[02/17] prmem: write rare for static allocation
diff mbox series

Message ID 20181023213504.28905-3-igor.stoppa@huawei.com
State New
Headers show
Series
  • prmem: protected memory
Related show

Commit Message

Igor Stoppa Oct. 23, 2018, 9:34 p.m. UTC
Implementation of write rare for statically allocated data, located in a
specific memory section through the use of the __write_rare label.

The basic functions are wr_memcpy() and wr_memset(): the write rare
counterparts of memcpy() and memset() respectively.

To minimize chances of attacks, this implementation does not unprotect
existing memory pages.
Instead, it remaps them, one by one, at random free locations, as writable.
Each page is mapped as writable strictly for the time needed to perform
changes in said page.
While a page is remapped, interrupts are disabled on the core performing
the write rare operation, to avoid being frozen mid-air by an attack
using interrupts for stretching the duration of the alternate mapping.
OTOH, to avoid introducing unpredictable delays, the interrupts are
re-enabled inbetween page remapping, when write operations are either
completed or not yet started, and there is not alternate, writable
mapping to exploit.

Signed-off-by: Igor Stoppa <igor.stoppa@huawei.com>
CC: Michal Hocko <mhocko@kernel.org>
CC: Vlastimil Babka <vbabka@suse.cz>
CC: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
CC: Andrew Morton <akpm@linux-foundation.org>
CC: Pavel Tatashin <pasha.tatashin@oracle.com>
CC: linux-mm@kvack.org
CC: linux-kernel@vger.kernel.org
---
 MAINTAINERS           |   7 ++
 include/linux/prmem.h | 213 ++++++++++++++++++++++++++++++++++++++++++
 mm/Makefile           |   1 +
 mm/prmem.c            |  10 ++
 4 files changed, 231 insertions(+)
 create mode 100644 include/linux/prmem.h
 create mode 100644 mm/prmem.c

Comments

Dave Hansen Oct. 25, 2018, 12:24 a.m. UTC | #1
> +static __always_inline bool __is_wr_after_init(const void *ptr, size_t size)
> +{
> +	size_t start = (size_t)&__start_wr_after_init;
> +	size_t end = (size_t)&__end_wr_after_init;
> +	size_t low = (size_t)ptr;
> +	size_t high = (size_t)ptr + size;
> +
> +	return likely(start <= low && low < high && high <= end);
> +}

size_t is an odd type choice for doing address arithmetic.

> +/**
> + * wr_memset() - sets n bytes of the destination to the c value
> + * @dst: beginning of the memory to write to
> + * @c: byte to replicate
> + * @size: amount of bytes to copy
> + *
> + * Returns true on success, false otherwise.
> + */
> +static __always_inline
> +bool wr_memset(const void *dst, const int c, size_t n_bytes)
> +{
> +	size_t size;
> +	unsigned long flags;
> +	uintptr_t d = (uintptr_t)dst;
> +
> +	if (WARN(!__is_wr_after_init(dst, n_bytes), WR_ERR_RANGE_MSG))
> +		return false;
> +	while (n_bytes) {
> +		struct page *page;
> +		uintptr_t base;
> +		uintptr_t offset;
> +		uintptr_t offset_complement;

Again, these are really odd choices for types.  vmap() returns a void*
pointer, on which you can do arithmetic.  Why bother keeping another
type to which you have to cast to and from?

BTW, our usual "pointer stored in an integer type" is 'unsigned long',
if a pointer needs to be manipulated.

> +		local_irq_save(flags);

Why are you doing the local_irq_save()?

> +		page = virt_to_page(d);
> +		offset = d & ~PAGE_MASK;
> +		offset_complement = PAGE_SIZE - offset;
> +		size = min(n_bytes, offset_complement);
> +		base = (uintptr_t)vmap(&page, 1, VM_MAP, PAGE_KERNEL);

Can you even call vmap() (which sleeps) with interrupts off?

> +		if (WARN(!base, WR_ERR_PAGE_MSG)) {
> +			local_irq_restore(flags);
> +			return false;
> +		}

You really need some kmap_atomic()-style accessors to wrap this stuff
for you.  This little pattern is repeated over and over.

