linux-kernel - [PATCH] fork: make whole stack

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Date:   Mon, 31 Oct 2016 15:04:01 +0100
From:   Jann Horn <jann@...jh.net>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>, Ingo Molnar <mingo@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Kees Cook <keescook@...omium.org>
Cc:     linux-kernel@...r.kernel.org, kernel-hardening@...ts.openwall.com,
        Daniel Micay <danielmicay@...il.com>
Subject: [PATCH] fork: make whole stack_canary random

On machines with sizeof(unsigned long)==8, this ensures that the more
significant 32 bits of stack_canary are random, too.
stack_canary is defined as unsigned long, all the architectures with stack
protector support already pick the stack_canary of init as a random
unsigned long, and get_random_long() should be as fast as get_random_int(),
so there seems to be no good reason against this.

This should help if someone tries to guess a stack canary with brute force.

(This change has been made in PaX already, with a different RNG.)

Signed-off-by: Jann Horn <jann@...jh.net>
---
 kernel/fork.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index 623259fc794d..d577e2c5d14f 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -518,7 +518,7 @@ static struct task_struct *dup_task_struct(struct task_struct *orig, int node)
 	set_task_stack_end_magic(tsk);

 #ifdef CONFIG_CC_STACKPROTECTOR
-	tsk->stack_canary = get_random_int();
+	tsk->stack_canary = get_random_long();
 #endif

 	/*
-- 
2.1.4