x86/fpu/signal: Move header zeroing out of xsave_to_user_sigframe()

There is no reason to have the header zeroing in the pagefault disabled
region. Do it upfront once.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20210908132525.621674721@linutronix.de

arch/x86/include/asm/fpu/internal.h

@@ -318,9 +318,12 @@ static inline void os_xrstor(struct xregs_state *xstate, u64 mask)
  * We don't use modified optimization because xrstor/xrstors might track
  * a different application.
  *
- * We don't use compacted format xsave area for
- * backward compatibility for old applications which don't understand
- * compacted format of xsave area.
+ * We don't use compacted format xsave area for backward compatibility for
+ * old applications which don't understand the compacted format of the
+ * xsave area.
+ *
+ * The caller has to zero buf::header before calling this because XSAVE*
+ * does not touch the reserved fields in the header.
  */
 static inline int xsave_to_user_sigframe(struct xregs_state __user *buf)
 {
@@ -334,14 +337,6 @@ static inline int xsave_to_user_sigframe(struct xregs_state __user *buf)
 	u32 hmask = mask >> 32;
 	int err;
 
-	/*
-	 * Clear the xsave header first, so that reserved fields are
-	 * initialized to zero.
-	 */
-	err = __clear_user(&buf->header, sizeof(buf->header));
-	if (unlikely(err))
-		return -EFAULT;
-
 	stac();
 	XSTATE_OP(XSAVE, buf, lmask, hmask, err);
 	clac();

arch/x86/kernel/fpu/signal.c

@@ -189,6 +189,18 @@ int copy_fpstate_to_sigframe(void __user *buf, void __user *buf_fx, int size)
 
 	if (!access_ok(buf, size))
 		return -EACCES;
+
+	if (use_xsave()) {
+		struct xregs_state __user *xbuf = buf_fx;
+
+		/*
+		 * Clear the xsave header first, so that reserved fields are
+		 * initialized to zero.
+		 */
+		ret = __clear_user(&xbuf->header, sizeof(xbuf->header));
+		if (unlikely(ret))
+			return ret;
+	}
 retry:
 	/*
 	 * Load the FPU registers if they are not valid for the current task.