...
> +const char WR_ERR_RANGE_MSG[] = "Write rare on invalid memory range.";
> +const char WR_ERR_PAGE_MSG[] = "Failed to remap write rare page.";

Doesn't the compiler de-duplicate duplicated strings for you?  Is there
any reason to declare these like this?
Peter Zijlstra Oct. 26, 2018, 9:41 a.m. UTC | #2
On Wed, Oct 24, 2018 at 12:34:49AM +0300, Igor Stoppa wrote:
> +static __always_inline

That's far too large for inline.

> +bool wr_memset(const void *dst, const int c, size_t n_bytes)
> +{
> +	size_t size;
> +	unsigned long flags;
> +	uintptr_t d = (uintptr_t)dst;
> +
> +	if (WARN(!__is_wr_after_init(dst, n_bytes), WR_ERR_RANGE_MSG))
> +		return false;
> +	while (n_bytes) {
> +		struct page *page;
> +		uintptr_t base;
> +		uintptr_t offset;
> +		uintptr_t offset_complement;
> +
> +		local_irq_save(flags);
> +		page = virt_to_page(d);
> +		offset = d & ~PAGE_MASK;
> +		offset_complement = PAGE_SIZE - offset;
> +		size = min(n_bytes, offset_complement);
> +		base = (uintptr_t)vmap(&page, 1, VM_MAP, PAGE_KERNEL);
> +		if (WARN(!base, WR_ERR_PAGE_MSG)) {
> +			local_irq_restore(flags);
> +			return false;
> +		}
> +		memset((void *)(base + offset), c, size);
> +		vunmap((void *)base);

BUG

> +		d += size;
> +		n_bytes -= size;
> +		local_irq_restore(flags);
> +	}
> +	return true;
> +}
> +
> +static __always_inline

Similarly large

> +bool wr_memcpy(const void *dst, const void *src, size_t n_bytes)
> +{
> +	size_t size;
> +	unsigned long flags;
> +	uintptr_t d = (uintptr_t)dst;
> +	uintptr_t s = (uintptr_t)src;
> +
> +	if (WARN(!__is_wr_after_init(dst, n_bytes), WR_ERR_RANGE_MSG))
> +		return false;
> +	while (n_bytes) {
> +		struct page *page;
> +		uintptr_t base;
> +		uintptr_t offset;
> +		uintptr_t offset_complement;
> +
> +		local_irq_save(flags);
> +		page = virt_to_page(d);
> +		offset = d & ~PAGE_MASK;
> +		offset_complement = PAGE_SIZE - offset;
> +		size = (size_t)min(n_bytes, offset_complement);
> +		base = (uintptr_t)vmap(&page, 1, VM_MAP, PAGE_KERNEL);
> +		if (WARN(!base, WR_ERR_PAGE_MSG)) {
> +			local_irq_restore(flags);
> +			return false;
> +		}
> +		__write_once_size((void *)(base + offset), (void *)s, size);
> +		vunmap((void *)base);

Similarly BUG.

> +		d += size;
> +		s += size;
> +		n_bytes -= size;
> +		local_irq_restore(flags);
> +	}
> +	return true;
> +}

> +static __always_inline

Guess what..

> +uintptr_t __wr_rcu_ptr(const void *dst_p_p, const void *src_p)
> +{
> +	unsigned long flags;
> +	struct page *page;
> +	void *base;
> +	uintptr_t offset;
> +	const size_t size = sizeof(void *);
> +
> +	if (WARN(!__is_wr_after_init(dst_p_p, size), WR_ERR_RANGE_MSG))
> +		return (uintptr_t)NULL;
> +	local_irq_save(flags);
> +	page = virt_to_page(dst_p_p);
> +	offset = (uintptr_t)dst_p_p & ~PAGE_MASK;
> +	base = vmap(&page, 1, VM_MAP, PAGE_KERNEL);
> +	if (WARN(!base, WR_ERR_PAGE_MSG)) {
> +		local_irq_restore(flags);
> +		return (uintptr_t)NULL;
> +	}
> +	rcu_assign_pointer((*(void **)(offset + (uintptr_t)base)), src_p);
> +	vunmap(base);

Also still bug.

