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authorAnton Blanchard <anton@samba.org>2009-02-22 01:50:05 +0000
committerBenjamin Herrenschmidt <benh@kernel.crashing.org>2009-02-23 15:53:21 +1100
commita5adc91a4b44b5d1706b9d906cc14fe4f312afe9 (patch)
treef0f5053b44b53fea4065bd05aeb9f0a53090d9dd /arch/powerpc/mm/mmap.c
parent912f9ee21c836081e3c96dfe61025841ebeb95da (diff)
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powerpc: Ensure random space between stack and mmaps
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 <anton@samba.org> Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Diffstat (limited to 'arch/powerpc/mm/mmap.c')
-rw-r--r--arch/powerpc/mm/mmap.c14
1 files changed, 11 insertions, 3 deletions
diff --git a/arch/powerpc/mm/mmap.c b/arch/powerpc/mm/mmap.c
index f7376dbb653b..75dc7faaf917 100644
--- a/arch/powerpc/mm/mmap.c
+++ b/arch/powerpc/mm/mmap.c
@@ -46,6 +46,14 @@ static inline int mmap_is_legacy(void)
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 @@ static unsigned long mmap_rnd(void)
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)