x86, xsave: clear the user buffer before doing fxsave/xsave

fxsave/xsave instructions will not touch all the bytes in the
fxsave/xsave frame. Clear the user buffer before doing fxsave/xsave
directly to user buffer during the sigcontext setup.

This is essentially needed in the context of xsave(for example,
some of the fields in the xsave header are not touched by the xsave
and defined as must be zero).

This will also present uniform and clean context to the user (from
which user can safely do fxrstor/xrstor).
Signed-off-by: NSuresh Siddha <suresh.b.siddha@intel.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

arch/x86/kernel/xsave.c

@@ -92,6 +92,12 @@ int save_i387_xstate(void __user *buf)
 		return 0;
 	clear_used_math(); /* trigger finit */
 	if (task_thread_info(tsk)->status & TS_USEDFPU) {
+		/*
+	 	 * Start with clearing the user buffer. This will present a
+	 	 * clean context for the bytes not touched by the fxsave/xsave.
+		 */
+		__clear_user(buf, sig_xstate_size);
+
 		if (task_thread_info(tsk)->status & TS_XSAVE)
 			err = xsave_user(buf);
 		else