x86/kexec: Fix kexec-reboot with CET active

The kexec_reloc() asm has an indirect jump to relocate onto the identity
trampoline.  While we clear CET in machine_crash_shutdown(), we fail to clear
CET for the non-crash path.  This in turn highlights that the same is true of
resetting the CPUID masking/faulting.

Move both pieces of logic from machine_crash_shutdown() to machine_kexec(),
the latter being common for all kexec transitions.  Adjust the condition for
CET being considered active to check in CR4, which is simpler and more robust.

Fixes: 311434bfc9 ("x86/setup: Rework MSR_S_CET handling for CET-IBT")
Fixes: b60ab42db2 ("x86/shstk: Activate Supervisor Shadow Stacks")
Fixes: 5ab9564c6f ("x86/cpu: Context switch cpuid masks and faulting state in context_switch()")
Reported-by: David Vrabel <dvrabel@amazon.co.uk>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: David Vrabel <dvrabel@amazon.co.uk>
(cherry picked from commit 7f5b2448bd)

xen/arch/x86/crash.c

@@ -197,16 +197,6 @@ void machine_crash_shutdown(void)
 
     nmi_shootdown_cpus();
 
-    /* Reset CPUID masking and faulting to the host's default. */
-    ctxt_switch_levelling(NULL);
-
-    /* Disable CET. */
-    if ( cpu_has_xen_shstk || cpu_has_xen_ibt )
-    {
-        wrmsrl(MSR_S_CET, 0);
-        write_cr4(read_cr4() & ~X86_CR4_CET);
-    }
-
     info = kexec_crash_save_info();
     info->xen_phys_start = xen_phys_start;
     info->dom0_pfn_to_mfn_frame_list_list =

xen/arch/x86/machine_kexec.c

@@ -173,6 +173,16 @@ void machine_kexec(struct kexec_image *image)
         _update_gate_addr_lower(&idt_tables[i][TRAP_machine_check], &trap_nop);
     }
 
+    /* Reset CPUID masking and faulting to the host's default. */
+    ctxt_switch_levelling(NULL);
+
+    /* Disable CET. */
+    if ( read_cr4() & X86_CR4_CET )
+    {
+        wrmsrl(MSR_S_CET, 0);
+        write_cr4(read_cr4() & ~X86_CR4_CET);
+    }
+
     /* Explicitly enable NMIs on this CPU.  Some crashdump kernels do
      * not like running with NMIs disabled. */
     enable_nmis();