[PATCH v2] x86/smpboot: Don't unconditionally call memguard_guard_stack() in cpu_smpboot_alloc()

[Andrew Cooper <andrew.cooper3@citrix.com>]

cpu_smpboot_alloc() is designed to be idempotent with respect to partially
initialised state.  This occurs for S3 and CPU parking, where enough state to
handle NMIs/#MCs needs to remain valid for the entire lifetime of Xen, even
when we otherwise want to offline the CPU.

For simplicity between various configuration, Xen always uses shadow stack
mappings (Read-only + Dirty) for the guard page, irrespective of whether
CET-SS is enabled.

Unfortunately, the CET-SS changes in memguard_guard_stack() broke idempotency
by first writing out the supervisor shadow stack tokens with plain writes,
then changing the mapping to being read-only.

This ordering is strictly necessary to configure the BSP, which cannot have
the supervisor tokens be written with WRSS.

Instead of calling memguard_guard_stack() unconditionally, call it only when
actually allocating a new stack.  Xenheap allocates are guaranteed to be
writeable, and the net result is idempotency WRT configuring stack_base[].

Fixes: 91d26ed304f ("x86/shstk: Create shadow stacks")
Reported-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
---
CC: Jan Beulich <JBeulich@suse.com>
CC: Roger Pau Monné <roger.pau@citrix.com>
CC: Wei Liu <wl@xen.org>

This can more easily be demonstrated with CPU hotplug than S3, and the absence
of bug reports goes to show how rarely hotplug is used.

v2:
 * Don't break S3/CPU parking in combination with CET-SS.  v1 would, for S3,
   turn the BSP shadow stack into regular mappings, and #DF as soon as the TLB
   shootdown completes.  For CPU Parking, it would invalidate the shadow stack
   of the parked CPUs, causing a #DF on the next NMI/#MC to hit the thread.
---
 xen/arch/x86/smpboot.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/xen/arch/x86/smpboot.c b/xen/arch/x86/smpboot.c
index 5708573c41..67e727cebd 100644
--- a/xen/arch/x86/smpboot.c
+++ b/xen/arch/x86/smpboot.c
@@ -997,16 +997,18 @@ static int cpu_smpboot_alloc(unsigned int cpu)
         memflags = MEMF_node(node);
 
     if ( stack_base[cpu] == NULL )
+    {
         stack_base[cpu] = alloc_xenheap_pages(STACK_ORDER, memflags);
-    if ( stack_base[cpu] == NULL )
-        goto out;
+        if ( !stack_base[cpu] )
+            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];
 
-    memguard_guard_stack(stack_base[cpu]);
-
     gdt = per_cpu(gdt, cpu) ?: alloc_xenheap_pages(0, memflags);
     if ( gdt == NULL )
         goto out;
-- 
2.11.0