[RFC PATCH v5 04/12] __wr_after_init: x86_64: randomize mapping offset

[Igor Stoppa <igor.stoppa@gmail.com>]

x86_64 specialized way of defining the base address for the alternate
mapping used by write-rare.

Since the kernel address space spans across 64TB and it is mapped into a
used address space of 128TB, the kernel address space can be shifted by a
random offset that is up to 64TB and page aligned.

This is accomplished by providing arch-specific version of the function
__init_wr_base()

Signed-off-by: Igor Stoppa <igor.stoppa@huawei.com>

CC: Andy Lutomirski <luto@amacapital.net>
CC: Nadav Amit <nadav.amit@gmail.com>
CC: Matthew Wilcox <willy@infradead.org>
CC: Peter Zijlstra <peterz@infradead.org>
CC: Kees Cook <keescook@chromium.org>
CC: Dave Hansen <dave.hansen@linux.intel.com>
CC: Mimi Zohar <zohar@linux.vnet.ibm.com>
CC: Thiago Jung Bauermann <bauerman@linux.ibm.com>
CC: Ahmed Soliman <ahmedsoliman@mena.vt.edu>
CC: linux-integrity@vger.kernel.org
CC: kernel-hardening@lists.openwall.com
CC: linux-mm@kvack.org
CC: linux-kernel@vger.kernel.org
---
 arch/x86/mm/Makefile      |  2 ++
 arch/x86/mm/prmem.c (new) | 20 ++++++++++++++++++++
 2 files changed, 22 insertions(+)

diff --git a/arch/x86/mm/Makefile b/arch/x86/mm/Makefile
index 4b101dd6e52f..66652de1e2c7 100644
--- a/arch/x86/mm/Makefile
+++ b/arch/x86/mm/Makefile
@@ -53,3 +53,5 @@ obj-$(CONFIG_PAGE_TABLE_ISOLATION)		+= pti.o
 obj-$(CONFIG_AMD_MEM_ENCRYPT)	+= mem_encrypt.o
 obj-$(CONFIG_AMD_MEM_ENCRYPT)	+= mem_encrypt_identity.o
 obj-$(CONFIG_AMD_MEM_ENCRYPT)	+= mem_encrypt_boot.o
+
+obj-$(CONFIG_PRMEM)		+= prmem.o
diff --git a/arch/x86/mm/prmem.c b/arch/x86/mm/prmem.c
new file mode 100644
index 000000000000..b04fc03f92fb
--- /dev/null
+++ b/arch/x86/mm/prmem.c
@@ -0,0 +1,20 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * prmem.c: Memory Protection Library - x86_64 backend
+ *
+ * (C) Copyright 2018-2019 Huawei Technologies Co. Ltd.
+ * Author: Igor Stoppa <igor.stoppa@huawei.com>
+ */
+
+#include <linux/mm.h>
+#include <linux/mmu_context.h>
+
+unsigned long __init __init_wr_base(void)
+{
+	/*
+	 * Place 64TB of kernel address space within 128TB of user address
+	 * space, at a random page aligned offset.
+	 */
+	return (((unsigned long)kaslr_get_random_long("WR Poke")) &
+		PAGE_MASK) % (64 * _BITUL(40));
+}
-- 
2.19.1