x86/boot: Disable interrupts when establishing SSP

Gitlab CI reported a crash on boot on Alder Lake hardware.  The bug is years
old, making it an incredibly rare occurance:

  (XEN) *** DOUBLE FAULT ***
  (XEN) ----[ Xen-4.22-unstable  x86_64  debug=y ubsan=y  Not tainted ]----
  (XEN) CPU:    0
  (XEN) RIP:    e008:[<ffff82d04077bbc4>] arch/x86/setup.c#reinit_bsp_stack+0xfa/0x160
  (XEN) RFLAGS: 0000000000010202   CONTEXT: hypervisor
  (XEN) rax: 0000000000000007   rbx: ffff83049a4b0000   rcx: 00000000000006a2
  (XEN) rdx: 0000000000000000   rsi: 0000000000000000   rdi: 0000000000000000
  (XEN) rbp: ffff83049a4b7f00   rsp: ffff83049a4b7ef8   r8:  ffff830497e47000
  (XEN) r9:  00000000ffffffff   r10: 00000000900c2121   r11: 000000009a392956
  (XEN) r12: ffff830497e47000   r13: ffff830497e49f40   r14: 0000000000000000
  (XEN) r15: ffff82d0407dad10   cr0: 0000000080050033   cr4: 0000000000f526e0
  (XEN) cr3: 0000000043c16000   cr2: fffffffffffffffc
  (XEN) fsb: 0000000000000000   gsb: 0000000000000000   gss: 0000000000000000
  (XEN) ds: 0000   es: 0000   fs: 0000   gs: 0000   ss: 0000   cs: e008
  (XEN) Xen code around <ffff82d04077bbc4> (arch/x86/setup.c#reinit_bsp_stack+0xfa/0x160):
  (XEN)  00 b9 a2 06 00 00 0f 30 <80> 3d 71 26 f1 ff 00 74 3e 48 8d 93 f8 5f 00 00
  (XEN) Valid stack range: ffff83049a4b6000-ffff83049a4b8000, sp=ffff83049a4b7ef8, tss.rsp0=ffff83049a4b7fb0
  (XEN) No stack overflow detected. Skipping stack trace.
  (XEN)
  (XEN) ****************************************
  (XEN) Panic on CPU 0:
  (XEN) DOUBLE FAULT -- system shutdown
  (XEN) ****************************************

This is on the instruction boundary after enabling CET (writing MSR_S_CET) and
prior to establishing SSP.  Despite identifying this as a critical window
where any fault was deadly (the CPU tries to push a shadow stack frame at 0,
hence the CR2 value wrapping around to the top of the address space), I
clearly forgot that this meant interrupts too, which are enabled.

Disable interrupts during the critical period.

Fixes: b60ab42db2 ("x86/shstk: Activate Supervisor Shadow Stacks")
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Teddy Astie <teddy.astie@vates.tech>

xen/arch/x86/setup.c

@@ -908,6 +908,13 @@ static void __init noreturn reinit_bsp_stack(void)
 
     if ( cpu_has_xen_shstk )
     {
+        /*
+         * Immediately after enabling CET, SSP is 0 and most interrupts and
+         * exceptions are fatal.  Like the SYSCALL/SYSENTER gaps, IST vectors
+         * (including NMI and #MC) are safe owing to IST switching the shstk.
+         */
+        local_irq_disable();
+
         wrmsrl(MSR_S_CET, xen_msr_s_cet_value());
 
         /*
@@ -932,6 +939,8 @@ static void __init noreturn reinit_bsp_stack(void)
         }
         else
             asm volatile ( "setssbsy" ::: "memory" );
+
+        local_irq_enable();
     }
 
     reset_stack_and_jump(init_done);