[PATCH] Xorg: Add a suid root wrapper
Peter Hutterer
peter.hutterer at who-t.net
Sun Mar 9 23:03:19 PDT 2014
On Fri, Mar 07, 2014 at 11:38:52AM +0100, Hans de Goede wrote:
> With the recent systemd-logind changes it is possible to install the Xorg
> binary without suid root rights and still have everything working as it
> should *if* the user only has cards which are supported by kms.
>
> This commit adds a little suid root wrapper, which is a bit weird, first we
> strip the suid-root bit of the Xorg binary, and then we add a wrapper ?
>
> The function of this wrapper is to see if a system still needs root-rights,
> if it does not (it supports kms and the kms drivers are properly loaded),
> then it will immediately drop all elevated rights before executing the real
> Xorg binary. If it finds (some) cards which don't support kms, or no cards
> at all, then it will execute the Xorg server with elevated rights so that
> ie the nvidia binary driver and the vesa driver can keep working normally.
>
> To make it possible for security concious users who don't need the root
> rights to completely remove the wrapper, Xorg is started in a 3 step process
> when the wrapper is enabled during build time:
>
> 1) A simple shell script which checks if the wrapper is there, if it is
> it executes the wrapper, if not it directly executes the real Xorg binary
>
> 2) The wrapper gets executed, does its checks, normally drops all elevated
> rights and then executes the real Xorg binary
>
> 3) The real Xorg binary does its thing
just thinking aloud here:
I wonder if we need a three-step process here, specifically if we need the
first shell script. Would a solution be useful where the wrapper simply
checks for its own permissions and skips most of the code if it doesn't have
root privs?
I guess this may be bad from a packaging POV because we'd have two packages
providing the same file, once with suid root set, once without.
or it could be an empty package that just triggers chmod.
> This allows distributions to put the wrapper binary in a separate package, and
> will allow users to remove this package. IE the plan with Fedora is to make
> "legacy" drivers depend on the wrapper pkg, and since our default install
> contains some legacy drivers it will be part of the default install, but
> users can later yum remove it (which will also automatically remove the
> legacy driver packages as those won't work without it anyways).
>
> The wrapper is loosely modelled after the existing Debian Xwrapper, it
> uses the same config-file + config-file format, and also allows restricting
> Xserver execution (through the wrapper) to console users only.
>
> There also is a new needs_root_rights config file directive, which can
> be used to override the auto-detection the wrapper does.
>
> Hopefully this will allow Debian to replace their own wrapper with this
> upstream one.
>
> Signed-off-by: Hans de Goede <hdegoede at redhat.com>
> ---
> configure.ac | 7 ++
> hw/xfree86/Makefile.am | 14 ++-
> hw/xfree86/Xorg.sh.in | 12 +++
> hw/xfree86/xorg-wrapper.c | 211 ++++++++++++++++++++++++++++++++++++++++++++++
> 4 files changed, 242 insertions(+), 2 deletions(-)
> create mode 100644 hw/xfree86/Xorg.sh.in
> create mode 100644 hw/xfree86/xorg-wrapper.c
>
> diff --git a/configure.ac b/configure.ac
> index 588ae59..6028ab1 100644
> --- a/configure.ac
> +++ b/configure.ac
> @@ -627,6 +627,7 @@ AC_ARG_ENABLE(pciaccess, AS_HELP_STRING([--enable-pciaccess], [Build Xorg with p
> AC_ARG_ENABLE(linux_acpi, AS_HELP_STRING([--disable-linux-acpi], [Disable building ACPI support on Linux (if available).]), [enable_linux_acpi=$enableval], [enable_linux_acpi=yes])
> AC_ARG_ENABLE(linux_apm, AS_HELP_STRING([--disable-linux-apm], [Disable building APM support on Linux (if available).]), [enable_linux_apm=$enableval], [enable_linux_apm=yes])
> AC_ARG_ENABLE(systemd-logind, AC_HELP_STRING([--enable-systemd-logind], [Build systemd-logind support (default: auto)]), [SYSTEMD_LOGIND=$enableval], [SYSTEMD_LOGIND=auto])
> +AC_ARG_ENABLE(suid-wrapper, AC_HELP_STRING([--enable-suid-wrapper], [Build suid-root wrapper for legacy driver support on rootless xserver systems (default: no)]), [SUID_WRAPPER=$enableval], [SUID_WRAPPER=no])
>
> dnl DDXes.
