x86emul: also put_fpu() on error paths

fail_if() and generate_exception_if() could theoretically bypass the
normal flow reaching put_fpu(), and not invoking it would leave the
fpu_exception_callback pointer in place, allowing for the callback to
be called at an unexpected time. Luckily the two
generate_exception_if()-s that would actually trigger this are
currently commented out, so this is not (yet) a (security) issue.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/x86_emulate/x86_emulate.c
+++ b/xen/arch/x86/x86_emulate/x86_emulate.c
@@ -670,10 +670,14 @@ do{ (_fic)->exn_raised = 0;             
     rc = ops->get_fpu(fpu_handle_exception, _fic, _type, ctxt); \
     if ( rc ) goto done;                                        \
 } while (0)
-#define put_fpu(_fic)                                           \
-do{                                                             \
+#define _put_fpu()                                              \
+do {                                                            \
     if ( ops->put_fpu != NULL )                                 \
-        ops->put_fpu(ctxt);                                     \
+        (ops->put_fpu)(ctxt);                                   \
+} while (0)
+#define put_fpu(_fic)                                           \
+do {                                                            \
+    _put_fpu();                                                 \
     generate_exception_if((_fic)->exn_raised, EXC_MF, -1);      \
 } while (0)
 
@@ -3787,6 +3791,7 @@ x86_emulate(
     *ctxt->regs = _regs;
 
  done:
+    _put_fpu();
     return rc;
 
  twobyte_insn:
@@ -4632,5 +4637,6 @@ x86_emulate(
     goto writeback;
 
  cannot_emulate:
+    _put_fpu();
     return X86EMUL_UNHANDLEABLE;
 }
--- a/xen/arch/x86/x86_emulate/x86_emulate.h
+++ b/xen/arch/x86/x86_emulate/x86_emulate.h
@@ -384,7 +384,11 @@ struct x86_emulate_ops
         enum x86_emulate_fpu_type type,
         struct x86_emulate_ctxt *ctxt);
 
-    /* put_fpu: Relinquish the FPU. Unhook from FPU/SIMD exception handlers. */
+    /*
+     * put_fpu: Relinquish the FPU. Unhook from FPU/SIMD exception handlers.
+     *  The handler, if installed, must be prepared to get called without
+     *  the get_fpu one having got called before!
+     */
     void (*put_fpu)(
         struct x86_emulate_ctxt *ctxt);