Query regarding randomization bits for a ASLR elf on PPC64

[Bhupesh Sharma]

Hi Experts,

I was recently looking at ways to extend the randomization range for a
ASLR elf on a PPC64LE system.

I basically have been using 28-bits of randomization on x86_64 for an
ASLR elf using appropriate ARCH_MMAP_RND_BITS_MIN and
ARCH_MMAP_RND_BITS_MAX values:

http://lxr.free-electrons.com/source/arch/x86/Kconfig#L192

And I understand from looking at the PPC64 code base that both
ARCH_MMAP_RND_BITS_MIN and ARCH_MMAP_RND_BITS_MAX are not used in the
current upstream code.

I am looking at ways to randomize the mmap, stack and brk ranges for a
ALSR elf on PPC64LE. Currently I am using a PAGE SIZE of 64K in my
config file and hence the randomization usually translates to
something like this for me:

mmap:
-------
http://lxr.free-electrons.com/source/arch/powerpc/mm/mmap.c#L67

rnd = get_random_long() % (1UL<<(30-PAGE_SHIFT));

Since PAGE_SHIFT is 16 for 64K page size, this computation reduces to:
rnd = get_random_long() % (1UL<<(14));

If I compare this to x86_64, I see there:

http://lxr.free-electrons.com/source/arch/x86/mm/mmap.c#L79

rnd = get_random_long() & ((1UL << mmap_rnd_bits) - 1);

So, if mmap_rnd_bits = 28, this equates to:
rnd = get_random_long() & ((1UL << 28) - 1);

Observations and Queries:
--------------------------------------

- So, x86_64 gives approx twice number of random bits for a ASLR elf
running on it as compared to PPC64 although both use a 48-bit VA.

- I also see this comment for PPC at various places, regarding 1GB
randomness spread for PPC64. Is this restricted by the hardware or the
kernel usage?:

/* 8MB for 32bit, 1GB for 64bit */
 64         if (is_32bit_task())
 65                 rnd = get_random_long() % (1<<(23-PAGE_SHIFT));
 66         else
 67                 rnd = get_random_long() % (1UL<<(30-PAGE_SHIFT));

- I tried to increase the randomness to 28 bits for PPC as well by
making the PPC mmap, brk code equivalent to x86_64 and it works fine
for my use case.

- But, I am not sure this is the right thing to do and whether the
PPC64 also supports the MIN and MAX ranges for randomization.

- If it does I would like to understand, test and push a patch to
implement the same for PPC64 in upstream.

Sorry for the long mail, but would really appreciate if someone can
help me understand the details here.

Thanks,
Bhupesh