[XEN PATCH] tools/xl: Add device_model_stubdomain_init_seclabel option to xl.cfg

Scott Davis posted 1 patch 2 years, 8 months ago
Failed in applying to current master (apply log)
There is a newer version of this series
docs/man/xl.cfg.5.pod.in             | 10 ++++++++++
tools/golang/xenlight/helpers.gen.go |  5 +++++
tools/golang/xenlight/types.gen.go   |  2 ++
tools/include/libxl.h                | 10 ++++++++++
tools/libs/light/libxl_create.c      | 28 ++++++++++++++++++++++++++--
tools/libs/light/libxl_dm.c          | 14 +++++++++-----
tools/libs/light/libxl_types.idl     |  2 ++
tools/xl/xl_parse.c                  | 12 +++++++++++-
8 files changed, 75 insertions(+), 8 deletions(-)
[XEN PATCH] tools/xl: Add device_model_stubdomain_init_seclabel option to xl.cfg
Posted by Scott Davis 2 years, 8 months ago
This adds an option to the xl domain configuration syntax for specifying
a build-time XSM security label for device-model stubdomains separate from
the run-time label specified by 'device_model_stubdomain_seclabel'. Fields
are also added to the 'libxl_domain_build_info' struct to contain the new
information, and a new call to 'xc_flask_relabel_domain' inserted to
affect the change at the appropriate time.

The implementation mirrors that of the 'seclabel' and 'init_seclabel'
options for user domains. When all used in concert, this enables the
creation of security policies that minimize run-time privileges between
the toolstack domain, device-model stubdomains, and user domains.

Signed-off-by: Scott Davis <scott.davis@starlab.io>
---
 docs/man/xl.cfg.5.pod.in             | 10 ++++++++++
 tools/golang/xenlight/helpers.gen.go |  5 +++++
 tools/golang/xenlight/types.gen.go   |  2 ++
 tools/include/libxl.h                | 10 ++++++++++
 tools/libs/light/libxl_create.c      | 28 ++++++++++++++++++++++++++--
 tools/libs/light/libxl_dm.c          | 14 +++++++++-----
 tools/libs/light/libxl_types.idl     |  2 ++
 tools/xl/xl_parse.c                  | 12 +++++++++++-
 8 files changed, 75 insertions(+), 8 deletions(-)

diff --git a/docs/man/xl.cfg.5.pod.in b/docs/man/xl.cfg.5.pod.in
index 56370a37db..3458d357fc 100644
--- a/docs/man/xl.cfg.5.pod.in
+++ b/docs/man/xl.cfg.5.pod.in
@@ -2762,6 +2762,16 @@ you have selected.
 
 Assign an XSM security label to the device-model stubdomain.
 
+=item B<device_model_stubdomain_init_seclabel="LABEL">
+
+Specify a temporary XSM security label for the device-model stubdomain used
+during creation of it and its associated guest. The stubdomain's XSM label will
+then be changed to the execution seclabel (as specified by
+B<device_model_stubdomain_seclabel>) once creation is complete, prior to
+unpausing the stubdomain's guest. With proper (re)labeling, a security policy
+can be constructed that minimizes run-time privileges between the toolstack
+domain, device-model stubdomains, and user domains.
+
 =item B<device_model_args=[ "ARG", "ARG", ...]>
 
