Re: git: b014e0f15bc7 - main - Enable ASLR by default for 64-bit executables
- In reply to: Shawn Webb : "Re: git: b014e0f15bc7 - main - Enable ASLR by default for 64-bit executables"
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Date: Wed, 17 Nov 2021 20:08:34 UTC
Hi,
śr., 17 lis 2021 o 06:40 Shawn Webb <shawn.webb@hardenedbsd.org> napisał(a):
>
> On Wed, Nov 17, 2021 at 10:42:12AM +1100, Kubilay Kocak wrote:
> > On 17/11/2021 9:26 am, Marcin Wojtas wrote:
> > > The branch main has been updated by mw:
> > >
> > > URL: https://cgit.FreeBSD.org/src/commit/?id=b014e0f15bc73d80ef49b64fd1f8c29f469467cb
> > >
> > > commit b014e0f15bc73d80ef49b64fd1f8c29f469467cb
> > > Author: Marcin Wojtas <mw@FreeBSD.org>
> > > AuthorDate: 2021-10-24 14:53:06 +0000
> > > Commit: Marcin Wojtas <mw@FreeBSD.org>
> > > CommitDate: 2021-11-16 22:16:09 +0000
> > >
> > > Enable ASLR by default for 64-bit executables
> > > Address Space Layout Randomization (ASLR) is an exploit mitigation
> > > technique implemented in the majority of modern operating systems.
> > > It involves randomly positioning the base address of an executable
> > > and the position of libraries, heap, and stack, in a process's address
> > > space. Although over the years ASLR proved to not guarantee full OS
> > > security on its own, this mechanism can make exploitation more difficult.
> > > Tests on the tier 1 64-bit architectures demonstrated that the ASLR is
> > > stable and does not result in noticeable performance degradation,
> > > therefore it should be safe to enable this mechanism by default.
> > > Moreover its effectiveness is increased for PIE (Position Independent
> > > Executable) binaries. Thanks to commit 9a227a2fd642 ("Enable PIE by
> > > default on 64-bit architectures"), building from src is not necessary
> > > to have PIE binaries. It is enough to control usage of ASLR in the
> > > OS solely by setting the appropriate sysctls.
> > > This patch toggles the kernel settings to use address map randomization
> > > for PIE & non-PIE 64-bit binaries. It also disables SBRK, in order
> > > to allow utilization of the bss grow region for mappings. The latter
> > > has no effect if ASLR is disabled, so apply it to all architectures.
> > > As for the drawbacks, a consequence of using the ASLR is more
> > > significant VM fragmentation, hence the issues may be encountered
> > > in the systems with a limited address space in high memory consumption
> > > cases, such as buildworld. As a result, although the tests on 32-bit
> > > architectures with ASLR enabled were mostly on par with what was
> > > observed on 64-bit ones, the defaults for the former are not changed
> > > at this time. Also, for the sake of safety keep the feature disabled
> > > for 32-bit executables on 64-bit machines, too.
> > > The committed change affects the overall OS operation, so the
> > > following should be taken into consideration:
> > > * Address space fragmentation.
> > > * A changed ABI due to modified layout of address space.
> > > * More complicated debugging due to:
> > > * Non-reproducible address space layout between runs.
> > > * Some debuggers automatically disable ASLR for spawned processes,
> > > making target's environment different between debug and
> > > non-debug runs.
> > > In order to confirm/rule-out the dependency of any encountered issue
> > > on ASLR it is strongly advised to re-run the test with the feature
> > > disabled - it can be done by setting the following sysctls
> > > in the /etc/sysctl.conf file:
> > > kern.elf64.aslr.enable=0
> > > kern.elf64.aslr.pie_enable=0
> > > Co-developed by: Dawid Gorecki <dgr@semihalf.com>
> > > Reviewed by: emaste, kib
> > > Obtained from: Semihalf
> > > Sponsored by: Stormshield
> > > MFC after: 1 month
> > > Differential revision: https://reviews.freebsd.org/D27666
> > > ---
> > > sys/kern/imgact_elf.c | 20 +++++++++++++++++---
> > > 1 file changed, 17 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/sys/kern/imgact_elf.c b/sys/kern/imgact_elf.c
> > > index 898f0f66a532..38ad61d8720b 100644
> > > --- a/sys/kern/imgact_elf.c
> > > +++ b/sys/kern/imgact_elf.c
> > > @@ -161,19 +161,33 @@ SYSCTL_NODE(__CONCAT(_kern_elf, __ELF_WORD_SIZE), OID_AUTO, aslr,
> > > "");
> > > #define ASLR_NODE_OID __CONCAT(__CONCAT(_kern_elf, __ELF_WORD_SIZE), _aslr)
> > > -static int __elfN(aslr_enabled) = 0;
> > > +/*
> > > + * While for 64-bit machines ASLR works properly, there are
> > > + * still some problems when using 32-bit architectures. For this
> > > + * reason ASLR is only enabled by default when running native
> > > + * 64-bit non-PIE executables.
> > > + */
> > > +static int __elfN(aslr_enabled) = __ELF_WORD_SIZE == 64;
> > > SYSCTL_INT(ASLR_NODE_OID, OID_AUTO, enable, CTLFLAG_RWTUN,
> > > &__elfN(aslr_enabled), 0,
> > > __XSTRING(__CONCAT(ELF, __ELF_WORD_SIZE))
> > > ": enable address map randomization");
> > > -static int __elfN(pie_aslr_enabled) = 0;
> > > +/*
> > > + * Enable ASLR only for 64-bit PIE binaries by default.
> > > + */
> > > +static int __elfN(pie_aslr_enabled) = __ELF_WORD_SIZE == 64;
> > > SYSCTL_INT(ASLR_NODE_OID, OID_AUTO, pie_enable, CTLFLAG_RWTUN,
> > > &__elfN(pie_aslr_enabled), 0,
> > > __XSTRING(__CONCAT(ELF, __ELF_WORD_SIZE))
> > > ": enable address map randomization for PIE binaries");
> >
> > The current description seems ambiguous with respect to the added comment.
> > If the sysctl (=1) applies ASLR "only" for PIE binaries, where the =0
> > (sysctl disabled) case applies it unconditionally, a better description
> > might be:
> >
> > "Enable address map randomization only for PIE binaries"
> >
> > What is the actual/correct behaviour of the control?
>
> It also doesn't make much sense to toggle AS{L}R for the different
> parts of an executable image. AS{L}R is an "all or nothing" thing.
> Really, there should be only a single toggle with four modes:
>
> 1. AS{L}R force disable
> 2. AS{L}R opt out
> 3. AS{L}R opt in
> 4. AS{L}R force enable
>
I think a single knob can simplify the situation, I'd need to take a
look if that's easily achievable.
What's the use case of the opt in/out options?
Best regards,
Marcin
> HardenedBSD has found that users get confused or are unsure of having
> too many toggles. "What happens when I do <X>?" In this case, you'd
> probably have to have deeper knowledge of how FreeBSD's AS{L}R is
> implemented. Having a single sysctl knob makes life easier for users
> and reduces implementation complexity.
>
> Thanks,
>
> --
> Shawn Webb
> Cofounder / Security Engineer
> HardenedBSD
>
> https://git.hardenedbsd.org/hardenedbsd/pubkeys/-/raw/master/Shawn_Webb/03A4CBEBB82EA5A67D9F3853FF2E67A277F8E1FA.pub.asc