x86/spec-ctrl: Defer CR4_PV32_RESTORE on the cstar_enter path

As stated (correctly) by the comment next to SPEC_CTRL_ENTRY_FROM_PV, between
the two hunks visible in the patch, RET's are not safe prior to this point.

CR4_PV32_RESTORE hides a CALL/RET pair in certain configurations (PV32
compiled in, SMEP or SMAP active), and the RET can be attacked with one of
several known speculative issues.

Furthermore, CR4_PV32_RESTORE also hides a reference to the cr4_pv32_mask
global variable, which is not safe when XPTI is active before restoring Xen's
full pagetables.

This crash has gone unnoticed because it is only AMD CPUs which permit the
SYSCALL instruction in compatibility mode, and these are not vulnerable to
Meltdown so don't activate XPTI by default.

This is XSA-429 / CVE-2022-42331

Fixes: 5e79629011 ("x86/entry: Organise the use of MSR_SPEC_CTRL at each entry/exit point")
Fixes: 5784de3e20 ("x86: Meltdown band-aid against malicious 64-bit PV guests")
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/x86_64/entry.S

@@ -288,7 +288,6 @@ ENTRY(cstar_enter)
         ALTERNATIVE "", "setssbsy", X86_FEATURE_XEN_SHSTK
 #endif
         push  %rax          /* Guest %rsp */
-        CR4_PV32_RESTORE
         movq  8(%rsp), %rax /* Restore guest %rax. */
         movq  $FLAT_USER_SS32, 8(%rsp) /* Assume a 64bit domain.  Compat handled lower. */
         pushq %r11
@@ -312,6 +311,8 @@ ENTRY(cstar_enter)
 .Lcstar_cr3_okay:
         sti
 
+        CR4_PV32_RESTORE
+
         movq  STACK_CPUINFO_FIELD(current_vcpu)(%rbx), %rbx
 
 #ifdef CONFIG_PV32