[src/trunk]: src/sys/arch/x86/x86 In cpu_uarea_{alloc,free}:

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

details:   https://anonhg.NetBSD.org/src/rev/cb5ab855937d
branches:  trunk
changeset: 973534:cb5ab855937d
user:      maxv <maxv%NetBSD.org@localhost>
date:      Fri Jul 03 16:17:24 2020 +0000

description:
In cpu_uarea_{alloc,free}:

 - My previous change in this file was not correct, kremove does not free
   the underlying PA, which caused a very slow leak under memory pressure.
   Rework to correctly free the PA.
 - Add a second redzone, this time after the stack, to catch several stack
   overflows. The main concern is read overflows which leak the heap that
   follows the stack.
 - UVM_KMF_WAITVA doesn't fail, so remove error check.
 - Add KASSERTs.

diffstat:

 sys/arch/x86/x86/vm_machdep.c |  53 ++++++++++++++++++++++++++++++++----------
 1 files changed, 40 insertions(+), 13 deletions(-)

diffs (86 lines):

diff -r 55c23c28c9fc -r cb5ab855937d sys/arch/x86/x86/vm_machdep.c
--- a/sys/arch/x86/x86/vm_machdep.c     Fri Jul 03 16:12:16 2020 +0000
+++ b/sys/arch/x86/x86/vm_machdep.c     Fri Jul 03 16:17:24 2020 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: vm_machdep.c,v 1.42 2020/03/17 17:18:49 maxv Exp $     */
+/*     $NetBSD: vm_machdep.c,v 1.43 2020/07/03 16:17:24 maxv Exp $     */
 
 /*-
  * Copyright (c) 1982, 1986 The Regents of the University of California.
@@ -80,7 +80,7 @@
  */
 
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: vm_machdep.c,v 1.42 2020/03/17 17:18:49 maxv Exp $");
+__KERNEL_RCSID(0, "$NetBSD: vm_machdep.c,v 1.43 2020/07/03 16:17:24 maxv Exp $");
 
 #include "opt_mtrr.h"
 
@@ -349,29 +349,56 @@
 }
 
 #ifdef __HAVE_CPU_UAREA_ROUTINES
+/*
+ * Layout of the uarea:
+ *    Page[0]        = PCB
+ *    Page[1]        = RedZone
+ *    Page[2]        = Stack
+ *    Page[...]      = Stack
+ *    Page[UPAGES-1] = Stack
+ *    Page[UPAGES]   = RedZone
+ * There is a redzone at the beginning of the stack, and another one at the
+ * end. The former is to protect against deep recursions that could corrupt
+ * the PCB, the latter to protect against severe stack overflows.
+ */
 void *
 cpu_uarea_alloc(bool system)
 {
-       vaddr_t va;
+       vaddr_t base, va;
+       paddr_t pa;
 
-       va = uvm_km_alloc(kernel_map, USPACE, 0, UVM_KMF_WIRED|UVM_KMF_WAITVA);
-       if (va == 0)
-               return NULL;
+       base = uvm_km_alloc(kernel_map, USPACE + PAGE_SIZE, 0,
+           UVM_KMF_WIRED|UVM_KMF_WAITVA);
 
-       /*
-        * The second page is unmapped, and acts as a guard page between the
-        * PCB (first page) and the stack (rest of the pages).
-        */
-       pmap_kremove(va + PAGE_SIZE, PAGE_SIZE);
+       /* Page[1] = RedZone */
+       va = base + PAGE_SIZE;
+       if (!pmap_extract(pmap_kernel(), va, &pa)) {
+               panic("%s: impossible, Page[1] unmapped", __func__);
+       }
+       pmap_kremove(va, PAGE_SIZE);
+       uvm_pagefree(PHYS_TO_VM_PAGE(pa));
+
+       /* Page[UPAGES] = RedZone */
+       va = base + USPACE;
+       if (!pmap_extract(pmap_kernel(), va, &pa)) {
+               panic("%s: impossible, Page[UPAGES] unmapped", __func__);
+       }
+       pmap_kremove(va, PAGE_SIZE);
+       uvm_pagefree(PHYS_TO_VM_PAGE(pa));
+
        pmap_update(pmap_kernel());
 
-       return (void *)va;
+       return (void *)base;
 }
 
 bool
 cpu_uarea_free(void *addr)
 {
-       uvm_km_free(kernel_map, (vaddr_t)addr, USPACE, UVM_KMF_WIRED);
+       vaddr_t base = (vaddr_t)addr;
+
+       KASSERT(!pmap_extract(pmap_kernel(), base + PAGE_SIZE, NULL));
+       KASSERT(!pmap_extract(pmap_kernel(), base + USPACE, NULL));
+       uvm_km_free(kernel_map, base, USPACE + PAGE_SIZE, UVM_KMF_WIRED);
        return true;
 }
 #endif /* __HAVE_CPU_UAREA_ROUTINES */