From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Resent-Message-ID: <20090222115855.GI12006@kryten> Message-Id: <20090222115332.669493518@samba.org> References: <20090222114957.213647384@samba.org> Date: Sun, 22 Feb 2009 22:50:05 +1100 From: Anton Blanchard To: linuxppc-dev@ozlabs.org Subject: [patch 08/10] powerpc: Ensure random space between stack and mmaps List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , get_random_int() returns the same value within a 1 jiffy interval. This means that the mmap and stack regions will almost always end up the same distance apart, making a relative offset based attack possible. To fix this, shift the randomness we use for the mmap region by 1 bit. Signed-off-by: Anton Blanchard --- Index: linux-2.6/arch/powerpc/mm/mmap.c =================================================================== --- linux-2.6.orig/arch/powerpc/mm/mmap.c 2009-02-22 11:58:54.000000000 +1100 +++ linux-2.6/arch/powerpc/mm/mmap.c 2009-02-22 12:05:01.000000000 +1100 @@ -46,6 +46,14 @@ return sysctl_legacy_va_layout; } +/* + * Since get_random_int() returns the same value within a 1 jiffy window, + * we will almost always get the same randomisation for the stack and mmap + * region. This will mean the relative distance between stack and mmap will + * be the same. + * + * To avoid this we can shift the randomness by 1 bit. + */ static unsigned long mmap_rnd(void) { unsigned long rnd = 0; @@ -53,11 +61,11 @@ if (current->flags & PF_RANDOMIZE) { /* 8MB for 32bit, 1GB for 64bit */ if (is_32bit_task()) - rnd = (long)(get_random_int() % (1<<(23-PAGE_SHIFT))); + rnd = (long)(get_random_int() % (1<<(22-PAGE_SHIFT))); else - rnd = (long)(get_random_int() % (1<<(30-PAGE_SHIFT))); + rnd = (long)(get_random_int() % (1<<(29-PAGE_SHIFT))); } - return rnd << PAGE_SHIFT; + return (rnd << PAGE_SHIFT) * 2; } static inline unsigned long mmap_base(void) --