[PATCH 3/6] Introduce a function to randomize the kernel text mapping address

[Baoquan He]

Kaslr extended kernel text mapping region size from 512M to 1G,
namely CONFIG_RANDOMIZE_BASE_MAX_OFFSET. This means kernel text
can be mapped to below region:

__START_KERNEL_map + LOAD_PHYSICAL_ADDR, __START_KERNEL_map + 1G]

Introduce a function find_random_virt_offset() to get random value
between LOAD_PHYSICAL_ADDR and CONFIG_RANDOMIZE_BASE_MAX_OFFSET.
This random value will be added to __START_KERNEL_map to get the
starting address which kernel text is mapped from.

Signed-off-by: Baoquan He <bhe@xxxxxxxxxx>
---
 arch/x86/boot/compressed/aslr.c | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/arch/x86/boot/compressed/aslr.c b/arch/x86/boot/compressed/aslr.c
index 9a7210c..091b118 100644
--- a/arch/x86/boot/compressed/aslr.c
+++ b/arch/x86/boot/compressed/aslr.c
@@ -1,4 +1,5 @@
 #include "misc.h"
+#include "../string.h"
 
 #include <asm/msr.h>
 #include <asm/archrandom.h>
@@ -295,6 +296,27 @@ static unsigned long find_random_addr(unsigned long minimum,
 	return slots_fetch_random();
 }
 
+static unsigned long find_random_virt_offset(unsigned long size)
+{
+	struct mem_vector region, img;
+
+	memset(slots, 0, sizeof(slots));
+	slot_max = 0;
+
+	region.start = LOAD_PHYSICAL_ADDR;
+	region.size = CONFIG_RANDOMIZE_BASE_MAX_OFFSET - region.start;
+
+	for (img.start = region.start, img.size = size;
+	     mem_contains(&region, &img);
+	     img.start += CONFIG_PHYSICAL_ALIGN) {
+		if (mem_avoid_overlap(&img))
+			continue;
+		slots_append(img.start);
+	}
+
+	return slots_fetch_random();
+}
+
 unsigned char *choose_kernel_location(unsigned char *input,
 				      unsigned long input_size,
 				      unsigned char *output,
-- 
1.9.3

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