x86/fpu: Fail ptrace() requests that try to set invalid MXCSR values

There is no benefit from accepting and silently changing an invalid MXCSR
value supplied via ptrace().  Instead, return -EINVAL on invalid input.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20210623121452.613614842@linutronix.de

arch/x86/kernel/fpu/regset.c

@@ -63,8 +63,9 @@ int xfpregs_set(struct task_struct *target, const struct user_regset *regset,
 	if (ret)
 		return ret;
 
-	/* Mask invalid MXCSR bits (for historical reasons). */
-	newstate.mxcsr &= mxcsr_feature_mask;
+	/* Do not allow an invalid MXCSR value. */
+	if (newstate.mxcsr & ~mxcsr_feature_mask)
+		return -EINVAL;
 
 	fpu__prepare_write(fpu);