libxl: add support for running bootloader in restricted mode

Much like the device model depriv mode, add the same kind of support for the
bootloader.  Such feature allows passing a UID as a parameter for the
bootloader to run as, together with the bootloader itself taking the necessary
actions to isolate.

Note that the user to run the bootloader as must have the right permissions to
access the guest disk image (in read mode only), and that the bootloader will
be run in non-interactive mode when restricted.

If enabled bootloader restrict mode will attempt to re-use the user(s) from the
QEMU depriv implementation if no user is provided on the configuration file or
the environment.  See docs/features/qemu-deprivilege.pandoc for more
information about how to setup those users.

Bootloader restrict mode is not enabled by default as it requires certain
setup to be done first (setup of the user(s) to use in restrict mode).

This is part of XSA-443 / CVE-2023-34325

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>

docs/man/xl.1.pod.in

@@ -1963,6 +1963,30 @@ ignored:
 
 =back
 
+=head1 ENVIRONMENT VARIABLES
+
+The following environment variables shall affect the execution of xl:
+
+=over 4
+
+=item LIBXL_BOOTLOADER_RESTRICT
+
+Equivalent to L<xl.cfg(5)> B<bootloader_restrict> option.  Provided for
+compatibility reasons.  Having this variable set is equivalent to enabling
+the option, even if the value is 0.
+
+If set takes precedence over L<xl.cfg(5)> and L<xl.conf(5)>
+B<bootloader_restrict> options.
+
+=item LIBXL_BOOTLOADER_USER
+
+Equivalent to L<xl.cfg(5)> B<bootloader_user> option.  Provided for
+compatibility reasons.
+
+If set takes precedence over L<xl.cfg(5)> B<bootloader_user> option.
+
+=back
+
 =head1 SEE ALSO
 
 The following man pages:

docs/man/xl.cfg.5.pod.in

@@ -1694,6 +1694,28 @@ Append B<ARG>s to the arguments to the B<bootloader>
 program. Alternatively if the argument is a simple string then it will
 be split into words at whitespace B<(this second option is deprecated)>.
 
+=item B<bootloader_restrict=BOOLEAN>
+
+Attempt to restrict the bootloader after startup, to limit the
+consequences of security vulnerabilities due to parsing guest
+owned image files.
+
+See docs/features/qemu-deprivilege.pandoc for more information
+on how to setup the unprivileged users.
+
+Note that running the bootloader in restricted mode also implies using
+non-interactive mode, and the disk image must be readable by the
+restricted user.
+
+=item B<bootloader_user=USERNAME>
+
+When using bootloader_restrict, run the bootloader as this user.  If not
+set the default QEMU restrict users will be used.
+
+NOTE: Each domain MUST have a SEPARATE username.
+
+See docs/features/qemu-deprivilege.pandoc for more information.
+
 =item B<e820_host=BOOLEAN>
 
 Selects whether to expose the host e820 (memory map) to the guest via
@@ -2736,6 +2758,27 @@ Append B<ARG>s to the arguments to the B<bootloader>
 program. Alternatively if the argument is a simple string then it will
 be split into words at whitespace B<(this second option is deprecated)>.
 
+=item B<bootloader_restrict=BOOLEAN>
+
+Attempt to restrict the bootloader after startup, to limit the
+consequences of security vulnerabilities due to parsing guest
+owned image files.
+
+See docs/features/qemu-deprivilege.pandoc for more information
+on how to setup the unprivileged users.
+
+Note that running the bootloader in restricted mode also implies using
+non-interactive mode, and the disk image must be readable by the
+restricted user.
+
+=item B<bootloader_user=USERNAME>
+
+When using bootloader_restrict, run the bootloader as this user.
+
+NOTE: Each domain MUST have a SEPARATE username.
+
+See docs/features/qemu-deprivilege.pandoc for more information.
+
 =item B<timer_mode="MODE">
 
 Specifies the mode for Virtual Timers. The valid values are as follows:

docs/man/xl.conf.5.pod.in

@@ -220,6 +220,12 @@ Due to bug(s), these options may not interact well with other options
 concerning CPU affinity. One example is CPU pools. Users should always double
 check that the required affinity has taken effect.
 
+=item B<bootloader_restrict=BOOLEAN>
+
+System wide default for whether the bootloader should be run in a restricted
+environment.  See L<xl.cfg(5)> B<bootloader_restrict> for more information on
+how to setup and use the option.
+
 =back
 
 =head1 SEE ALSO

tools/include/libxl.h

@@ -600,6 +600,14 @@
  * first ABI incompatible change in a development branch.
  */
 