> +	local_irq_restore(flags);
> +	return (uintptr_t)src_p;
> +}

Also, I see an amount of duplication here that shows you're not nearly
lazy enough.
Igor Stoppa Oct. 29, 2018, 6:03 p.m. UTC | #3
On 25/10/2018 01:24, Dave Hansen wrote:
>> +static __always_inline bool __is_wr_after_init(const void *ptr, size_t size)
>> +{
>> +	size_t start = (size_t)&__start_wr_after_init;
>> +	size_t end = (size_t)&__end_wr_after_init;
>> +	size_t low = (size_t)ptr;
>> +	size_t high = (size_t)ptr + size;
>> +
>> +	return likely(start <= low && low < high && high <= end);
>> +}
> 
> size_t is an odd type choice for doing address arithmetic.

it seemed more portable than unsigned long

>> +/**
>> + * wr_memset() - sets n bytes of the destination to the c value
>> + * @dst: beginning of the memory to write to
>> + * @c: byte to replicate
>> + * @size: amount of bytes to copy
>> + *
>> + * Returns true on success, false otherwise.
>> + */
>> +static __always_inline
>> +bool wr_memset(const void *dst, const int c, size_t n_bytes)
>> +{
>> +	size_t size;
>> +	unsigned long flags;
>> +	uintptr_t d = (uintptr_t)dst;
>> +
>> +	if (WARN(!__is_wr_after_init(dst, n_bytes), WR_ERR_RANGE_MSG))
>> +		return false;
>> +	while (n_bytes) {
>> +		struct page *page;
>> +		uintptr_t base;
>> +		uintptr_t offset;
>> +		uintptr_t offset_complement;
> 
> Again, these are really odd choices for types.  vmap() returns a void*
> pointer, on which you can do arithmetic.  

I wasn't sure of how much I could rely on the compiler not doing some 
unwanted optimizations.

> Why bother keeping another
> type to which you have to cast to and from?

For the above reason. If I'm worrying unnecessarily, I can switch back 
to void *
It certainly is easier to use.

> BTW, our usual "pointer stored in an integer type" is 'unsigned long',
> if a pointer needs to be manipulated.

yes, I noticed that, but it seemed strange ...
size_t corresponds to unsigned long, afaik

but it seems that I have not fully understood where to use it

anyway, I can stick to the convention with unsigned long

> 
>> +		local_irq_save(flags);
> 
> Why are you doing the local_irq_save()?

The idea was to avoid the case where an attack would somehow freeze the 
core doing the write-rare operation, while the temporary mapping is 
accessible.

I have seen comments about using mappings that are private to the 
current core (and I will reply to those comments as well), but this 
approach seems architecture-dependent, while I was looking for a 
solution that, albeit not 100% reliable, would work on any system with 
an MMU. This would not prevent each arch to come up with own custom 
implementation that provides better coverage, performance, etc.

>> +		page = virt_to_page(d);
>> +		offset = d & ~PAGE_MASK;
>> +		offset_complement = PAGE_SIZE - offset;
>> +		size = min(n_bytes, offset_complement);
>> +		base = (uintptr_t)vmap(&page, 1, VM_MAP, PAGE_KERNEL);
> 
> Can you even call vmap() (which sleeps) with interrupts off?

I accidentally disabled sleeping while atomic debugging and I totally 
missed this problem :-(

However, to answer your question, nothing exploded while I was testing 
(without that type of debugging).

I suspect I was just "lucky". Or maybe I was simply not triggering the 
sleeping sub-case.

As I understood the code, sleeping _might_ happen, but it's not going to 
happen systematically.

I wonder if I could split vmap() into two parts: first the sleeping one, 
with interrupts enabled, then the non sleeping one, with interrupts 
disabled.

I need to read the code more carefully, but it seems that sleeping might 
happen when memory for the mapping meta data is not immediately available.

BTW, wouldn't the might_sleep() call belong more to the part which 
really sleeps, instead than to the whole vmap() ?

>> +		if (WARN(!base, WR_ERR_PAGE_MSG)) {
>> +			local_irq_restore(flags);
>> +			return false;
>> +		}
> 
> You really need some kmap_atomic()-style accessors to wrap this stuff
> for you.  This little pattern is repeated over and over.

I really need to learn more about the way the kernel works and is 
structured. It's a work in progress. Thanks for the advice.