> AC_ARG_ENABLE(xorg, AS_HELP_STRING([--enable-xorg], [Build Xorg server (default: auto)]), [XORG=$enableval], [XORG=auto])
> @@ -922,6 +923,11 @@ if test "x$SYSTEMD_LOGIND" = xyes; then
> fi
> AM_CONDITIONAL(SYSTEMD_LOGIND, [test "x$SYSTEMD_LOGIND" = xyes])
>
> +if test "x$SUID_WRAPPER" = xyes; then
> + SETUID="no"
> +fi
> +AM_CONDITIONAL(SUID_WRAPPER, [test "x$SUID_WRAPPER" = xyes])
> +
> if test "x$NEED_DBUS" = xyes; then
> AC_DEFINE(NEED_DBUS, 1, [Enable D-Bus core])
> fi
> @@ -2486,6 +2492,7 @@ dri3/Makefile
> present/Makefile
> hw/Makefile
> hw/xfree86/Makefile
> +hw/xfree86/Xorg.sh
> hw/xfree86/common/Makefile
> hw/xfree86/common/xf86Build.h
> hw/xfree86/ddc/Makefile
> diff --git a/hw/xfree86/Makefile.am b/hw/xfree86/Makefile.am
> index 9672904..59ba9d8 100644
> --- a/hw/xfree86/Makefile.am
> +++ b/hw/xfree86/Makefile.am
> @@ -76,9 +76,14 @@ Xorg_DEPENDENCIES = $(LOCAL_LIBS)
>
> Xorg_LDFLAGS = $(LD_EXPORT_SYMBOLS_FLAG)
>
> +if SUID_WRAPPER
> +bin_PROGRAMS += Xorg.wrap
> +Xorg_wrap_SOURCES = xorg-wrapper.c
> +endif
> +
> BUILT_SOURCES = xorg.conf.example
> DISTCLEANFILES = xorg.conf.example
> -EXTRA_DIST = xorgconf.cpp
> +EXTRA_DIST = xorgconf.cpp Xorg.sh.in
>
> # Without logdir, X will post an error on the terminal and will not start
> install-data-local:
> @@ -93,6 +98,11 @@ if INSTALL_SETUID
> chown root $(DESTDIR)$(bindir)/Xorg
> chmod u+s $(DESTDIR)$(bindir)/Xorg
> endif
> +if SUID_WRAPPER
> + mv $(DESTDIR)$(bindir)/Xorg $(DESTDIR)$(bindir)/Xorg.bin
> + ${INSTALL} -m 755 Xorg.sh $(DESTDIR)$(bindir)/Xorg
> + -chown root $(DESTDIR)$(bindir)/Xorg.wrap && chmod u+s $(DESTDIR)$(bindir)/Xorg.wrap
> +endif
>
> uninstall-local:
> if CYGWIN
> @@ -114,7 +124,7 @@ xorg.conf.example: xorgconf.cpp
> relink:
> $(AM_V_at)rm -f Xorg$(EXEEXT) && $(MAKE) Xorg$(EXEEXT)
>
> -CLEANFILES = sdksyms.c sdksyms.dep
> +CLEANFILES = sdksyms.c sdksyms.dep Xorg.sh
> EXTRA_DIST += sdksyms.sh
>
> sdksyms.dep sdksyms.c: sdksyms.sh
> diff --git a/hw/xfree86/Xorg.sh.in b/hw/xfree86/Xorg.sh.in
> new file mode 100644
> index 0000000..99f1d5b
> --- /dev/null
> +++ b/hw/xfree86/Xorg.sh.in
> @@ -0,0 +1,12 @@
> +#!/bin/sh
> +#
> +# See if Xorg.wrap is installed and if it is execute it, otherwise execute
> +# Xorg.bin directly. This allows distros to put the suid wrapper in a
> +# separate package.
> +
> +bindir=@PROJECTROOT@/bin
> +if [ -x "$bindir"/Xorg.wrap ]; then
> + exec "$bindir"/Xorg.wrap "$@"
> +else
> + exec "$bindir"/Xorg.bin "$@"
> +fi
I think libexecdir would be the best directory to put the wrapper script and
the binary.
