x86/shstk: Fix use of shadow stacks with XPTI active

The call to setup_cpu_root_pgt(0) in smp_prepare_cpus() is too early.  It
clones the BSP's stack while the .data mapping is still in use, causing all
mappings to be fully read read/write (and with no guard pages either).  This
ultimately causes #DF when trying to enter the dom0 kernel for the first time.

Defer setting up BSPs XPTI pagetable until reinit_bsp_stack() after we've set
up proper shadow stack permissions.

Fixes: 6001660 ("x86/shstk: Rework the stack layout to support shadow stacks")
Fixes: b60ab42 ("x86/shstk: Activate Supervisor Shadow Stacks")
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Release-Acked-by: Ian Jackson <iwj@xenproject.org>

xen/arch/x86/setup.c

@@ -666,6 +666,7 @@ static void noreturn init_done(void)
 static void __init noreturn reinit_bsp_stack(void)
 {
     unsigned long *stack = (void*)(get_stack_bottom() & ~(STACK_SIZE - 1));
+    int rc;
 
     /* Update TSS and ISTs */
     load_system_tables();
@@ -676,6 +677,10 @@ static void __init noreturn reinit_bsp_stack(void)
     stack_base[0] = stack;
     memguard_guard_stack(stack);
 
+    rc = setup_cpu_root_pgt(0);
+    if ( rc )
+        panic("Error %d setting up PV root page table\n", rc);
+
     if ( IS_ENABLED(CONFIG_XEN_SHSTK) && cpu_has_xen_shstk )
     {
         wrmsrl(MSR_PL0_SSP,

xen/arch/x86/smpboot.c

@@ -821,7 +821,7 @@ static root_pgentry_t common_pgt;
 
 extern const char _stextentry[], _etextentry[];
 
-static int setup_cpu_root_pgt(unsigned int cpu)
+int setup_cpu_root_pgt(unsigned int cpu)
 {
     root_pgentry_t *rpt;
     unsigned int off;
@@ -1138,8 +1138,6 @@ static struct notifier_block cpu_smpboot_nfb = {
 
 void __init smp_prepare_cpus(void)
 {
-    int rc;
-
     register_cpu_notifier(&cpu_smpboot_nfb);
 
     mtrr_aps_sync_begin();
@@ -1153,10 +1151,7 @@ void __init smp_prepare_cpus(void)
 
     stack_base[0] = (void *)((unsigned long)stack_start & ~(STACK_SIZE - 1));
 
-    rc = setup_cpu_root_pgt(0);
-    if ( rc )
-        panic("Error %d setting up PV root page table\n", rc);
-    if ( per_cpu(root_pgt, 0) )
+    if ( opt_xpti_hwdom || opt_xpti_domu )
     {
         get_cpu_info()->pv_cr3 = 0;
 

xen/include/xen/smp.h

@@ -70,5 +70,6 @@ int alloc_cpu_id(void);
 extern void *stack_base[NR_CPUS];
 
 void initialize_cpu_data(unsigned int cpu);
+int setup_cpu_root_pgt(unsigned int cpu);
 
 #endif /* __XEN_SMP_H__ */