> ...
>> +const char WR_ERR_RANGE_MSG[] = "Write rare on invalid memory range.";
>> +const char WR_ERR_PAGE_MSG[] = "Failed to remap write rare page.";
> 
> Doesn't the compiler de-duplicate duplicated strings for you?  Is there
> any reason to declare these like this?

I noticed I have made some accidental modifications in a couple of 
cases, when replicating the command.

So I thought that if I really want to use the same string, why not doing 
it explicitly? It seemed also easier, in case I want to tweak the 
message. I need to do it only in one place.

--
igor
Igor Stoppa Oct. 29, 2018, 8:01 p.m. UTC | #4
On 26/10/2018 10:41, Peter Zijlstra wrote:
> On Wed, Oct 24, 2018 at 12:34:49AM +0300, Igor Stoppa wrote:
>> +static __always_inline
> 
> That's far too large for inline.

The reason for it is that it's supposed to minimize the presence of 
gadgets that might be used in JOP attacks.
I am ready to stand corrected, if I'm wrong, but this is the reason why 
I did it.

Regarding the function being too large, yes, I would not normally choose 
it for inlining.

Actually, I would not normally use "__always_inline" and instead I would 
limit myself to plain "inline", at most.

> 
>> +bool wr_memset(const void *dst, const int c, size_t n_bytes)
>> +{
>> +	size_t size;
>> +	unsigned long flags;
>> +	uintptr_t d = (uintptr_t)dst;
>> +
>> +	if (WARN(!__is_wr_after_init(dst, n_bytes), WR_ERR_RANGE_MSG))
>> +		return false;
>> +	while (n_bytes) {
>> +		struct page *page;
>> +		uintptr_t base;
>> +		uintptr_t offset;
>> +		uintptr_t offset_complement;
>> +
>> +		local_irq_save(flags);
>> +		page = virt_to_page(d);
>> +		offset = d & ~PAGE_MASK;
>> +		offset_complement = PAGE_SIZE - offset;
>> +		size = min(n_bytes, offset_complement);
>> +		base = (uintptr_t)vmap(&page, 1, VM_MAP, PAGE_KERNEL);
>> +		if (WARN(!base, WR_ERR_PAGE_MSG)) {
>> +			local_irq_restore(flags);
>> +			return false;
>> +		}
>> +		memset((void *)(base + offset), c, size);
>> +		vunmap((void *)base);
> 
> BUG

yes, somehow I managed to drop this debug configuration from the debug 
builds I made.

[...]

> Also, I see an amount of duplication here that shows you're not nearly
> lazy enough.

I did notice a certain amount of duplication, but I didn't know how to 
exploit it.

--
igor

Patch
diff mbox series

diff --git a/MAINTAINERS b/MAINTAINERS
index b2f710eee67a..e566c5d09faf 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -9454,6 +9454,13 @@  F:	kernel/sched/membarrier.c
 F:	include/uapi/linux/membarrier.h
 F:	arch/powerpc/include/asm/membarrier.h
 