> diff --git a/hw/xfree86/xorg-wrapper.c b/hw/xfree86/xorg-wrapper.c
> new file mode 100644
> index 0000000..9e9bc45
> --- /dev/null
> +++ b/hw/xfree86/xorg-wrapper.c
> @@ -0,0 +1,211 @@
> +/*
> + * Copyright © 2014 Red Hat, Inc.
> + *
> + * Permission is hereby granted, free of charge, to any person obtaining a
> + * copy of this software and associated documentation files (the "Software"),
> + * to deal in the Software without restriction, including without limitation
> + * the rights to use, copy, modify, merge, publish, distribute, sublicense,
> + * and/or sell copies of the Software, and to permit persons to whom the
> + * Software is furnished to do so, subject to the following conditions:
> + *
> + * The above copyright notice and this permission notice (including the next
> + * paragraph) shall be included in all copies or substantial portions of the
> + * Software.
> + *
> + * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
> + * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
> + * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
> + * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
> + * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
> + * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
> + * DEALINGS IN THE SOFTWARE.
> + *
> + * Author: Hans de Goede <hdegoede at redhat.com>
> + */
> +
> +#include "dix-config.h"
> +
> +#include <fcntl.h>
> +#include <limits.h>
> +#include <stdint.h>
> +#include <stdio.h>
> +#include <stdlib.h>
> +#include <string.h>
> +#include <sys/ioctl.h>
> +#include <sys/stat.h>
> +#include <sys/types.h>
> +#include <unistd.h>
> +#include <drm/drm.h>
> +
> +#define CONFIG_FILE "/etc/X11/Xwrapper.config"
this should probably use SYSCONFDIR
> +
> +enum { ROOT_ONLY, CONSOLE_ONLY, ANYBODY };
> +
> +static int allowed = CONSOLE_ONLY;
> +static int needs_root_rights = -1;
> +
> +/* KISS non locale / LANG parsing isspace version */
> +static int is_space(char c)
> +{
> + return c == ' ' || c == '\t' || c == '\n';
> +}
> +
> +static char *strip(char *s)
> +{
> + int i;
> +
> + /* Strip leading whitespace */
> + while (s[0] && is_space(s[0]))
> + s++;
> +
> + /* Strip trailing whitespace */
> + i = strlen(s) - 1;
> + while (i >= 0 && is_space(s[i])) {
> + s[i] = 0;
> + i--;
> + }
> +
> + return s;
> +}
> +
> +static void parse_config(void)
> +{
> + FILE *f;
> + char buf[1024];
> + char *stripped, *equals, *key, *value;
> + int line = 0;
> +
> + f = fopen(CONFIG_FILE, "r");
> + if (!f)
> + return;
> +
> + while (fgets(buf, sizeof(buf), f)) {
> + line++;
> +
> + /* Skip comments and empty lines */
> + stripped = strip(buf);
> + if (stripped[0] == '#' || stripped[0] == 0)
> + continue;
> +
> + /* Split in a key + value pair */
> + equals = strchr(stripped, '=');
> + if (!equals) {
> + fprintf(stderr, "Syntax error at %s line %d\n", CONFIG_FILE, line);
> + exit(1);
> + }
> + *equals = 0;
> + key = strip(stripped); /* To remove trailing whitespace from key */
> + value = strip(equals + 1); /* To remove leading whitespace from val */
> +
> + /* And finally process */
> + if (strcmp(key, "allowed_users") == 0) {
> + if (strcmp(value, "rootonly") == 0)
> + allowed = ROOT_ONLY;
> + else if (strcmp(value, "console") == 0)
> + allowed = CONSOLE_ONLY;
> + else if (strcmp(value, "anybody") == 0)
> + allowed = ANYBODY;
> + else {
> + fprintf(stderr,
> + "Unknown value '%s' for 'allowed_users' at %s line %d\n",
> + value, CONFIG_FILE, line);
> + exit(1);
> + }
> + }
> + else if (strcmp(key, "needs_root_rights") == 0) {
> + needs_root_rights = atoi(value);
I'd use strtol here, because otherwise "y", "yes", "true", "prettyplease",
etc. all resolve to false. or, my preference, strcmp to "1" and "0" and
whatever else you want to support and bail out with an error otherwise.
btw: a man page describing what this wrapper does and what keywords are
allowed is definitely needed.
