* Kees Cook <keescook@xxxxxxxxxxxx> wrote: > On Mon, Mar 2, 2015 at 11:31 PM, Ingo Molnar <mingo@xxxxxxxxxx> wrote: > > > > * Kees Cook <keescook@xxxxxxxxxxxx> wrote: > > > >> To address the "offset2lib" ASLR weakness[1], this separates ET_DYN > >> ASLR from mmap ASLR, as already done on s390. The architectures > >> that are already randomizing mmap (arm, arm64, mips, powerpc, s390, > >> and x86), have their various forms of arch_mmap_rnd() made available > >> via the new CONFIG_ARCH_HAS_ELF_RANDOMIZE. For these architectures, > >> arch_randomize_brk() is collapsed as well. > >> > >> This is an alternative to the solutions in: > >> https://lkml.org/lkml/2015/2/23/442 > > > > Looks good so far: > > > > Reviewed-by: Ingo Molnar <mingo@xxxxxxxxxx> > > > > While reviewing this series I also noticed that the following code > > could be factored out from architecture mmap code as well: > > > > - arch_pick_mmap_layout() uses very similar patterns across the > > platforms, with only few variations. Many architectures use > > the same duplicated mmap_is_legacy() helper as well. There's > > usually just trivial differences between mmap_legacy_base() > > approaches as well. > > I was nervous to start refactoring this code, but it's true: most of > it is the same. Well, it still needs to be done if we want to add new randomization features: code fractured over multiple architectures is a receipe for bugs, as this series demonstrates. So it first has to be made more maintainable. > > - arch_mmap_rnd(): the PF_RANDOMIZE checks are needlessly > > exposed to the arch routine - the arch routine should only > > concentrate on arch details, not generic flags like > > PF_RANDOMIZE. > > Yeah, excellent point. I will send a follow-up patch to move this > into binfmt_elf instead. I'd like to avoid removing it in any of the > other patches since each was attempting a single step in the > refactoring. Finegrained patches are ideal! > > In theory the mmap layout could be fully parametrized as well: > > i.e. no callback functions to architectures by default at all: > > just declarations of bits of randomization desired (or, available > > address space bits), and perhaps an arch helper to allow 32-bit > > vs. 64-bit address space distinctions. > > Yeah, I was considering that too, since each architecture has a > nearly identical arch_mmap_rnd() at this point. Only the size of the > entropy was changing. > > > 'Weird' architectures could provide special routines, but only by > > overriding the default behavior, which should be generic, safe and > > robust. > > Yeah, quite true. Should entropy size be a #define like > ELF_ET_DYN_BASE? Something like ASLR_MMAP_ENTROPY and > ASLR_MMAP_ENTROPY_32? [...] That would work I suspect. > [...] Is there a common function for determining a compat task? That > seemed to be per-arch too. Maybe arch_mmap_entropy()? Compat flags are a bit of a mess, and since they often tie into arch low level assembly code, they are hard to untangle. So maybe as an intermediate step add an is_compat() generic method, and make that obvious and self-defined function a per arch thing? But I'm just handwaving here - I suspect it has to be tried to see all the complications and to determine whether that's the best structure and whether it's a win ... Only one thing is certain: the current code is not compact and reviewable enough, and VM bits hiding in arch/*/mm/mmap.c tends to reduce net attention paid to these details. Thanks, Ingo
