xen/arm: Turn on SILO mode by default on Arm

On Arm, exclusive load-store atomics should only be used between trusted
thread. As not all the guests are trusted, it may be possible to DoS Xen
when updating shared memory with guest atomically.

Recent patches introduced new helpers to update shared memory with guest
atomically. Those helpers relies on a memory region to be be shared with
Xen and a single guest.

At the moment, nothing prevent a guest sharing a page with Xen and as
well with another guest (e.g via grant table).

For the scope of the XSA, the quickest way is to deny communications
between unprivileged guest. So this patch is enabling and using SILO
mode by default on Arm.

Users wanted finer graine policy could wrote their own Flask policy.

This is part of XSA-295.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

diff --git a/xen/arch/arm/setup.c b/xen/arch/arm/setup.c
index 6116caa7ec8392d5e8b93ad2ee9dca3507c40531..64143d34c5baa74b560bfe5f63f062fa4118021a 100644 (file)
--- a/xen/arch/arm/setup.c
+++ b/xen/arch/arm/setup.c
@@ -38,6 +38,7 @@
 #include <xen/vmap.h>
 #include <xen/libfdt/libfdt.h>
 #include <xen/acpi.h>
+#include <xen/warning.h>
 #include <asm/alternative.h>
 #include <asm/page.h>
 #include <asm/current.h>
@@ -801,8 +802,11 @@ void __init start_xen(unsigned long boot_phys_offset,
 
     tasklet_subsys_init();
 
-
-    xsm_dt_init();
+    if ( xsm_dt_init() != 1 )
+        warning_add("WARNING: SILO mode is not enabled.\n"
+                    "It has implications on the security of the system,\n"
+                    "unless the communications have been forbidden between\n"
+                    "untrusted domains.\n");
 
     init_maintenance_interrupt();
     init_timer_interrupt();

diff --git a/xen/common/Kconfig b/xen/common/Kconfig
index ac93928207c89aca6867dbcd5e1aa4053974c550..618e252351264ee5d60d637df77fb11e054a2355 100644 (file)
--- a/xen/common/Kconfig
+++ b/xen/common/Kconfig
@@ -87,7 +87,7 @@ config XENOPROF
 
 config XSM
        bool "Xen Security Modules support"
-       default n
+       default ARM
        ---help---
          Enables the security framework known as Xen Security Modules which
          allows administrators fine-grained control over a Xen domain and
@@ -152,6 +152,7 @@ config XSM_SILO
 choice
        prompt "Default XSM implementation"
        depends on XSM
+       default XSM_SILO_DEFAULT if XSM_SILO && ARM
        default XSM_FLASK_DEFAULT if XSM_FLASK
        default XSM_SILO_DEFAULT if XSM_SILO
        default XSM_DUMMY_DEFAULT

diff --git a/xen/include/xsm/xsm.h b/xen/include/xsm/xsm.h
index 900d7d5f6edc71574f208ffefa6c224a68ca6b4e..06e7c226326ebb4a582b7d63c869781e9ed866d0 100644 (file)
--- a/xen/include/xsm/xsm.h
+++ b/xen/include/xsm/xsm.h
@@ -743,6 +743,11 @@ extern int xsm_multiboot_policy_init(unsigned long *module_map,
 #endif
 
 #ifdef CONFIG_HAS_DEVICE_TREE
+/*
+ * Initialize XSM
+ *
+ * On success, return 1 if using SILO mode else 0.
+ */
 extern int xsm_dt_init(void);
 extern int xsm_dt_policy_init(void **policy_buffer, size_t *policy_size);
 extern bool has_xsm_magic(paddr_t);

diff --git a/xen/xsm/xsm_core.c b/xen/xsm/xsm_core.c
index 91ef8a70b77bf057b8ffa40d78222573ceec3418..2a2100929d12416295ce11988b3ef128964a393e 100644 (file)
--- a/xen/xsm/xsm_core.c
+++ b/xen/xsm/xsm_core.c
@@ -164,7 +164,7 @@ int __init xsm_dt_init(void)
 
     xfree(policy_buffer);
 
-    return ret;
+    return ret ?: (xsm_bootparam == XSM_BOOTPARAM_SILO);
 }
 
 /**