x86/cet: Remove writeable mapping of the BSPs shadow stack

An unintended consequence of the BSP using cpu0_stack[] is that writeable
mappings to the BSPs shadow stacks are retained in the bss.  This renders
CET-SS almost useless, as an attacker can update both return addresses and the
ret will not fault.

We specifically don't want to shatter the superpage mapping .data and .bss, so
the only way to fix this is to not have the BSP stack in the main Xen image.

Break cpu_alloc_stack() out of cpu_smpboot_alloc(), and dynamically allocate
the BSP stack as early as reasonable in __start_xen().  As a consequence,
there is no need to delay the BSP's memguard_guard_stack() call.

Copy the top of cpu info block just before switching to use the new stack.
Fix a latent bug by setting %rsp to info->guest_cpu_user_regs rather than
->es; this would be buggy if reinit_bsp_stack() called schedule() (which
rewrites the GPR block) directly, but luckily it doesn't.

Finally, move cpu0_stack[] into .init, so it can be reclaimed after boot.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/include/asm/smp.h

@@ -85,6 +85,8 @@ extern cpumask_t **socket_cpumask;
 extern unsigned int disabled_cpus;
 extern bool unaccounted_cpus;
 
+void *cpu_alloc_stack(unsigned int cpu);
+
 #endif /* !__ASSEMBLY__ */
 
 #endif

xen/arch/x86/setup.c

@@ -148,7 +148,7 @@ cpumask_t __read_mostly cpu_present_map;
 
 unsigned long __read_mostly xen_phys_start;
 
-char __section(".bss.stack_aligned") __aligned(STACK_SIZE)
+char __section(".init.bss.stack_aligned") __aligned(STACK_SIZE)
     cpu0_stack[STACK_SIZE];
 
 /* Used by the BSP/AP paths to find the higher half stack mapping to use. */
@@ -712,7 +712,6 @@ static void __init noreturn reinit_bsp_stack(void)
     percpu_traps_init();
 
     stack_base[0] = stack;
-    memguard_guard_stack(stack);
 
     rc = setup_cpu_root_pgt(0);
     if ( rc )
@@ -886,6 +885,8 @@ void __init noreturn __start_xen(unsigned long mbi_p)
 {
     char *memmap_type = NULL;
     char *cmdline, *kextra, *loader;
+    void *bsp_stack;
+    struct cpu_info *info = get_cpu_info(), *bsp_info;
     unsigned int initrdidx, num_parked = 0;
     multiboot_info_t *mbi;
     module_t *mod;
@@ -918,7 +919,7 @@ void __init noreturn __start_xen(unsigned long mbi_p)
     /* Full exception support from here on in. */
 
     rdmsrl(MSR_EFER, this_cpu(efer));
-    asm volatile ( "mov %%cr4,%0" : "=r" (get_cpu_info()->cr4) );
+    asm volatile ( "mov %%cr4,%0" : "=r" (info->cr4) );
 
     /* Enable NMIs.  Our loader (e.g. Tboot) may have left them disabled. */
     enable_nmis();
@@ -1703,6 +1704,10 @@ void __init noreturn __start_xen(unsigned long mbi_p)
      */
     vm_init();
 
+    bsp_stack = cpu_alloc_stack(0);
+    if ( !bsp_stack )
+        panic("No memory for BSP stack\n");
+
     console_init_ring();
     vesa_init();
 
@@ -1974,17 +1979,18 @@ void __init noreturn __start_xen(unsigned long mbi_p)
 
     if ( bsp_delay_spec_ctrl )
     {
-        struct cpu_info *info = get_cpu_info();
-
         info->spec_ctrl_flags &= ~SCF_use_shadow;
         barrier();
         wrmsrl(MSR_SPEC_CTRL, default_xen_spec_ctrl);
         info->last_spec_ctrl = default_xen_spec_ctrl;
     }
 
-    /* Jump to the 1:1 virtual mappings of cpu0_stack. */
+    /* Copy the cpu info block, and move onto the BSP stack. */
+    bsp_info = get_cpu_info_from_stack((unsigned long)bsp_stack);
+    *bsp_info = *info;
+
     asm volatile ("mov %[stk], %%rsp; jmp %c[fn]" ::
-                  [stk] "g" (__va(__pa(get_stack_bottom()))),
+                  [stk] "g" (&bsp_info->guest_cpu_user_regs),
                   [fn] "i" (reinit_bsp_stack) : "memory");
     unreachable();
 }

xen/arch/x86/smpboot.c

@@ -1023,6 +1023,23 @@ static void cpu_smpboot_free(unsigned int cpu, bool remove)
     }
 }
 
+void *cpu_alloc_stack(unsigned int cpu)
+{
+    nodeid_t node = cpu_to_node(cpu);
+    unsigned int memflags = 0;
+    void *stack;
+
+    if ( node != NUMA_NO_NODE )
+        memflags = MEMF_node(node);
+
+    stack = alloc_xenheap_pages(STACK_ORDER, memflags);
+
+    if ( stack )
+        memguard_guard_stack(stack);
+
+    return stack;
+}
+
 static int cpu_smpboot_alloc(unsigned int cpu)
 {
     struct cpu_info *info;
@@ -1035,15 +1052,10 @@ static int cpu_smpboot_alloc(unsigned int cpu)
     if ( node != NUMA_NO_NODE )
         memflags = MEMF_node(node);
 
-    if ( stack_base[cpu] == NULL )
-    {
-        stack_base[cpu] = alloc_xenheap_pages(STACK_ORDER, memflags);
-        if ( !stack_base[cpu] )
+    if ( stack_base[cpu] == NULL &&
+         (stack_base[cpu] = cpu_alloc_stack(cpu)) == NULL )
             goto out;
 
-        memguard_guard_stack(stack_base[cpu]);
-    }
-
     info = get_cpu_info_from_stack((unsigned long)stack_base[cpu]);
     info->processor_id = cpu;
     info->per_cpu_offset = __per_cpu_offset[cpu];

xen/arch/x86/xen.lds.S

@@ -235,6 +235,7 @@ SECTIONS
   } PHDR(text)
   DECL_SECTION(.init.data) {
 #endif
+       *(.init.bss.stack_aligned)
 
        . = ALIGN(POINTER_ALIGN);
        __initdata_cf_clobber_start = .;
@@ -348,7 +349,6 @@ SECTIONS
 
   DECL_SECTION(.bss) {
        __bss_start = .;
-       *(.bss.stack_aligned)
        *(.bss.page_aligned*)
        . = ALIGN(PAGE_SIZE);
        __per_cpu_start = .;