linux-kernel - [PATCH 1/2] x86: introduce kaslr

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Message-ID: <alpine.LNX.2.00.1504271334300.16113@pobox.suse.cz>
Date:	Mon, 27 Apr 2015 16:28:07 +0200 (CEST)
From:	Jiri Kosina <jkosina@...e.cz>
To:	x86@...nel.org, Borislav Petkov <bp@...en8.de>,
	Kees Cook <keescook@...omium.org>,
	Josh Poimboeuf <jpoimboe@...hat.com>,
	Seth Jennings <sjenning@...hat.com>,
	Vojtech Pavlik <vojtech@...e.cz>
cc:	linux-kernel@...r.kernel.org, live-patching@...r.kernel.org
Subject: [PATCH 1/2] x86: introduce kaslr_offset()

Offset that has been chosen for kaslr during kernel decompression can be 
easily computed as a difference between _text and __START_KERNEL. We are 
already making use of this in dump_kernel_offset() notifier.

Introduce kaslr_offset() that makes this computation instead of 
hard-coding it, so that other kernel code (such as live patching) can make 
use of it.

Signed-off-by: Jiri Kosina <jkosina@...e.cz>
---
 arch/x86/include/asm/setup.h | 6 ++++++
 arch/x86/kernel/setup.c      | 2 +-
 2 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/setup.h b/arch/x86/include/asm/setup.h
index f69e06b..785ac2f 100644
--- a/arch/x86/include/asm/setup.h
+++ b/arch/x86/include/asm/setup.h
@@ -65,12 +65,18 @@ static inline void x86_ce4100_early_setup(void) { }
  * This is set up by the setup-routine at boot-time
  */
 extern struct boot_params boot_params;
+extern char _text[];
 
 static inline bool kaslr_enabled(void)
 {
 	return !!(boot_params.hdr.loadflags & KASLR_FLAG);
 }
 
+static inline unsigned long kaslr_offset(void)
+{
+	return (unsigned long)&_text - __START_KERNEL;
+}
+
 /*
  * Do NOT EVER look at the BIOS memory size location.
  * It does not work on many machines.
diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
index d74ac33..5056d3c 100644
--- a/arch/x86/kernel/setup.c
+++ b/arch/x86/kernel/setup.c
@@ -834,7 +834,7 @@ dump_kernel_offset(struct notifier_block *self, unsigned long v, void *p)
 {
 	if (kaslr_enabled()) {
 		pr_emerg("Kernel Offset: 0x%lx from 0x%lx (relocation range: 0x%lx-0x%lx)\n",
-			 (unsigned long)&_text - __START_KERNEL,
+			 kaslr_offset(),
 			 __START_KERNEL,
 			 __START_KERNEL_map,
 			 MODULES_VADDR-1);
-- 
Jiri Kosina
SUSE Labs
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