[PATCH v9 5/5] x86/KASLR: Allow randomization below load address

From: Kees Cook
Date: Wed May 25 2016 - 18:46:16 EST

From: Yinghai Lu <yinghai@xxxxxxxxxx>

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@xxxxxxxxxx>
[kees: rewrote changelog, refactor to use min()]
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
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 {