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[Xen-devel] [PATCH 17/22] xl, libxl: Provide dm_restrict

This functionality is still quite imperfect, but it will be useful in
certain restricted use cases.

Signed-off-by: Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>
 docs/man/xl.cfg.pod.5.in    | 86 +++++++++++++++++++++++++++++++++++++++++++++
 tools/libxl/libxl_create.c  |  1 +
 tools/libxl/libxl_dm.c      |  9 +++++
 tools/libxl/libxl_types.idl |  1 +
 tools/xl/xl_parse.c         |  3 ++
 5 files changed, 100 insertions(+)

diff --git a/docs/man/xl.cfg.pod.5.in b/docs/man/xl.cfg.pod.5.in
index 79cb2ea..e3a73bc 100644
--- a/docs/man/xl.cfg.pod.5.in
+++ b/docs/man/xl.cfg.pod.5.in
@@ -2045,6 +2045,92 @@ specified, enabling the use of XenServer PV drivers in 
the guest.
 This parameter only takes effect when device_model_version=qemu-xen.
 See B<xen-pci-device-reservations(7)> for more information.
+=item B<dm_restrict=BOOLEAN>
+Restrict the HVM device model after startup,
+to limit the consequencese of security vulnerabilities in qemu.
+With this feature enabled,
+a compromise of the device model,
+via such a vulnerability,
+will not provide a privilege escalation attack on the whole system.
+This feature is a B<technology preview>.
+There are some significant limitations:
+=over 4
+You must have a new enough qemu.
+In particular,
+if your qemu does not have the commit
+B<xen: restrict: use xentoolcore_restrict_all>
+the restriction request will be silently ineffective!
+The mechanisms used are not effective against
+denial of service problems.
+A compromised qemu can probably still impair
+or perhaps even prevent
+the proper functioning of the whole system,
+(at the very least, but not limited to,
+through resource exhaustion).
+It is not known whether the protection is
+effective when a domain is migrated.
+Some domain management functions do not work.
+For example, cdrom insert will fail.
+You must create user(s) for qemu to run as.
+Currently, you should either create
+for every $domid from 1 to 32751 inclusive,
+(in which case different guests will not
+be protected against each other).
+And if you do not create the user,
+the restriction request will be silently ineffective!
+There are no countermeasures taken against reuse
+of the same unix user (uid)
+for subsequent domains,
+even if the B<xen-qemuuser-domid$domid> users are created.
+So a past domain with the same domid may be able to
+interferer with future domains.
+Possibly, even after a reboot.
+A compromised qemu will be able to read world-readable
+files in the dom0 operating system.
+Because of these limitations, this functionality,
+while it may enhance your security,
+should not be relied on.
+Any further limitations discovered in the current version
+will B<not> be handled via the Xen Project Security Process.
+In the future as we enhance this feature to improve the security,
+we may break backward compatibility.
 =head2 Device-Model Options
diff --git a/tools/libxl/libxl_create.c b/tools/libxl/libxl_create.c
index 9123585..c757651 100644
--- a/tools/libxl/libxl_create.c
+++ b/tools/libxl/libxl_create.c
@@ -322,6 +322,7 @@ int libxl__domain_build_info_setdefault(libxl__gc *gc,
         libxl_defbool_setdefault(&b_info->u.hvm.altp2m,             false);
         libxl_defbool_setdefault(&b_info->u.hvm.usb,                false);
         libxl_defbool_setdefault(&b_info->u.hvm.xen_platform_pci,   true);
+        libxl_defbool_setdefault(&b_info->u.hvm.dm_restrict,        false);
         libxl_defbool_setdefault(&b_info->u.hvm.spice.enable, false);
         if (!libxl_defbool_val(b_info->u.hvm.spice.enable) &&
diff --git a/tools/libxl/libxl_dm.c b/tools/libxl/libxl_dm.c
index e0e6a99..472a42b 100644
--- a/tools/libxl/libxl_dm.c
+++ b/tools/libxl/libxl_dm.c
@@ -641,6 +641,12 @@ static int libxl__build_device_model_args_old(libxl__gc 
             flexarray_append(dm_args, "-nographic");
+    if (libxl_defbool_val(b_info->u.hvm.dm_restrict)) {
+        LOGD(ERROR, domid,
+             "dm_restrict not supported by qemu-xen-traditional");
+        return ERROR_INVAL;
+    }
     if (state->saved_state) {
         flexarray_vappend(dm_args, "-loadvm", state->saved_state, NULL);
@@ -1396,6 +1402,9 @@ static int libxl__build_device_model_args_new(libxl__gc 
+    if (libxl_defbool_val(b_info->u.hvm.dm_restrict))
+        flexarray_append(dm_args, "-xen-domid-restrict");
     if (state->saved_state) {
         /* This file descriptor is meant to be used by QEMU */
         *dm_state_fd = open(state->saved_state, O_RDONLY);
diff --git a/tools/libxl/libxl_types.idl b/tools/libxl/libxl_types.idl
index 173d70a..fc39b32 100644
--- a/tools/libxl/libxl_types.idl
+++ b/tools/libxl/libxl_types.idl
@@ -565,6 +565,7 @@ libxl_domain_build_info = Struct("domain_build_info",[
                                        ("rdm", libxl_rdm_reserve),
                                        ("rdm_mem_boundary_memkb", MemKB),
                                        ("mca_caps",         uint64),
+                                       ("dm_restrict",      libxl_defbool),
                  ("pv", Struct(None, [("kernel", string),
                                       ("slack_memkb", MemKB),
diff --git a/tools/xl/xl_parse.c b/tools/xl/xl_parse.c
index 02ddd2e..2c86527 100644
--- a/tools/xl/xl_parse.c
+++ b/tools/xl/xl_parse.c
@@ -2085,6 +2085,9 @@ skip_usbdev:
             b_info->u.hvm.vendor_device = d;
+        xlu_cfg_get_defbool(config, "dm_restrict",
+                            &b_info->u.hvm.dm_restrict, 0);
     if (!xlu_cfg_get_string (config, "gic_version", &buf, 1)) {

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