linux-kernel - [PATCH v3 3/3] Replace invocation of weak PRNG in arch/x86/mm/kaslr.c

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Date:   Sun, 18 Dec 2022 20:19:00 +0200
From:   david.keisarschm@...l.huji.ac.il
To:     linux-kernel@...r.kernel.org,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>
Cc:     Jason@...c4.com,
        David Keisar Schmidt <david.keisarschm@...l.huji.ac.il>,
        aksecurity@...il.com, ilay.bahat1@...il.com
Subject: [PATCH v3 3/3] Replace invocation of weak PRNG in arch/x86/mm/kaslr.c

From: David Keisar Schmidt <david.keisarschm@...l.huji.ac.il>

This third series add some changes to the commit messages,
and also replaces get_random_u32 with get_random_u32_below,
in a case a modulo operation is done on the result.

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, this PRNG turned out to be weak, as noted in commit c51f8f88d705
To fix it, we have changed the invocation of prandom_bytes_state to get_random_bytes.

Unlike get_random_bytes which maintains its own state, prandom_bytes state needs to be seeded,
thus, we have omitted the call to the seeding function, since it is not needed anymore.

# Changes since v2

* edited commit message.


Signed-off-by: David Keisar Schmidt <david.keisarschm@...l.huji.ac.il>
---
 arch/x86/mm/kaslr.c | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/arch/x86/mm/kaslr.c b/arch/x86/mm/kaslr.c
index 557f0fe25..9ef8993d5 100644
--- a/arch/x86/mm/kaslr.c
+++ b/arch/x86/mm/kaslr.c
@@ -66,7 +66,6 @@ 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;
 
@@ -113,8 +112,6 @@ 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 +120,7 @@ void __init kernel_randomize_memory(void)
 		 * available.
 		 */
 		entropy = remain_entropy / (ARRAY_SIZE(kaslr_regions) - i);
-		prandom_bytes_state(&rand_state, &rand, sizeof(rand));
+		get_random_bytes(&rand, sizeof(rand));
 		entropy = (rand % (entropy + 1)) & PUD_MASK;
 		vaddr += entropy;
 		*kaslr_regions[i].base = vaddr;
-- 
2.38.0