x86/fpu: Restrict xsaves()/xrstors() to independent states

These interfaces are really only valid for features which are independently
managed and not part of the task context state for various reasons.

Tighten the checks and adjust the misleading comments.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20211015011538.608492174@linutronix.de

arch/x86/kernel/fpu/xstate.c

@@ -1175,20 +1175,14 @@ int copy_sigframe_from_user_to_xstate(struct xregs_state *xsave,
 	return copy_uabi_to_xstate(xsave, NULL, ubuf);
 }
 
-static bool validate_xsaves_xrstors(u64 mask)
+static bool validate_independent_components(u64 mask)
 {
 	u64 xchk;
 
 	if (WARN_ON_FPU(!cpu_feature_enabled(X86_FEATURE_XSAVES)))
 		return false;
-	/*
-	 * Validate that this is either a task->fpstate related component
-	 * subset or an independent one.
-	 */
-	if (mask & xfeatures_mask_independent())
-		xchk = ~xfeatures_mask_independent();
-	else
-		xchk = ~xfeatures_mask_all;
+
+	xchk = ~xfeatures_mask_independent();
 
 	if (WARN_ON_ONCE(!mask || mask & xchk))
 		return false;
@@ -1206,14 +1200,13 @@ static bool validate_xsaves_xrstors(u64 mask)
  * buffer should be zeroed otherwise a consecutive XRSTORS from that buffer
  * can #GP.
  *
- * The feature mask must either be a subset of the independent features or
- * a subset of the task->fpstate related features.
+ * The feature mask must be a subset of the independent features.
  */
 void xsaves(struct xregs_state *xstate, u64 mask)
 {
 	int err;
 
-	if (!validate_xsaves_xrstors(mask))
+	if (!validate_independent_components(mask))
 		return;
 
 	XSTATE_OP(XSAVES, xstate, (u32)mask, (u32)(mask >> 32), err);
@@ -1231,14 +1224,13 @@ void xsaves(struct xregs_state *xstate, u64 mask)
  * Proper usage is to restore the state which was saved with
  * xsaves() into @xstate.
  *
- * The feature mask must either be a subset of the independent features or
- * a subset of the task->fpstate related features.
+ * The feature mask must be a subset of the independent features.
  */
 void xrstors(struct xregs_state *xstate, u64 mask)
 {
 	int err;
 
-	if (!validate_xsaves_xrstors(mask))
+	if (!validate_independent_components(mask))
 		return;
 
 	XSTATE_OP(XRSTORS, xstate, (u32)mask, (u32)(mask >> 32), err);