linux-kernel - [PATCH v8 4/4] x86/KASLR: Allow randomization below load address

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Message-Id: <1462900755-20005-5-git-send-email-keescook@chromium.org>
Date:	Tue, 10 May 2016 10:19:15 -0700
From:	Kees Cook <keescook@...omium.org>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Kees Cook <keescook@...omium.org>, Yinghai Lu <yinghai@...nel.org>,
	Borislav Petkov <bp@...e.de>, Baoquan He <bhe@...hat.com>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...en8.de>,
	Vivek Goyal <vgoyal@...hat.com>,
	Andy Lutomirski <luto@...nel.org>, lasse.collin@...aani.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Young <dyoung@...hat.com>,
	kernel-hardening@...ts.openwall.com,
	LKML <linux-kernel@...r.kernel.org>
Subject: [PATCH v8 4/4] x86/KASLR: Allow randomization below load address

From: Yinghai Lu <yinghai@...nel.org>

Currently the physical randomization's lower boundary is the original
kernel load address. For bootloaders that load kernels into very high
memory (e.g. kexec), this means randomization takes place in a very small
window at the top of memory, ignoring the large region of physical memory
below the load address.

Since mem_avoid is already correctly tracking the regions that must be
avoided, this patch changes the minimum address to whatever is less:
512M (to conservatively avoid unknown things in lower memory) or the
load address. Now, for example, if the kernel is loaded at 8G, [512M,
8G) will be added into possible physical memory positions.

Signed-off-by: Yinghai Lu <yinghai@...nel.org>
[kees: rewrote changelog, refactor to use min()]
Signed-off-by: Kees Cook <keescook@...omium.org>
---
 arch/x86/boot/compressed/kaslr.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/arch/x86/boot/compressed/kaslr.c b/arch/x86/boot/compressed/kaslr.c
index d0a823df183b..304c5c369aff 100644
--- a/arch/x86/boot/compressed/kaslr.c
+++ b/arch/x86/boot/compressed/kaslr.c
@@ -492,7 +492,7 @@ void choose_random_location(unsigned long input,
 			    unsigned long output_size,
 			    unsigned long *virt_addr)
 {
-	unsigned long random_addr;
+	unsigned long random_addr, min_addr;
 
 	/* By default, keep output position unchanged. */
 	*virt_addr = *output;
@@ -517,8 +517,11 @@ void choose_random_location(unsigned long input,
 	/* Record the various known unsafe memory ranges. */
 	mem_avoid_init(input, input_size, *output);
 
+	/* Low end should be the smaller of 512M or initial location. */
+	min_addr = min(*output, 512UL << 20);
+
 	/* Walk e820 and find a random address. */
-	random_addr = find_random_phys_addr(*output, output_size);
+	random_addr = find_random_phys_addr(min_addr, output_size);
 	if (!random_addr) {
 		warn("KASLR disabled: could not find suitable E820 region!");
 	} else {
-- 
2.6.3