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 d23c9e82c5f71bf74e826175b389bb3a1c83f418..6207d88b658ead69d58ed85587e86df242f7daf5 100644 (file)
--- a/xen/arch/arm/setup.c
+++ b/xen/arch/arm/setup.c
@@ -37,6 +37,7 @@
 #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>
@@ -798,8 +799,11 @@ 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 c129cfbdc24e193d4475b09ed032ed7ca2ab0780..4c3bbb2de4d308dfc9a86499b71287024a5bd81e 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 ef6ff3a78653f82028a8fff12f4546979d22156c..1281302471ec61b1a6d2267a08841782f3033a05 100644 (file)
--- a/xen/include/xsm/xsm.h
+++ b/xen/include/xsm/xsm.h
@@ -719,6 +719,11 @@ 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);
 }
 
 /**