x86/fpu: Mask out the invalid MXCSR bits properly

This is a fix for the fix (yeah, /facepalm).

The correct mask to use is not the negation of the MXCSR_MASK but the
actual mask which contains the supported bits in the MXCSR register.

Reported and debugged by Ville Syrjälä <[email protected]>

Fixes: d298b03506 ("x86/fpu: Restore the masking out of reserved MXCSR bits")
Signed-off-by: Borislav Petkov <[email protected]>
Tested-by: Ville Syrjälä <[email protected]>
Tested-by: Ser Olmy <[email protected]>
Cc: <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

arch/x86/kernel/fpu/signal.c

@@ -385,7 +385,7 @@ static int __fpu_restore_sig(void __user *buf, void __user *buf_fx,
 				return -EINVAL;
 		} else {
 			/* Mask invalid bits out for historical reasons (broken hardware). */
-			fpu->state.fxsave.mxcsr &= ~mxcsr_feature_mask;
+			fpu->state.fxsave.mxcsr &= mxcsr_feature_mask;
 		}
 
 		/* Enforce XFEATURE_MASK_FPSSE when XSAVE is enabled */