[PATCH v6 11/11] x86/KASLR: Allow randomization below load address

[Kees Cook <keescook@chromium.org>]

From: Yinghai Lu <yinghai@kernel.org>

Currently the physical randomization's lower boundary is the 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 which ever 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@kernel.org>
[kees: rewrote changelog, refactor to use min()]
Signed-off-by: Kees Cook <keescook@chromium.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 8cf705826bdc..ace41822d16f 100644
--- a/arch/x86/boot/compressed/kaslr.c
+++ b/arch/x86/boot/compressed/kaslr.c
@@ -464,7 +464,7 @@ void choose_random_location(unsigned char *input_ptr,
 	 */
 	unsigned long input = (unsigned long)input_ptr;
 	unsigned long output = (unsigned long)*output_ptr;
-	unsigned long random_addr;
+	unsigned long random_addr, min_addr;
 
 	/* By default, keep output position unchanged. */
 	*virt_addr = *output_ptr;
@@ -486,8 +486,11 @@ void choose_random_location(unsigned char *input_ptr,
 	/* 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