Re: git: b014e0f15bc7 - main - Enable ASLR by default for 64-bit executables
- In reply to: Kubilay Kocak : "Re: git: b014e0f15bc7 - main - Enable ASLR by default for 64-bit executables"
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Date: Wed, 17 Nov 2021 20:17:51 UTC
Hi,
śr., 17 lis 2021 o 00:42 Kubilay Kocak <koobs@freebsd.org> napisał(a):
>
> 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?
>
The lower comment gets probably more context when read read together
with the one preceding `aslr_enabled` variable.
According to `man security`:
kern.elf64.aslr.enable 64bit binaries ASLR control.
kern.elf64.aslr.pie_enable 64bit PIE binaries ASLR control.
So both are enabled by setting to 1 and responsible for the different
kind of binaries.
> Might aslr_enabled_pie_only also be a better OID name? Perhaps not worth
> the churn, but long term it would be great if OID names reflected what
> they are/do, rather than what they're not/don't do.
>
I agree that aslr_enable + pie_aslr_enable is somewhat confusing. And
if it's confusing for the kernel developers, we cannot expect it's
easier for the end users.
> > -static int __elfN(aslr_honor_sbrk) = 1;
> > +/*
> > + * Sbrk is now deprecated and it can be assumed, that in most
> > + * cases it will not be used anyway. This setting is valid only
> > + * for the ASLR enabled and allows for utilizing the bss grow region.
> > + */
> > +static int __elfN(aslr_honor_sbrk) = 0;
> > SYSCTL_INT(ASLR_NODE_OID, OID_AUTO, honor_sbrk, CTLFLAG_RW,
> > &__elfN(aslr_honor_sbrk), 0,
> > __XSTRING(__CONCAT(ELF, __ELF_WORD_SIZE)) ": assume sbrk is used");
> >
>
> Can we add (DEPRECATED) to the control description, and/or otherwise
> mark the control as deprecated if the sysctl framework supports an
> attribute marking them as such?
>
Do you mean the SBRK is deprecated or the tunable itself?
Best regards,
Marcin