 Pass additional arbitrary options on the device-model command
diff --git a/tools/golang/xenlight/helpers.gen.go b/tools/golang/xenlight/helpers.gen.go
index db82537b42..e961cb5f75 100644
--- a/tools/golang/xenlight/helpers.gen.go
+++ b/tools/golang/xenlight/helpers.gen.go
@@ -1022,6 +1022,8 @@ x.StubdomainRamdisk = C.GoString(xc.stubdomain_ramdisk)
 x.DeviceModel = C.GoString(xc.device_model)
 x.DeviceModelSsidref = uint32(xc.device_model_ssidref)
 x.DeviceModelSsidLabel = C.GoString(xc.device_model_ssid_label)
+x.DeviceModelExecSsidref = uint32(xc.device_model_exec_ssidref)
+x.DeviceModelExecSsidLabel = C.GoString(xc.device_model_exec_ssid_label)
 x.DeviceModelUser = C.GoString(xc.device_model_user)
 if err := x.Extra.fromC(&xc.extra);err != nil {
 return fmt.Errorf("converting field Extra: %v", err)
@@ -1351,6 +1353,9 @@ xc.device_model = C.CString(x.DeviceModel)}
 xc.device_model_ssidref = C.uint32_t(x.DeviceModelSsidref)
 if x.DeviceModelSsidLabel != "" {
 xc.device_model_ssid_label = C.CString(x.DeviceModelSsidLabel)}
+xc.device_model_exec_ssidref = C.uint32_t(x.DeviceModelExecSsidref)
+if x.DeviceModelExecSsidLabel != "" {
+xc.device_model_exec_ssid_label = C.CString(x.DeviceModelExecSsidLabel)}
 if x.DeviceModelUser != "" {
 xc.device_model_user = C.CString(x.DeviceModelUser)}
 if err := x.Extra.toC(&xc.extra); err != nil {
diff --git a/tools/golang/xenlight/types.gen.go b/tools/golang/xenlight/types.gen.go
index a214dd9df6..45061d1afa 100644
--- a/tools/golang/xenlight/types.gen.go
+++ b/tools/golang/xenlight/types.gen.go
@@ -487,6 +487,8 @@ StubdomainRamdisk string
 DeviceModel string
 DeviceModelSsidref uint32
 DeviceModelSsidLabel string
+DeviceModelExecSsidref uint32
+DeviceModelExecSsidLabel string
 DeviceModelUser string
 Extra StringList
 ExtraPv StringList
diff --git a/tools/include/libxl.h b/tools/include/libxl.h
index ae7fe27c1f..62b69222f6 100644
--- a/tools/include/libxl.h
+++ b/tools/include/libxl.h
@@ -1069,6 +1069,16 @@ typedef struct libxl__ctx libxl_ctx;
  */
 #define LIBXL_HAVE_SSID_LABEL 1
 
+/*
+ * LIBXL_HAVE_BUILDINFO_DEVICE_MODEL_STUBDOMAIN_EXEC_SSID
+ *
+ * If this is defined, then the libxl_domain_build_info structure will
+ * contain 'device_model_exec_ssidref' and 'device_model_exec_ssid_label' for
+ * specifying a run-time XSM security label separate from the build-time label
+ * specified in 'device_model_ssidref' and 'device_model_ssid_label'.
+ */
+#define LIBXL_HAVE_BUILDINFO_DEVICE_MODEL_STUBDOMAIN_EXEC_SSID 1
+
 /*
  * LIBXL_HAVE_CPUPOOL_NAME
  *
diff --git a/tools/libs/light/libxl_create.c b/tools/libs/light/libxl_create.c
index e356b2106d..a12da5531d 100644
--- a/tools/libs/light/libxl_create.c
+++ b/tools/libs/light/libxl_create.c
@@ -1060,13 +1060,31 @@ int libxl__domain_config_setdefault(libxl__gc *gc,
         char *s = d_config->b_info.device_model_ssid_label;
         ret = libxl_flask_context_to_sid(ctx, s, strlen(s),
                                          &d_config->b_info.device_model_ssidref);
+        if (ret) {
+            if (errno == ENOSYS) {
+                LOGD(WARN, domid,
+                     "XSM Disabled: device_model_stubdomain_init_seclabel not supported");
+                ret = 0;
+            } else {
+                LOGD(ERROR, domid,
+                     "Invalid device_model_stubdomain_init_seclabel: %s", s);
+                goto error_out;
+            }
+        }
+    }
+
+    if (d_config->b_info.device_model_exec_ssid_label) {
+        char *s = d_config->b_info.device_model_exec_ssid_label;
+        ret = libxl_flask_context_to_sid(ctx, s, strlen(s),
+                                         &d_config->b_info.device_model_exec_ssidref);
         if (ret) {
             if (errno == ENOSYS) {
                 LOGD(WARN, domid,
                      "XSM Disabled: device_model_stubdomain_seclabel not supported");
                 ret = 0;
             } else {
-                LOGD(ERROR, domid, "Invalid device_model_stubdomain_seclabel: %s", s);
+                LOGD(ERROR, domid,
+                     "Invalid device_model_stubdomain_seclabel: %s", s);
                 goto error_out;
             }
         }
@@ -1935,7 +1953,13 @@ static void domcreate_complete(libxl__egc *egc,
     libxl__domain_build_state_dispose(&dcs->build_state);
 
