CVS commit: src/sys/arch/x86/x86

[Taylor R Campbell]

Module Name:    src
Committed By:   riastradh
Date:           Mon Jul 20 16:37:35 UTC 2020

Modified Files:
        src/sys/arch/x86/x86: fpu.c

Log Message:
Revert 1.67 "Restore the lwp's fpu state, not zeros, and leave with fpu 
enabled."

This didn't actually avoid double-restore, and it doesn't solve the
problem anyway, and made it harder to detect in-kernel fpu abuse.


To generate a diff of this commit:
cvs rdiff -u -r1.68 -r1.69 src/sys/arch/x86/x86/fpu.c

Please note that diffs are not public domain; they are subject to the
copyright notices on the relevant files.


Modified files:

Index: src/sys/arch/x86/x86/fpu.c
diff -u src/sys/arch/x86/x86/fpu.c:1.68 src/sys/arch/x86/x86/fpu.c:1.69
--- src/sys/arch/x86/x86/fpu.c:1.68	Mon Jul 13 16:51:51 2020
+++ src/sys/arch/x86/x86/fpu.c	Mon Jul 20 16:37:34 2020
@@ -1,4 +1,4 @@
-/*	$NetBSD: fpu.c,v 1.68 2020/07/13 16:51:51 riastradh Exp $	*/
+/*	$NetBSD: fpu.c,v 1.69 2020/07/20 16:37:34 riastradh Exp $	*/
 
 /*
  * Copyright (c) 2008, 2019 The NetBSD Foundation, Inc.  All
@@ -96,7 +96,7 @@
  */
 
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: fpu.c,v 1.68 2020/07/13 16:51:51 riastradh Exp $");
+__KERNEL_RCSID(0, "$NetBSD: fpu.c,v 1.69 2020/07/20 16:37:34 riastradh Exp $");
 
 #include "opt_multiprocessor.h"
 
@@ -421,9 +421,6 @@ void
 fpu_kern_leave(void)
 {
 	static const union savefpu zero_fpu __aligned(64);
-	const union savefpu *savefpu;
-	struct lwp *l = curlwp;
-	struct pcb *pcb;
 	struct cpu_info *ci = curcpu();
 	int s;
 
@@ -431,18 +428,17 @@ fpu_kern_leave(void)
 	KASSERT(ci->ci_kfpu_spl != -1);
 
 	/*
-	 * Restore the FPU state immediately to avoid leaking any
-	 * kernel secrets, or zero it if this is a kthread.
+	 * Zero the fpu registers; otherwise we might leak secrets
+	 * through Spectre-class attacks to userland, even if there are
+	 * no bugs in fpu state management.
 	 */
-	if ((l->l_pflag & LP_INTR) && (l->l_switchto != NULL))
-		l = l->l_switchto;
-	if (l->l_flag & LW_SYSTEM) {
-		savefpu = &zero_fpu;
-	} else {
-		pcb = lwp_getpcb(l);
-		savefpu = &pcb->pcb_savefpu;
-	}
-	fpu_area_restore(savefpu, x86_xsave_features);
+	fpu_area_restore(&zero_fpu, x86_xsave_features);
+
+	/*
+	 * Set CR0_TS again so that the kernel can't accidentally use
+	 * the FPU.
+	 */
+	stts();
 
 	s = ci->ci_kfpu_spl;
 	ci->ci_kfpu_spl = -1;