[PATCH] mm: Add the ram_latent_entropy kernel parameter

[Kees Cook <keescook@chromium.org>]

From: Emese Revfy <re.emese@gmail.com>

When "ram_latent_entropy" is passed on the kernel command line, entropy
will be extracted from up to the first 4GB of RAM while the runtime memory
allocator is being initialized. This entropy isn't cryptographically
secure, but does help provide additional unpredictability on otherwise
low-entropy systems.

Based on work created by the PaX Team.

Signed-off-by: Emese Revfy <re.emese@gmail.com>
[kees: renamed parameter, dropped relationship with plugin, updated log]
Signed-off-by: Kees Cook <keescook@chromium.org>
---
This patch has been extracted from the latent_entropy gcc plugin, as
suggested by Linus: https://lkml.org/lkml/2016/8/8/840
---
 Documentation/kernel-parameters.txt |  5 +++++
 mm/page_alloc.c                     | 21 +++++++++++++++++++++
 2 files changed, 26 insertions(+)

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 46c030a49186..9d054984370f 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -3245,6 +3245,11 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 	raid=		[HW,RAID]
 			See Documentation/md.txt.
 
+	ram_latent_entropy
+			Enable a very simple form of latent entropy extraction
+			from the first 4GB of memory as the bootmem allocator
+			passes the memory pages to the buddy allocator.
+
 	ramdisk_size=	[RAM] Sizes of RAM disks in kilobytes
 			See Documentation/blockdev/ramdisk.txt.
 
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index fb975cec3518..1de94f0ff29d 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -64,6 +64,7 @@
 #include <linux/page_owner.h>
 #include <linux/kthread.h>
 #include <linux/memcontrol.h>
+#include <linux/random.h>
 
 #include <asm/sections.h>
 #include <asm/tlbflush.h>
@@ -1236,6 +1237,15 @@ static void __free_pages_ok(struct page *page, unsigned int order)
 	local_irq_restore(flags);
 }
 
+bool __meminitdata ram_latent_entropy;
+
+static int __init setup_ram_latent_entropy(char *str)
+{
+	ram_latent_entropy = true;
+	return 0;
+}
+early_param("ram_latent_entropy", setup_ram_latent_entropy);
+
 static void __init __free_pages_boot_core(struct page *page, unsigned int order)
 {
 	unsigned int nr_pages = 1 << order;
@@ -1251,6 +1261,17 @@ static void __init __free_pages_boot_core(struct page *page, unsigned int order)
 	__ClearPageReserved(p);
 	set_page_count(p, 0);
 
+	if (ram_latent_entropy && !PageHighMem(page) &&
+		page_to_pfn(page) < 0x100000) {
+		u64 hash = 0;
+		size_t index, end = PAGE_SIZE * nr_pages / sizeof(hash);
+		const u64 *data = lowmem_page_address(page);
+
+		for (index = 0; index < end; index++)
+			hash ^= hash + data[index];
+		add_device_randomness((const void *)&hash, sizeof(hash));
+	}
+
 	page_zone(page)->managed_pages += nr_pages;
 	set_page_refcounted(page);
 	__free_pages(page, order);
-- 
2.7.4


-- 
Kees Cook
Nexus Security