     if (!rc && d_config->b_info.exec_ssidref)
-        rc = xc_flask_relabel_domain(CTX->xch, dcs->guest_domid, d_config->b_info.exec_ssidref);
+        rc = xc_flask_relabel_domain(CTX->xch, dcs->guest_domid,
+                                     d_config->b_info.exec_ssidref);
+
+    if (!rc && dcs->sdss.pvqemu.guest_domid != INVALID_DOMID &&
+        d_config->b_info.device_model_exec_ssidref)
+        rc = xc_flask_relabel_domain(CTX->xch, dcs->sdss.pvqemu.guest_domid,
+                                     d_config->b_info.device_model_exec_ssidref);
 
     bool retain_domain = !rc || rc == ERROR_ABORTED;
 
diff --git a/tools/libs/light/libxl_dm.c b/tools/libs/light/libxl_dm.c
index dbd3c7f278..2b69b207c4 100644
--- a/tools/libs/light/libxl_dm.c
+++ b/tools/libs/light/libxl_dm.c
@@ -2300,20 +2300,24 @@ void libxl__spawn_stub_dm(libxl__egc *egc, libxl__stub_dm_spawn_state *sdss)
     sdss->pvqemu.guest_domid = INVALID_DOMID;
 
     libxl_domain_create_info_init(&dm_config->c_info);
+    libxl_domain_build_info_init(&dm_config->b_info);
+    libxl_domain_build_info_init_type(&dm_config->b_info, LIBXL_DOMAIN_TYPE_PV);
+
     dm_config->c_info.type = LIBXL_DOMAIN_TYPE_PV;
     dm_config->c_info.name = libxl__stub_dm_name(gc,
                                     libxl__domid_to_name(gc, guest_domid));
-    /* When we are here to launch stubdom, ssidref is a valid value
-     * already, no need to parse it again.
+
+    /* When we are here to launch stubdom, ssidrefs are valid values already,
+     * no need to parse them again.
      */
     dm_config->c_info.ssidref = guest_config->b_info.device_model_ssidref;
     dm_config->c_info.ssid_label = NULL;
+    dm_config->b_info.exec_ssidref =
+        guest_config->b_info.device_model_exec_ssidref;
+    dm_config->b_info.exec_ssid_label = NULL;
 
     libxl_uuid_generate(&dm_config->c_info.uuid);
 
-    libxl_domain_build_info_init(&dm_config->b_info);
-    libxl_domain_build_info_init_type(&dm_config->b_info, LIBXL_DOMAIN_TYPE_PV);
-
     dm_config->b_info.shadow_memkb = 0;
     dm_config->b_info.max_vcpus = 1;
     dm_config->b_info.max_memkb = guest_config->b_info.stubdomain_memkb;
diff --git a/tools/libs/light/libxl_types.idl b/tools/libs/light/libxl_types.idl
index f45adddab0..b483729b9c 100644
--- a/tools/libs/light/libxl_types.idl
+++ b/tools/libs/light/libxl_types.idl
@@ -528,6 +528,8 @@ libxl_domain_build_info = Struct("domain_build_info",[
     ("device_model",     string),
     ("device_model_ssidref", uint32),
     ("device_model_ssid_label", string),
+    ("device_model_exec_ssidref",    uint32),
+    ("device_model_exec_ssid_label", string),
     ("device_model_user", string),
 
     # extra parameters pass directly to qemu, NULL terminated
diff --git a/tools/xl/xl_parse.c b/tools/xl/xl_parse.c
index 9fb0791429..236f8b2fc0 100644
--- a/tools/xl/xl_parse.c
+++ b/tools/xl/xl_parse.c
@@ -2523,10 +2523,20 @@ skip_usbdev:
     xlu_cfg_get_defbool (config, "device_model_stubdomain_override",
                          &b_info->device_model_stubdomain, 0);
 