+MEMORY HARDENING
+M:	Igor Stoppa <igor.stoppa@gmail.com>
+L:	kernel-hardening@lists.openwall.com
+S:	Maintained
+F:	include/linux/prmem.h
+F:	mm/prmem.c
+
 MEMORY MANAGEMENT
 L:	linux-mm@kvack.org
 W:	http://www.linux-mm.org
diff --git a/include/linux/prmem.h b/include/linux/prmem.h
new file mode 100644
index 000000000000..3ba41d76a582
--- /dev/null
+++ b/include/linux/prmem.h
@@ -0,0 +1,213 @@ 
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * prmem.h: Header for memory protection library
+ *
+ * (C) Copyright 2018 Huawei Technologies Co. Ltd.
+ * Author: Igor Stoppa <igor.stoppa@huawei.com>
+ *
+ * Support for:
+ * - statically allocated write rare data
+ */
+
+#ifndef _LINUX_PRMEM_H
+#define _LINUX_PRMEM_H
+
+#include <linux/set_memory.h>
+#include <linux/mm.h>
+#include <linux/vmalloc.h>
+#include <linux/string.h>
+#include <linux/slab.h>
+#include <linux/mutex.h>
+#include <linux/compiler.h>
+#include <linux/irqflags.h>
+#include <linux/set_memory.h>
+
+/* ============================ Write Rare ============================ */
+
+extern const char WR_ERR_RANGE_MSG[];
+extern const char WR_ERR_PAGE_MSG[];
+
+/*
+ * The following two variables are statically allocated by the linker
+ * script at the the boundaries of the memory region (rounded up to
+ * multiples of PAGE_SIZE) reserved for __wr_after_init.
+ */
+extern long __start_wr_after_init;
+extern long __end_wr_after_init;
+
+static __always_inline bool __is_wr_after_init(const void *ptr, size_t size)
+{
+	size_t start = (size_t)&__start_wr_after_init;
+	size_t end = (size_t)&__end_wr_after_init;
+	size_t low = (size_t)ptr;
+	size_t high = (size_t)ptr + size;
+
+	return likely(start <= low && low < high && high <= end);
+}
+
+/**
+ * wr_memset() - sets n bytes of the destination to the c value
+ * @dst: beginning of the memory to write to
+ * @c: byte to replicate
+ * @size: amount of bytes to copy
+ *
+ * Returns true on success, false otherwise.
+ */
+static __always_inline
+bool wr_memset(const void *dst, const int c, size_t n_bytes)
+{
+	size_t size;
+	unsigned long flags;
+	uintptr_t d = (uintptr_t)dst;
+
+	if (WARN(!__is_wr_after_init(dst, n_bytes), WR_ERR_RANGE_MSG))
+		return false;
+	while (n_bytes) {
+		struct page *page;
+		uintptr_t base;
+		uintptr_t offset;
+		uintptr_t offset_complement;
+
+		local_irq_save(flags);
+		page = virt_to_page(d);
+		offset = d & ~PAGE_MASK;
+		offset_complement = PAGE_SIZE - offset;
+		size = min(n_bytes, offset_complement);
+		base = (uintptr_t)vmap(&page, 1, VM_MAP, PAGE_KERNEL);
+		if (WARN(!base, WR_ERR_PAGE_MSG)) {
+			local_irq_restore(flags);
+			return false;
+		}
+		memset((void *)(base + offset), c, size);
+		vunmap((void *)base);
+		d += size;
+		n_bytes -= size;
+		local_irq_restore(flags);
+	}
+	return true;
+}
+
+/**
+ * wr_memcpy() - copyes n bytes from source to destination
+ * @dst: beginning of the memory to write to
+ * @src: beginning of the memory to read from
+ * @n_bytes: amount of bytes to copy
+ *
+ * Returns true on success, false otherwise.
+ */
+static __always_inline
+bool wr_memcpy(const void *dst, const void *src, size_t n_bytes)
+{
+	size_t size;
+	unsigned long flags;
+	uintptr_t d = (uintptr_t)dst;
+	uintptr_t s = (uintptr_t)src;
+
+	if (WARN(!__is_wr_after_init(dst, n_bytes), WR_ERR_RANGE_MSG))
+		return false;
+	while (n_bytes) {
+		struct page *page;
+		uintptr_t base;
+		uintptr_t offset;
+		uintptr_t offset_complement;
+
+		local_irq_save(flags);
+		page = virt_to_page(d);
+		offset = d & ~PAGE_MASK;
+		offset_complement = PAGE_SIZE - offset;
+		size = (size_t)min(n_bytes, offset_complement);
+		base = (uintptr_t)vmap(&page, 1, VM_MAP, PAGE_KERNEL);
+		if (WARN(!base, WR_ERR_PAGE_MSG)) {
