[Xen-changelog] [xen stable-4.8] xen/arm: Turn on SILO mode by default on Arm

[patchbot@xxxxxxx]

commit 7feb3ccccab3db1da12bb14be4eecbfe2bfffbb2
Author:     Julien Grall <julien.grall@xxxxxxx>
AuthorDate: Mon Apr 29 15:05:25 2019 +0100
Commit:     Julien Grall <julien.grall@xxxxxxx>
CommitDate: Fri Jun 14 15:46:00 2019 +0100

    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@xxxxxxx>
    Acked-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 xen/arch/arm/setup.c  | 8 ++++++--
 xen/common/Kconfig    | 3 ++-
 xen/include/xsm/xsm.h | 5 +++++
 xen/xsm/xsm_core.c    | 2 +-
 4 files changed, 14 insertions(+), 4 deletions(-)

diff --git a/xen/arch/arm/setup.c b/xen/arch/arm/setup.c
index 6116caa7ec..64143d34c5 100644
--- 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 ac93928207..618e252351 100644
--- 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 900d7d5f6e..06e7c22632 100644
--- 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 91ef8a70b7..2a2100929d 100644
--- 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);
 }
 
 /**
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.8

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