-    if (!xlu_cfg_get_string (config, "device_model_stubdomain_seclabel",
+    if (!xlu_cfg_get_string (config, "device_model_stubdomain_init_seclabel",
                              &buf, 0))
+        xlu_cfg_replace_string(config, "device_model_stubdomain_init_seclabel",
+                               &b_info->device_model_ssid_label, 0);
+
+    if (!xlu_cfg_get_string (config, "device_model_stubdomain_seclabel",
+                             &buf, 0)) {
+        if (b_info->device_model_ssid_label)
+            xlu_cfg_replace_string(config, "device_model_stubdomain_seclabel",
+                                   &b_info->device_model_exec_ssid_label, 0);
+        else
             xlu_cfg_replace_string(config, "device_model_stubdomain_seclabel",
                                    &b_info->device_model_ssid_label, 0);
+    }
 
     xlu_cfg_replace_string(config, "device_model_user",
                            &b_info->device_model_user, 0);
-- 
2.25.1


Re: [XEN PATCH] tools/xl: Add device_model_stubdomain_init_seclabel option to xl.cfg
Posted by Jason Andryuk 2 years, 8 months ago
On Fri, Jul 23, 2021 at 12:47 AM Scott Davis <scottwd@gmail.com> wrote:
>
> This adds an option to the xl domain configuration syntax for specifying
> a build-time XSM security label for device-model stubdomains separate from
> the run-time label specified by 'device_model_stubdomain_seclabel'. Fields
> are also added to the 'libxl_domain_build_info' struct to contain the new
> information, and a new call to 'xc_flask_relabel_domain' inserted to
> affect the change at the appropriate time.
>
> The implementation mirrors that of the 'seclabel' and 'init_seclabel'
> options for user domains. When all used in concert, this enables the
> creation of security policies that minimize run-time privileges between
> the toolstack domain, device-model stubdomains, and user domains.

Cool stuff!

> Signed-off-by: Scott Davis <scott.davis@starlab.io>
> ---

> @@ -1935,7 +1953,13 @@ static void domcreate_complete(libxl__egc *egc,
>      libxl__domain_build_state_dispose(&dcs->build_state);
>
>      if (!rc && d_config->b_info.exec_ssidref)
> -        rc = xc_flask_relabel_domain(CTX->xch, dcs->guest_domid, d_config->b_info.exec_ssidref);
> +        rc = xc_flask_relabel_domain(CTX->xch, dcs->guest_domid,
> +                                     d_config->b_info.exec_ssidref);
> +
> +    if (!rc && dcs->sdss.pvqemu.guest_domid != INVALID_DOMID &&
> +        d_config->b_info.device_model_exec_ssidref)
> +        rc = xc_flask_relabel_domain(CTX->xch, dcs->sdss.pvqemu.guest_domid,
> +                                     d_config->b_info.device_model_exec_ssidref);

The build/create logic is complicated, so I'm asking the question in
case you already know.  This looks like domcreate_complete runs once
and relabels both the guest domain and the stubdom.  I thought it
would get called for each of stubdom and guest, so they would be
labeled according to exec_ssidref which you set for the stubdom b_info
below.  I looked around some and it seems like domcreate_complete is
only called for the guest.

Sort of relatedly, is stubdom unpaused before the guest gets
relabeled?  Quickly looking, I think stubdom is unpaused.  I would
think you want them both relabeled before either is unpaused.  If the
stubdom starts with the exec_label, but it sees the guest with the
init_label, it may get an unexpected denial?  On the other hand,
delayed unpausing of stubdom would slow down booting.

With the stubdom getting unpaused before relabel, do you have to give
the stubdom some extra flask policy permissions to handle that case?

