[src/trunk]: src/sys/arch/x86/x86 Fix race in fpu save with fpu_kern_enter in...

[riastradh <riastradh%NetBSD.org@localhost>]

details:   https://anonhg.NetBSD.org/src/rev/c705b1d4f26b
branches:  trunk
changeset: 1011643:c705b1d4f26b
user:      riastradh <riastradh%NetBSD.org@localhost>
date:      Mon Jul 06 01:08:15 2020 +0000

description:
Fix race in fpu save with fpu_kern_enter in softint.

Likely source of:

https://mail-index.netbsd.org/current-users/2020/07/02/msg039051.html

diffstat:

 sys/arch/x86/x86/fpu.c |  34 +++++++++++++++++++++++++++++-----
 1 files changed, 29 insertions(+), 5 deletions(-)

diffs (78 lines):

diff -r 973d08ca11ad -r c705b1d4f26b sys/arch/x86/x86/fpu.c
--- a/sys/arch/x86/x86/fpu.c    Sun Jul 05 20:41:50 2020 +0000
+++ b/sys/arch/x86/x86/fpu.c    Mon Jul 06 01:08:15 2020 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: fpu.c,v 1.65 2020/06/14 16:12:05 riastradh Exp $       */
+/*     $NetBSD: fpu.c,v 1.66 2020/07/06 01:08:15 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 1.65 2020/06/14 16:12:05 riastradh Exp $");
+__KERNEL_RCSID(0, "$NetBSD: fpu.c,v 1.66 2020/07/06 01:08:15 riastradh Exp $");
 
 #include "opt_multiprocessor.h"
 
@@ -126,6 +126,8 @@
 #define stts() HYPERVISOR_fpu_taskswitch(1)
 #endif
 
+static void fpu_area_do_save(void *, uint64_t);
+
 void fpu_handle_deferred(void);
 void fpu_switch(struct lwp *, struct lwp *);
 
@@ -155,8 +157,24 @@
        kpreempt_disable();
        if (l->l_md.md_flags & MDL_FPU_IN_CPU) {
                KASSERT((l->l_flag & LW_SYSTEM) == 0);
-               fpu_area_save(area, x86_xsave_features);
+
+               /*
+                * Order is important, in case we are interrupted and
+                * the interrupt calls fpu_kern_enter, triggering
+                * reentry of fpu_save_lwp:
+                *
+                * 1. Save FPU state.
+                * 2. Note FPU state has been saved.
+                * 3. Disable FPU access so the kernel doesn't
+                *    accidentally use it.
+                *
+                * Steps (1) and (2) are both idempotent until step
+                * (3), after which point attempting to save the FPU
+                * state will trigger #NM/fpudna fault.
+                */
+               fpu_area_do_save(area, x86_xsave_features);
                l->l_md.md_flags &= ~MDL_FPU_IN_CPU;
+               stts();
        }
        kpreempt_enable();
 }
@@ -245,8 +263,8 @@
        fldummy();
 }
 
-void
-fpu_area_save(void *area, uint64_t xsave_features)
+static void
+fpu_area_do_save(void *area, uint64_t xsave_features)
 {
        switch (x86_fpu_save) {
        case FPU_SAVE_FSAVE:
@@ -262,7 +280,13 @@
                xsaveopt(area, xsave_features);
                break;
        }
+}
 
+void
+fpu_area_save(void *area, uint64_t xsave_features)
+{
+
+       fpu_area_do_save(area, xsave_features);
        stts();
 }