+#define LIBXL_HAVE_BOOTLOADER_RESTRICT 1
+/*
+ * LIBXL_HAVE_BOOTLOADER_RESTRICT indicates the presence of the
+ * bootloader_restrict and bootloader_user fields in libxl_domain_build_info.
+ * Such fields signal the need to pass a --runas parameter to the bootloader
+ * executable in order to not run it as the same user as libxl.
+ */
+
 /*
  * libxl memory management
  *

tools/libs/light/libxl_bootloader.c

@@ -14,6 +14,7 @@
 
 #include "libxl_osdeps.h" /* must come before any other headers */
 
+#include <pwd.h>
 #include <termios.h>
 #ifdef HAVE_UTMP_H
 #include <utmp.h>
@@ -42,8 +43,71 @@ static void bootloader_arg(libxl__bootloader_state *bl, const char *arg)
     bl->args[bl->nargs++] = arg;
 }
 
-static void make_bootloader_args(libxl__gc *gc, libxl__bootloader_state *bl,
-                                 const char *bootloader_path)
+static int bootloader_uid(libxl__gc *gc, domid_t guest_domid,
+                          const char *user, uid_t *intended_uid)
+{
+    struct passwd *user_base, user_pwbuf;
+    int rc;
+
+    if (user) {
+        rc = userlookup_helper_getpwnam(gc, user, &user_pwbuf, &user_base);
+        if (rc) return rc;
+
+        if (!user_base) {
+            LOGD(ERROR, guest_domid, "Couldn't find user %s", user);
+            return ERROR_INVAL;
+        }
+
+        *intended_uid = user_base->pw_uid;
+        return 0;
+    }
+
+    /* Re-use QEMU user range for the bootloader. */
+    rc = userlookup_helper_getpwnam(gc, LIBXL_QEMU_USER_RANGE_BASE,
+                                    &user_pwbuf, &user_base);
+    if (rc) return rc;
+
+    if (user_base) {
+        struct passwd *user_clash, user_clash_pwbuf;
+        uid_t temp_uid = user_base->pw_uid + guest_domid;
+
+        rc = userlookup_helper_getpwuid(gc, temp_uid, &user_clash_pwbuf,
+                                        &user_clash);
+        if (rc) return rc;
+
+        if (user_clash) {
+            LOGD(ERROR, guest_domid,
+                 "wanted to use uid %ld (%s + %d) but that is user %s !",
+                 (long)temp_uid, LIBXL_QEMU_USER_RANGE_BASE,
+                 guest_domid, user_clash->pw_name);
+            return ERROR_INVAL;
+        }
+
+        *intended_uid = temp_uid;
+        return 0;
+    }
+
+    rc = userlookup_helper_getpwnam(gc, LIBXL_QEMU_USER_SHARED, &user_pwbuf,
+                                    &user_base);
+    if (rc) return rc;
+
+    if (user_base) {
+        LOGD(WARN, guest_domid, "Could not find user %s, falling back to %s",
+             LIBXL_QEMU_USER_RANGE_BASE, LIBXL_QEMU_USER_SHARED);
+        *intended_uid = user_base->pw_uid;
+
+        return 0;
+    }
+
+    LOGD(ERROR, guest_domid,
+    "Could not find user %s or range base pseudo-user %s, cannot restrict",
+         LIBXL_QEMU_USER_SHARED, LIBXL_QEMU_USER_RANGE_BASE);
+
+    return ERROR_INVAL;
+}
+
+static int make_bootloader_args(libxl__gc *gc, libxl__bootloader_state *bl,
+                                const char *bootloader_path)
 {
     const libxl_domain_build_info *info = bl->info;
 
@@ -61,6 +125,22 @@ static void make_bootloader_args(libxl__gc *gc, libxl__bootloader_state *bl,
         ARG(GCSPRINTF("--ramdisk=%s", info->ramdisk));
     if (info->cmdline && *info->cmdline != '\0')
         ARG(GCSPRINTF("--args=%s", info->cmdline));
+    if (libxl_defbool_val(info->bootloader_restrict)) {
+        uid_t uid = -1;
+        int rc = bootloader_uid(gc, bl->domid, info->bootloader_user,
+                                &uid);
+
+        if (rc) return rc;
+
+        assert(uid != -1);
+        if (!uid) {
+            LOGD(ERROR, bl->domid, "bootloader restrict UID is 0 (root)!");
+            return ERROR_INVAL;
+        }
+        LOGD(DEBUG, bl->domid, "using uid %ld", (long)uid);
+        ARG(GCSPRINTF("--runas=%ld", (long)uid));
+        ARG("--quiet");
+    }
 
     ARG(GCSPRINTF("--output=%s", bl->outputpath));
     ARG("--output-format=simple0");
@@ -79,6 +159,7 @@ static void make_bootloader_args(libxl__gc *gc, libxl__bootloader_state *bl,
     /* Sentinel for execv */
     ARG(NULL);
 
+    return 0;
 #undef ARG
 }
 
@@ -443,7 +524,8 @@ static void bootloader_disk_attached_cb(libxl__egc *egc,
             bootloader = bltmp;
     }
 