>      bool retain_domain = !rc || rc == ERROR_ABORTED;
>
> diff --git a/tools/libs/light/libxl_dm.c b/tools/libs/light/libxl_dm.c
> index dbd3c7f278..2b69b207c4 100644
> --- a/tools/libs/light/libxl_dm.c
> +++ b/tools/libs/light/libxl_dm.c
> @@ -2300,20 +2300,24 @@ void libxl__spawn_stub_dm(libxl__egc *egc, libxl__stub_dm_spawn_state *sdss)
>      sdss->pvqemu.guest_domid = INVALID_DOMID;
>
>      libxl_domain_create_info_init(&dm_config->c_info);
> +    libxl_domain_build_info_init(&dm_config->b_info);
> +    libxl_domain_build_info_init_type(&dm_config->b_info, LIBXL_DOMAIN_TYPE_PV);
> +

Is there a particular need for moving these lines here?

>      dm_config->c_info.type = LIBXL_DOMAIN_TYPE_PV;
>      dm_config->c_info.name = libxl__stub_dm_name(gc,
>                                      libxl__domid_to_name(gc, guest_domid));
> -    /* When we are here to launch stubdom, ssidref is a valid value
> -     * already, no need to parse it again.
> +
> +    /* When we are here to launch stubdom, ssidrefs are valid values already,
> +     * no need to parse them again.
>       */
>      dm_config->c_info.ssidref = guest_config->b_info.device_model_ssidref;
>      dm_config->c_info.ssid_label = NULL;
> +    dm_config->b_info.exec_ssidref =
> +        guest_config->b_info.device_model_exec_ssidref;
> +    dm_config->b_info.exec_ssid_label = NULL;

At first glance, it seems only these additions are strictly necessary.
But if only domcreate_complete is doing the relabel, then they are
unused?

Thanks,
Jason

Re: [XEN PATCH] tools/xl: Add device_model_stubdomain_init_seclabel option to xl.cfg
Posted by Andrew Cooper 2 years, 8 months ago
On 23/07/2021 05:47, Scott Davis wrote:
> diff --git a/tools/libs/light/libxl_create.c b/tools/libs/light/libxl_create.c
> index e356b2106d..a12da5531d 100644
> --- a/tools/libs/light/libxl_create.c
> +++ b/tools/libs/light/libxl_create.c
> @@ -1060,13 +1060,31 @@ int libxl__domain_config_setdefault(libxl__gc *gc,
>          char *s = d_config->b_info.device_model_ssid_label;
>          ret = libxl_flask_context_to_sid(ctx, s, strlen(s),
>                                           &d_config->b_info.device_model_ssidref);
> +        if (ret) {
> +            if (errno == ENOSYS) {
> +                LOGD(WARN, domid,
> +                     "XSM Disabled: device_model_stubdomain_init_seclabel not supported");
> +                ret = 0;

Surely this wants to be a hard error?

Not specifying a label is one thing, but specifying a label and having
it not take effect because code was compiled out of the hypervisor
sounds like a security hole.

I see this is a pattern copied from elsewhere, but it seems very short
signed.

~Andrew


Re: [XEN PATCH] tools/xl: Add device_model_stubdomain_init_seclabel option to xl.cfg
Posted by Ian Jackson 2 years, 8 months ago
Andrew Cooper writes ("Re: [XEN PATCH] tools/xl: Add device_model_stubdomain_init_seclabel option to xl.cfg"):
> On 23/07/2021 05:47, Scott Davis wrote:
...
> >          ret = libxl_flask_context_to_sid(ctx, s, strlen(s),
> >                                           &d_config->b_info.device_model_ssidref);
> > +        if (ret) {
> > +            if (errno == ENOSYS) {
> > +                LOGD(WARN, domid,
> > +                     "XSM Disabled: device_model_stubdomain_init_seclabel not supported");
> > +                ret = 0;
> 
> Surely this wants to be a hard error?
> 
> Not specifying a label is one thing, but specifying a label and having
> it not take effect because code was compiled out of the hypervisor
> sounds like a security hole.
> 
> I see this is a pattern copied from elsewhere, but it seems very short
> signed.

I wonder if this is to try to make it possible to boot a system whose
config specifies XSM labels but with XSM disabled.

Marek, or someone, can you advise ?

My initial thoughts are to agree with Andrew that ignoring this error
seems to me to be a bad plan, but maybe there is a good reason.

If we do want to improve this, maybe we need to update all the
corresponding call sites.

Thanks,
Ian.