+			local_irq_restore(flags);
+			return false;
+		}
+		__write_once_size((void *)(base + offset), (void *)s, size);
+		vunmap((void *)base);
+		d += size;
+		s += size;
+		n_bytes -= size;
+		local_irq_restore(flags);
+	}
+	return true;
+}
+
+/*
+ * rcu_assign_pointer is a macro, which takes advantage of being able to
+ * take the address of the destination parameter "p", so that it can be
+ * passed to WRITE_ONCE(), which is called in one of the branches of
+ * rcu_assign_pointer() and also, being a macro, can rely on the
+ * preprocessor for taking the address of its parameter.
+ * For the sake of staying compatible with the API, also
+ * wr_rcu_assign_pointer() is a macro that accepts a pointer as parameter,
+ * instead of the address of said pointer.
+ * However it is simply a wrapper to __wr_rcu_ptr(), which receives the
+ * address of the pointer.
+ */
+static __always_inline
+uintptr_t __wr_rcu_ptr(const void *dst_p_p, const void *src_p)
+{
+	unsigned long flags;
+	struct page *page;
+	void *base;
+	uintptr_t offset;
+	const size_t size = sizeof(void *);
+
+	if (WARN(!__is_wr_after_init(dst_p_p, size), WR_ERR_RANGE_MSG))
+		return (uintptr_t)NULL;
+	local_irq_save(flags);
+	page = virt_to_page(dst_p_p);
+	offset = (uintptr_t)dst_p_p & ~PAGE_MASK;
+	base = vmap(&page, 1, VM_MAP, PAGE_KERNEL);
+	if (WARN(!base, WR_ERR_PAGE_MSG)) {
+		local_irq_restore(flags);
+		return (uintptr_t)NULL;
+	}
+	rcu_assign_pointer((*(void **)(offset + (uintptr_t)base)), src_p);
+	vunmap(base);
+	local_irq_restore(flags);
+	return (uintptr_t)src_p;
+}
+
+#define wr_rcu_assign_pointer(p, v)	__wr_rcu_ptr(&p, v)
+
+#define __wr_simple(dst_ptr, src_ptr)					\
+	wr_memcpy(dst_ptr, src_ptr, sizeof(*(src_ptr)))
+
+#define __wr_safe(dst_ptr, src_ptr,					\
+		  unique_dst_ptr, unique_src_ptr)			\
+({									\
+	typeof(dst_ptr) unique_dst_ptr = (dst_ptr);			\
+	typeof(src_ptr) unique_src_ptr = (src_ptr);			\
+									\
+	wr_memcpy(unique_dst_ptr, unique_src_ptr,			\
+		  sizeof(*(unique_src_ptr)));				\
+})
+
+#define __safe_ops(dst, src)	\
+	(__typecheck(dst, src) && __no_side_effects(dst, src))
+
+/**
+ * wr - copies an object over another of same type and size
+ * @dst_ptr: address of the destination object
+ * @src_ptr: address of the source object
+ */
+#define wr(dst_ptr, src_ptr)						\
+	__builtin_choose_expr(__safe_ops(dst_ptr, src_ptr),		\
+			      __wr_simple(dst_ptr, src_ptr),		\
+			      __wr_safe(dst_ptr, src_ptr,		\
+						__UNIQUE_ID(__dst_ptr),	\
+						__UNIQUE_ID(__src_ptr)))
+
+/**
+ * wr_ptr() - alters a pointer in write rare memory
+ * @dst: target for write
+ * @val: new value
+ *
+ * Returns true on success, false otherwise.
+ */
+static __always_inline
+bool wr_ptr(const void *dst, const void *val)
+{
+	return wr_memcpy(dst, &val, sizeof(val));
+}
+#endif
diff --git a/mm/Makefile b/mm/Makefile
index 26ef77a3883b..215c6a6d7304 100644
--- a/mm/Makefile
+++ b/mm/Makefile
@@ -64,6 +64,7 @@  obj-$(CONFIG_SPARSEMEM)	+= sparse.o
 obj-$(CONFIG_SPARSEMEM_VMEMMAP) += sparse-vmemmap.o
 obj-$(CONFIG_SLOB) += slob.o
 obj-$(CONFIG_MMU_NOTIFIER) += mmu_notifier.o
+obj-$(CONFIG_PRMEM) += prmem.o
 obj-$(CONFIG_KSM) += ksm.o
 obj-$(CONFIG_PAGE_POISONING) += page_poison.o
 obj-$(CONFIG_SLAB) += slab.o
diff --git a/mm/prmem.c b/mm/prmem.c
new file mode 100644
index 000000000000..de9258f5f29a
--- /dev/null
+++ b/mm/prmem.c
@@ -0,0 +1,10 @@ 
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * prmem.c: Memory Protection Library
+ *
+ * (C) Copyright 2017-2018 Huawei Technologies Co. Ltd.
+ * Author: Igor Stoppa <igor.stoppa@huawei.com>
+ */
+
+const char WR_ERR_RANGE_MSG[] = "Write rare on invalid memory range.";
+const char WR_ERR_PAGE_MSG[] = "Failed to remap write rare page.";