-    make_bootloader_args(gc, bl, bootloader);
+    rc = make_bootloader_args(gc, bl, bootloader);
+    if (rc) goto out;
 
     bl->openpty.ao = ao;
     bl->openpty.callback = bootloader_gotptys;

tools/libs/light/libxl_create.c

@@ -482,6 +482,17 @@ int libxl__domain_build_info_setdefault(libxl__gc *gc,
         return -ERROR_INVAL;
     }
 
+    /* Assume that providing a bootloader user implies enabling restrict. */
+    libxl_defbool_setdefault(&b_info->bootloader_restrict,
+                             !!b_info->bootloader_user);
+    /* ENV takes precedence over provided domain_build_info. */
+    if (getenv("LIBXL_BOOTLOADER_RESTRICT") ||
+        getenv("LIBXL_BOOTLOADER_USER"))
+        libxl_defbool_set(&b_info->bootloader_restrict, true);
+    if(getenv("LIBXL_BOOTLOADER_USER"))
+        b_info->bootloader_user =
+            libxl__strdup(gc, getenv("LIBXL_BOOTLOADER_USER"));
+
     return 0;
 }
 

tools/libs/light/libxl_dm.c

@@ -80,10 +80,10 @@ static int libxl__create_qemu_logfile(libxl__gc *gc, char *name)
  *  On error, return a libxl-style error code.
  */
 #define DEFINE_USERLOOKUP_HELPER(NAME,SPEC_TYPE,STRUCTNAME,SYSCONF)     \
-    static int userlookup_helper_##NAME(libxl__gc *gc,                  \
-                                        SPEC_TYPE spec,                 \
-                                        struct STRUCTNAME *resultbuf,   \
-                                        struct STRUCTNAME **out)        \
+    int userlookup_helper_##NAME(libxl__gc *gc,                         \
+                                 SPEC_TYPE spec,                        \
+                                 struct STRUCTNAME *resultbuf,          \
+                                 struct STRUCTNAME **out)               \
     {                                                                   \
         struct STRUCTNAME *resultp = NULL;                              \
         char *buf = NULL;                                               \

tools/libs/light/libxl_internal.h

@@ -4874,6 +4874,14 @@ struct libxl__cpu_policy {
     struct xc_msr *msr;
 };
 
+struct passwd;
+_hidden int userlookup_helper_getpwnam(libxl__gc*, const char *user,
+                                       struct passwd *res,
+                                       struct passwd **out);
+_hidden int userlookup_helper_getpwuid(libxl__gc*, uid_t uid,
+                                       struct passwd *res,
+                                       struct passwd **out);
+
 #endif
 
 /*

tools/libs/light/libxl_types.idl

@@ -624,6 +624,8 @@ libxl_domain_build_info = Struct("domain_build_info",[
     ("acpi",             libxl_defbool),
     ("bootloader",       string),
     ("bootloader_args",  libxl_string_list),
+    ("bootloader_restrict", libxl_defbool),
+    ("bootloader_user",  string),
     ("timer_mode",       libxl_timer_mode),
     ("nested_hvm",       libxl_defbool),
     ("apic",             libxl_defbool),

tools/xl/xl.c

@@ -57,6 +57,7 @@ int max_grant_frames = -1;
 int max_maptrack_frames = -1;
 int max_grant_version = LIBXL_MAX_GRANT_DEFAULT;
 libxl_domid domid_policy = INVALID_DOMID;
+libxl_defbool bootloader_restrict;
 
 xentoollog_level minmsglevel = minmsglevel_default;
 
@@ -253,6 +254,9 @@ static void parse_global_config(const char *configfile,
             fprintf(stderr, "invalid domid_policy option");
     }
 
+    xlu_cfg_get_defbool(config, "bootloader_restrict",
+                        &bootloader_restrict, 0);
+
     xlu_cfg_destroy(config);
 }
 

tools/xl/xl.h

@@ -288,6 +288,7 @@ extern libxl_bitmap global_vm_affinity_mask;
 extern libxl_bitmap global_hvm_affinity_mask;
 extern libxl_bitmap global_pv_affinity_mask;
 extern libxl_domid domid_policy;
+extern libxl_defbool bootloader_restrict;
 
 enum output_format {
     OUTPUT_FORMAT_JSON,

tools/xl/xl_parse.c

@@ -1700,6 +1700,13 @@ void parse_config_data(const char *config_source,
         exit(-ERROR_FAIL);
     }
 #endif
+    xlu_cfg_get_defbool(config, "bootloader_restrict",
+                        &b_info->bootloader_restrict, 0);
+    if (!libxl_defbool_is_default(bootloader_restrict))
+        libxl_defbool_setdefault(&b_info->bootloader_restrict,
+                                 libxl_defbool_val(bootloader_restrict));
+    xlu_cfg_replace_string(config, "bootloader_user",
+                           &b_info->bootloader_user, 0);
 
     switch (xlu_cfg_get_list_as_string_list(config, "bootloader_args",
                                             &b_info->bootloader_args, 1)) {