Commit a5adc91a authored by Anton Blanchard's avatar Anton Blanchard Committed by Benjamin Herrenschmidt

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: default avatarAnton Blanchard <>
Signed-off-by: default avatarBenjamin Herrenschmidt <>
parent 912f9ee2
......@@ -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)));
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)
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