but "
"support for the sandboxing method wasn't found.")

View file

@ -519,7 +519,7 @@ AM_CONDITIONAL([COND_DOC], [test x$enable_doc != xno])
AC_MSG_CHECKING([if sandboxing should be used])
AC_ARG_ENABLE([sandbox], [AS_HELP_STRING([--enable-sandbox=METHOD],
[Sandboxing METHOD can be
'auto', 'no', 'capsicum', or 'pledge'.
'auto', 'no', 'capsicum', 'pledge', or 'landlock'.
The default is 'auto' which enables sandboxing if
a supported sandboxing method is found.])],
[], [enable_sandbox=auto])
@ -527,12 +527,12 @@ case $enable_sandbox in
auto)
AC_MSG_RESULT([maybe (autodetect)])
;;
no | capsicum | pledge)
no | capsicum | pledge | landlock)
AC_MSG_RESULT([$enable_sandbox])
;;
*)
AC_MSG_RESULT([])
AC_MSG_ERROR([--enable-sandbox only accepts 'auto', 'no', 'capsicum', or 'pledge'.])
AC_MSG_ERROR([--enable-sandbox only accepts 'auto', 'no', 'capsicum', 'pledge', or 'landlock'.])
;;
esac
@ -1059,6 +1059,11 @@ AS_CASE([$enable_sandbox],
AC_CHECK_FUNCS([pledge], [enable_sandbox=found])
]
)
AS_CASE([$enable_sandbox],
[auto | landlock], [
AC_CHECK_HEADERS([linux/landlock.h], [enable_sandbox=found])
]
)
# If a specific sandboxing method was explicitly requested and it wasn't
# found, give an error.

View file

@ -33,6 +33,11 @@ static bool warn_fchown;
# include <sys/capsicum.h>
#endif
#ifdef HAVE_LINUX_LANDLOCK_H
# include <linux/landlock.h>
# include <sys/syscall.h>
#endif
#include "tuklib_open_stdxxx.h"
#ifdef _MSC_VER
@ -253,6 +258,59 @@ io_sandbox_enter(int src_fd)
(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 openend. 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

View file

@ -13,6 +13,13 @@
#include "private.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().
static enum exit_status_type exit_status = E_SUCCESS;
@ -156,6 +163,18 @@ main(int argc, char **argv)
}
#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__)
InitializeCriticalSection(&exit_status_cs);
#endif

View file

@ -52,7 +52,8 @@
# define STDERR_FILENO (fileno(stderr))
#endif
#if defined(HAVE_CAP_RIGHTS_LIMIT) || defined(HAVE_PLEDGE)
#if defined(HAVE_CAP_RIGHTS_LIMIT) || defined(HAVE_PLEDGE) \
|| defined(HAVE_LINUX_LANDLOCK_H)
# define ENABLE_SANDBOX 1
#endif