[PATCH v3 08/22] x86/traps: Skip Supervisor Shadow Stack tokens in FRED mode

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

FRED doesn't use Supervisor Shadow Stack tokens.  Skip setting them up.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>

v3:
 * Move further still

v2:
 * New
---
 xen/arch/x86/mm.c    | 12 +++++++++---
 xen/arch/x86/setup.c |  8 ++++----
 2 files changed, 13 insertions(+), 7 deletions(-)

diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
index b929d15d0050..043e6aa9d73a 100644
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -129,6 +129,7 @@
 #include <asm/shadow.h>
 #include <asm/shared.h>
 #include <asm/trampoline.h>
+#include <asm/traps.h>
 #include <asm/x86_emulate.h>
 
 #include <public/memory.h>
@@ -6441,8 +6442,13 @@ static void write_sss_token(unsigned long *ptr)
 
 void memguard_guard_stack(void *p)
 {
-    /* IST Shadow stacks.  4x 1k in stack page 0. */
-    if ( IS_ENABLED(CONFIG_XEN_SHSTK) )
+    /*
+     * IST Shadow stacks.  4x 1k in stack page 0.
+     *
+     * With IDT delivery, we need Supervisor Shadow Stack tokens at the base
+     * of each stack.  With FRED delivery, these no longer exist.
+     */
+    if ( IS_ENABLED(CONFIG_XEN_SHSTK) && !opt_fred )
     {
         write_sss_token(p + (IST_MCE * IST_SHSTK_SIZE) - 8);
         write_sss_token(p + (IST_NMI * IST_SHSTK_SIZE) - 8);
@@ -6453,7 +6459,7 @@ void memguard_guard_stack(void *p)
 
     /* Primary Shadow Stack.  1x 4k in stack page 5. */
     p += PRIMARY_SHSTK_SLOT * PAGE_SIZE;
-    if ( IS_ENABLED(CONFIG_XEN_SHSTK) )
+    if ( IS_ENABLED(CONFIG_XEN_SHSTK) && !opt_fred )
         write_sss_token(p + PAGE_SIZE - 8);
 
     map_pages_to_xen((unsigned long)p, virt_to_mfn(p), 1, 
PAGE_HYPERVISOR_SHSTK);
diff --git a/xen/arch/x86/setup.c b/xen/arch/x86/setup.c
index 44da5efa1d20..160a9611f456 100644
--- a/xen/arch/x86/setup.c
+++ b/xen/arch/x86/setup.c
@@ -1923,10 +1923,6 @@ void asmlinkage __init noreturn __start_xen(void)
 
     system_state = SYS_STATE_boot;
 
-    bsp_stack = cpu_alloc_stack(0);
-    if ( !bsp_stack )
-        panic("No memory for BSP stack\n");
-
     console_init_ring();
     vesa_init();
 
@@ -2111,6 +2107,10 @@ void asmlinkage __init noreturn __start_xen(void)
 
     console_init_postirq();
 
+    bsp_stack = cpu_alloc_stack(0); /* Needs to know IDT vs FRED */
+    if ( !bsp_stack )
+        panic("No memory for BSP stack\n");
+
     system_state = SYS_STATE_smp_boot;
 
     do_presmp_initcalls();
-- 
2.39.5