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

[patchbot@xxxxxxx]

commit a7952a320c1e202a218702bfdb14f75132f04894
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Mon Oct 5 12:46:30 2020 +0100
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Fri Oct 16 11:56:59 2020 +0100

    x86/smpboot: Don't unconditionally call memguard_guard_stack() in 
cpu_smpboot_alloc()
    
    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@xxxxxxxxxxxxxxxxxxxxxx>
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 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;
--
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