> + }
> + else if (strcmp(key, "nice_value") == 0) {
> + /* Backwart compat with older Debian Xwrapper, ignore */
typo: backward.
> + }
> + else {
> + fprintf(stderr, "Unknown keyword '%s' at %s line %d\n", key,
> + CONFIG_FILE, line);
> + exit(1);
> + }
> + }
> + fclose(f);
> +}
> +
> +int main(int argc, char *argv[])
> +{
> + struct drm_mode_card_res res;
> + struct stat st;
> + char buf[PATH_MAX];
> + int i, r, fd;
> + int kms_cards = 0;
> + int total_cards = 0;
> +
> + parse_config();
fwiw, parse_config is simple enough and only has two outputs, so instead of
having global variables you could just return them here. Less chance of
surprise.
> +
> + /* For non root users check if they are allowed to run the X server */
> + if (getuid() != 0) {
> + switch (allowed) {
> + case ROOT_ONLY:
> + /* Already checked above */
> + fprintf(stderr, "%s: Only root is allowed to run the X server\n", argv[0]);
> + exit(1);
> + break;
> + case CONSOLE_ONLY:
> + /* Some of stdin / stdout / stderr maybe redirected to a file */
> + for (i = STDIN_FILENO; i <= STDERR_FILENO; i++) {
> + r = fstat(i, &st);
> + if (r == 0 && S_ISCHR(st.st_mode) && major(st.st_rdev) == 4)
> + break;
> + }
I honestly don't know this: what's the difference to this approach vs using
isatty() and is one preferred over the other?
> + if (i > STDERR_FILENO) {
> + fprintf(stderr, "%s: Only console users are allowed to run the X server\n", argv[0]);
> + exit(1);
> + }
please add a break statement here, no effect, but it does avoid potential confusion.
> + case ANYBODY:
> + break;
> + }
> + }
> +
> + /* Detect if we need root rights, except when overriden by the config */
> + if (needs_root_rights == -1) {
> + for (i = 0; i < 16; i++) {
> + snprintf(buf, PATH_MAX, "/dev/dri/card%d", i);
sizeof(buf) please, even if it has no effective difference here.
also, should we change this to a #define, Mark said that there are other
paths on other systems?
> + fd = open(buf, O_RDWR);
> + if (fd == -1)
> + continue;
> +
> + total_cards++;
> +
> + memset(&res, 0, sizeof(struct drm_mode_card_res));
> + r = ioctl(fd, DRM_IOCTL_MODE_GETRESOURCES, &res);
> + if (r == 0 && res.count_connectors > 0)
> + kms_cards++;
> +
> + close(fd);
> + }
> + }
> +
> + /* If we've found cards, and all cards support kms, drop root rights */
> + if (needs_root_rights == 0 || (total_cards && kms_cards == total_cards)) {
> + gid_t realgid = getgid();
> + uid_t realuid = getuid();
> +
> + if (setresgid(-1, realgid, realgid) != 0) {
> + perror("Could not drop setgid privileges");
> + exit(1);
> + }
> + if (setresuid(-1, realuid, realuid) != 0) {
> + perror("Could not drop setuid privileges");
> + exit(1);
> + }
> + }
> +
> + snprintf(buf, PATH_MAX, "%s/bin/Xorg.bin", PROJECTROOT);
> +
> + /* Check if the server is exetable by our real uid */
typo: executable
> + if (access(buf, X_OK) != 0) {
> + perror("Missing execute permissions for " PROJECTROOT "/bin/Xorg.bin");
> + exit(1);
> + }
> +
> + argv[0] = buf;
> + (void) execv(argv[0], argv);
> + perror("Failed to execute " PROJECTROOT "/bin/Xorg.bin");
> + exit(1);
Having the paths hardcoded here will likely bite us. IMO we should
just #define this in whatever config.h.in applies for this part of the
server and let it be set by autotools.
Cheers,
Peter
> +}
> --
> 1.9.0
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