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>
(cherry picked from commit 1f762642d2cad1a40634e3280361928109d902f1)

diff --git a/docs/man/xl.1.pod.in b/docs/man/xl.1.pod.in
index 45e1430aeb7446999ea25cde8b99c1c3fb980c99..96e6fb1c32a3ce9c58b26c66dd0fb0642badda59 100644 (file)
--- a/docs/man/xl.1.pod.in
+++ b/docs/man/xl.1.pod.in
@@ -1976,6 +1976,39 @@ ignored:
 
 =back
 
+=head1 ENVIRONMENT VARIABLES
+
+The following environment variables shall affect the execution of xl:
+
+=over 4
+
+=item LIBXL_BOOTLOADER_RESTRICT
+
+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.
+
+Having this variable set is equivalent to enabling the option, even if the
+value is 0.
+
+=item LIBXL_BOOTLOADER_USER
+
+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.
+
+=back
+
 =head1 SEE ALSO
 
 The following man pages:

diff --git a/tools/libs/light/libxl_bootloader.c b/tools/libs/light/libxl_bootloader.c
index 18e9ebd7148c4e584e83074ecafe56603f4fefe7..97d9bf4ddc0aaee31b9610456dced5eb7915a6db 100644 (file)
--- a/tools/libs/light/libxl_bootloader.c
+++ b/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>
@@ -46,8 +47,71 @@ 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;
 
@@ -65,6 +129,23 @@ 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 (getenv("LIBXL_BOOTLOADER_RESTRICT") ||
+        getenv("LIBXL_BOOTLOADER_USER")) {
+        uid_t uid = -1;
+        int rc = bootloader_uid(gc, bl->domid, getenv("LIBXL_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");
@@ -83,6 +164,7 @@ static void make_bootloader_args(libxl__gc *gc, libxl__bootloader_state *bl,
     /* Sentinel for execv */
     ARG(NULL);
 
+    return 0;
 #undef ARG
 }
 
@@ -447,7 +529,8 @@ 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;

diff --git a/tools/libs/light/libxl_dm.c b/tools/libs/light/libxl_dm.c
index b86e8ccc858f4e01c219a642e947be005264eff5..59de5c1ae22f27cd2a94517360bc063e83750b23 100644 (file)
--- a/tools/libs/light/libxl_dm.c
+++ b/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;                                               \

diff --git a/tools/libs/light/libxl_internal.h b/tools/libs/light/libxl_internal.h
index cc27c72ecf305d462004f20e388ac0d399bb6044..8415d1feed163ebe47f19e447019072fb2790d18 100644 (file)
--- a/tools/libs/light/libxl_internal.h
+++ b/tools/libs/light/libxl_internal.h
@@ -4864,6 +4864,14 @@ 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
 
 /*