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Date: Mon,  9 May 2016 13:22:12 -0700
From: Kees Cook <>
To: Ingo Molnar <>
Cc: Kees Cook <>,
	Yinghai Lu <>,
	Borislav Petkov <>,
	Baoquan He <>,
	Ingo Molnar <>,
	"H. Peter Anvin" <>,
	Borislav Petkov <>,
	Vivek Goyal <>,
	Andy Lutomirski <>,,
	Andrew Morton <>,
	Dave Young <>,,
	LKML <>
Subject: [PATCH v7 9/9] x86/KASLR: Allow randomization below load address

From: Yinghai Lu <>

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 <>
[kees: rewrote changelog, refactor to use min()]
Signed-off-by: Kees Cook <>
 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 def1da9e7863..d972a026d44c 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;
@@ -514,8 +514,11 @@ 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 {

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