Date: Thu, 20 Apr 2017 21:22:22 +1000 From: Michael Ellerman <mpe@...erman.id.au> To: Kees Cook <keescook@...omium.org> Cc: "linuxppc-dev\@ozlabs.org" <linuxppc-dev@...abs.org>, Bhupesh Sharma <bhsharma@...hat.com>, "kernel-hardening\@lists.openwall.com" <kernel-hardening@...ts.openwall.com> Subject: Re: Re: [PATCH] powerpc/mm: Add support for runtime configuration of ASLR limits Kees Cook <keescook@...omium.org> writes: > On Wed, Apr 19, 2017 at 7:29 AM, Michael Ellerman <mpe@...erman.id.au> wrote: >> Add powerpc support for mmap_rnd_bits and mmap_rnd_compat_bits, which are two >> sysctls that allow a user to configure the number of bits of randomness used for >> ASLR. >> >> Because of the way the Kconfig for ARCH_MMAP_RND_BITS is defined, we have to >> construct at least the MIN value in Kconfig, vs in a header which would be more >> natural. Given that we just go ahead and do it all in Kconfig. >> >> At least according to the code (the documentation makes no mention of it), the >> value is defined as the number of bits of randomisation *of the page*, not the >> address. This makes some sense, with larger page sizes more of the low bits are >> forced to zero, which would reduce the randomisation if we didn't take the >> PAGE_SIZE into account. However it does mean the min/max values have to change >> depending on the PAGE_SIZE in order to actually limit the amount of address >> space consumed by the randomisation. >> >> The result of that is that we have to define the default values based on both >> 32-bit vs 64-bit, but also the configured PAGE_SIZE. Furthermore now that we >> have 128TB address space support on Book3S, we also have to take that into >> account. >> >> Finally we can wire up the value in arch_mmap_rnd(). >> >> Signed-off-by: Michael Ellerman <mpe@...erman.id.au> > > Maybe add a Suggested-by: for the earlier patches? Sure. >> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig >> index 97a8bc8a095c..608ee0b7b79f 100644 >> --- a/arch/powerpc/Kconfig >> +++ b/arch/powerpc/Kconfig >> @@ -22,6 +22,48 @@ config MMU >> bool >> default y >> >> +config ARCH_MMAP_RND_BITS_MAX >> + # On Book3S 64, the default virtual address space for 64-bit processes >> + # is 2^47 (128TB). As a maximum, allow randomisation to consume up to >> + # 32T of address space (2^45), which should ensure a reasonable gap >> + # between bottom-up and top-down allocations for applications that >> + # consume "normal" amounts of address space. Book3S 64 only supports 64K >> + # and 4K page sizes. >> + default 29 if PPC_BOOK3S_64 && PPC_64K_PAGES # 29 = 45 (32T) - 16 (64K) >> + default 33 if PPC_BOOK3S_64 # 33 = 45 (32T) - 12 (4K) >> + # >> + # On all other 64-bit platforms (currently only Book3E), the virtual >> + # address space is 2^46 (64TB). Allow randomisation to consume up to 16T >> + # of address space (2^44). Only 4K page sizes are supported. >> + default 32 if 64BIT # 32 = 44 (16T) - 12 (4K) >> + # >> + # For 32-bit, use the compat values, as they're the same. >> + default ARCH_MMAP_RND_COMPAT_BITS_MIN > > Shouldn't the default case be ..._MAX? Yes, ooops! Thanks for the review. > Yay! Ever closer to being able to extract arch_mmap_rnd() out of arch/ ;) Hah, you are an optimist :) cheers
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