[PATCH v6 3/3] arch/x86/mm/kaslr: use siphash instead of prandom_bytes_state

[david . keisarschm]

From: David Keisar Schmidt <david.keisarschm@xxxxxxxxxxxxxxx>

The memory randomization of the virtual address space
of kernel memory regions (physical memory mapping, vmalloc & vmemmap) inside
arch/x86/mm/kaslr.c is based on the function prandom_bytes_state which uses
the prandom_u32 PRNG.

However, the seeding here is done by calling prandom_seed_state,
which effectively uses only 32bits of the seed, which means that observing ONE
region's offset (say 30 bits) can provide the attacker with 2 possible seeds
(from which the attacker can calculate the remaining two regions)

In order to fix it,  we have replaced the two invocations of prandom_bytes_state and prandom_seed_state
with siphash, which is considered more secure.
Besides, the original code used the same pseudo-random number in every iteration,
so to add some additional randomization
we call siphash every iteration, hashing the iteration index with the described key.

Signed-off-by: David Keisar Schmidt <david.keisarschm@xxxxxxxxxxxxxxx>
---
Changes since v5:
* deleted irrelevant changes which were appended accidentally.

Changes since v4:
* replaced the call to prandom_bytes_state and prandom_seed_state,
    with siphash.

Changes since v2:
* edited commit message.

 arch/x86/mm/kaslr.c | 21 +++++++++++++++------
 1 file changed, 15 insertions(+), 6 deletions(-)

diff --git a/arch/x86/mm/kaslr.c b/arch/x86/mm/kaslr.c
index 557f0fe25..fb551796c 100644
--- a/arch/x86/mm/kaslr.c
+++ b/arch/x86/mm/kaslr.c
@@ -25,6 +25,7 @@
 #include <linux/random.h>
 #include <linux/memblock.h>
 #include <linux/pgtable.h>
+#include <linux/siphash.h>
 
 #include <asm/setup.h>
 #include <asm/kaslr.h>
@@ -66,9 +67,14 @@ void __init kernel_randomize_memory(void)
 	size_t i;
 	unsigned long vaddr_start, vaddr;
 	unsigned long rand, memory_tb;
-	struct rnd_state rand_state;
 	unsigned long remain_entropy;
 	unsigned long vmemmap_size;
+	/*
+	 * Create a Siphash key. We use a mask of PI digits to add some
+	 * randomness to the key.
+	 */
+	u64 seed = (u64) kaslr_get_random_long("Memory");
+	siphash_key_t key = {{seed, seed ^ 0x3141592653589793UL}};
 
 	vaddr_start = pgtable_l5_enabled() ? __PAGE_OFFSET_BASE_L5 : __PAGE_OFFSET_BASE_L4;
 	vaddr = vaddr_start;
@@ -94,7 +100,7 @@ void __init kernel_randomize_memory(void)
 	 */
 	BUG_ON(kaslr_regions[0].base != &page_offset_base);
 	memory_tb = DIV_ROUND_UP(max_pfn << PAGE_SHIFT, 1UL << TB_SHIFT) +
-		CONFIG_RANDOMIZE_MEMORY_PHYSICAL_PADDING;
+		    CONFIG_RANDOMIZE_MEMORY_PHYSICAL_PADDING;
 
 	/* Adapt physical memory region size based on available memory */
 	if (memory_tb < kaslr_regions[0].size_tb)
@@ -105,7 +111,7 @@ void __init kernel_randomize_memory(void)
 	 * boundary.
 	 */
 	vmemmap_size = (kaslr_regions[0].size_tb << (TB_SHIFT - PAGE_SHIFT)) *
-			sizeof(struct page);
+		       sizeof(struct page);
 	kaslr_regions[2].size_tb = DIV_ROUND_UP(vmemmap_size, 1UL << TB_SHIFT);
 
 	/* Calculate entropy available between regions */
@@ -113,8 +119,6 @@ void __init kernel_randomize_memory(void)
 	for (i = 0; i < ARRAY_SIZE(kaslr_regions); i++)
 		remain_entropy -= get_padding(&kaslr_regions[i]);
 
-	prandom_seed_state(&rand_state, kaslr_get_random_long("Memory"));
-
 	for (i = 0; i < ARRAY_SIZE(kaslr_regions); i++) {
 		unsigned long entropy;
 
@@ -123,7 +127,12 @@ void __init kernel_randomize_memory(void)
 		 * available.
 		 */
 		entropy = remain_entropy / (ARRAY_SIZE(kaslr_regions) - i);
-		prandom_bytes_state(&rand_state, &rand, sizeof(rand));
+		/*
+		 * Use Siphash to generate a pseudo-random number every
+		 * iteration
+		 *
+		 */
+		rand = siphash_1u64(i, &key);
 		entropy = (rand % (entropy + 1)) & PUD_MASK;
 		vaddr += entropy;
 		*kaslr_regions[i].base = vaddr;
-- 
2.37.3