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