[PATCH] x86/smpboot: Unconditionally call memguard_unguard_stack() in cpu_smpboot_free()

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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.

memguard_guard_stack() writes shadow stack tokens with plain writes.  This is
necessary to configure the BSP shadow stack correctly, and cannot be
implemented with WRSS.

Therefore, unconditionally call memguard_unguard_stack() to return the
mappings to fully writeable, so a subsequent call to memguard_guard_stack()
will succeed.

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

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.
---
 xen/arch/x86/smpboot.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/xen/arch/x86/smpboot.c b/xen/arch/x86/smpboot.c
index 5708573c41..c193cc0fb8 100644
--- a/xen/arch/x86/smpboot.c
+++ b/xen/arch/x86/smpboot.c
@@ -971,16 +971,16 @@ static void cpu_smpboot_free(unsigned int cpu, bool 
remove)
     if ( IS_ENABLED(CONFIG_PV32) )
         FREE_XENHEAP_PAGE(per_cpu(compat_gdt, cpu));
 
+    if ( stack_base[cpu] )
+        memguard_unguard_stack(stack_base[cpu]);
+
     if ( remove )
     {
         FREE_XENHEAP_PAGE(per_cpu(gdt, cpu));
         FREE_XENHEAP_PAGE(idt_tables[cpu]);
 
         if ( stack_base[cpu] )
-        {
-            memguard_unguard_stack(stack_base[cpu]);
             FREE_XENHEAP_PAGES(stack_base[cpu], STACK_ORDER);
-        }
     }
 }
 
-- 
2.11.0