Date: Wed, 25 Jan 2017 17:38:44 -0800 From: Kees Cook <keescook@...omium.org> To: Bhupesh Sharma <bhsharma@...hat.com> Cc: "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>, Daniel Cashman <dcashman@...gle.com>, "kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>, Michael Ellerman <mpe@...erman.id.au> Subject: Re: Query regarding randomization bits for a ASLR elf on PPC64 On Sun, Jan 22, 2017 at 9:34 PM, Bhupesh Sharma <bhsharma@...hat.com> wrote: > 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. Yeah, looks like PPC could use it. If you've got hardware to test with, please add it. :) > 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: Just to be clear: 64K pages will lose you 4 bits of entropy when compared to 4K on x86_64. (Assuming I'm doing the math right...) > 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)); Yeah, I'm not sure about this. The comments above the MIN_GAP* macros seem to talk about making sure there is the 1GB stack gap, but that shouldn't limit mmap. Stack base is randomized in fs/binfmt_elf.c randomize_stack_top() which uses STACK_RND_MASK (and PAGE_SHIFT). x86: /* 1GB for 64bit, 8MB for 32bit */ #define STACK_RND_MASK (test_thread_flag(TIF_ADDR32) ? 0x7ff : 0x3fffff) powerpc: /* 1GB for 64bit, 8MB for 32bit */ #define STACK_RND_MASK (is_32bit_task() ? \ (0x7ff >> (PAGE_SHIFT - 12)) : \ (0x3ffff >> (PAGE_SHIFT - 12))) So, in the 64k page case, stack randomization entropy is reduced, but otherwise identical to x86. x86 and powerpc both use arch_mmap_rnd() for both mmap and ET_DYN (with different bases). x86 uses ELF_ET_DYN_BASE as TASK_SIZE / 3 * 2 (which the ELF loader pushes back up the nearest PAGE_SIZE alignment: 0x555555555000), though powerpc uses 0x20000000, so it should have significantly more space for mmap and ET_DYN ASLR than x86. > - 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. The PPC brk randomization on powerpc doesn't use the more common randomize_page() way other archs do it... /* 8MB for 32bit, 1GB for 64bit */ if (is_32bit_task()) rnd = (get_random_long() % (1UL<<(23-PAGE_SHIFT))); else rnd = (get_random_long() % (1UL<<(30-PAGE_SHIFT))); return rnd << PAGE_SHIFT; x86 uses 0x02000000 (via randomize_page()), which, if I'm doing the math right is 14 bits, regardless of 32/64-bit. arm64 uses 0x40000000 (20 bits) on 64-bit processes and the same as x86 (14) for 32-bit processes. Looks like powerpc uses either 13 or 20 for 4k pages, which is close to the same. > - 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. It can support it once you implement the Kconfigs for it. :) > - 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. Hopefully this helped a bit. I would literally draw out the memory map, and double-check nothing can collide at your max values. -Kees -- Kees Cook Nexus Security
Powered by blists - more mailing lists
Confused about mailing lists and their use? Read about mailing lists on Wikipedia and check out these guidelines on proper formatting of your messages.