[OpenWrt-Devel] MIPS stack security and other problems

[Rosen Penev]

On Tue, Jan 1, 2019 at 9:44 AM Hauke Mehrtens <hauke at hauke-m.de> wrote:
>
> On 12/18/18 12:46 PM, Hauke Mehrtens wrote:
> > On 12/17/18 1:54 AM, Dave Taht wrote:
> >>
> >> A pretty deep look at home MIPS and arm routers, and a surprising bug in Linux/MIPS - by mudge and co:
> >>
> >> https://cyber-itl.org/2018/12/07/a-look-at-home-routers-and-linux-mips.html
> >>
> >> I have no idea if current openwrt, or what prior releases... are subject to
> >> the problems they outline.
> >
> > In the second paper "Build Safety of Software in 28 Popular Home Router"
> > [0] they checked the "security" of multiple popular devices, by checking
> > if they activate ASLR, Non stack Exec, Relro and stack guards. The best
> > device was the Linksys wrt32x and this is based on OpenWrt with not so
> > many modifications. ;-) Just something like Samba downgrade to 3.0.37.
> > The paper also wonders why the other Linksys devices like the wrt1900ac
> > are much worse, but they probably do not use OpenWrt or a much older
> > version. The GPL source code tar.gz of the Linksys wrt32x, begins with
> > cloning from https://github.com/openwrt/openwrt.git
> >
> >
> > It is also interesting how different this approve to security checking
> > is to what the German BSI published in the "BSI TR-03148: Secure
> > Broadband Router:" [1].
> > You can build a device which scores 100% in the one and 0% in the other,
> > there is no overlap. ;-)
> >
> > Hauke
> >
> >
> > [0]:
> > https://cyber-itl.org/assets/papers/2018/build_safety_of_software_in_28_popular_home_routers.pdf
> > [1]:
> > https://www.bsi.bund.de/SharedDocs/Downloads/DE/BSI/Publikationen/TechnischeRichtlinien/TR03148/TR03148.pdf?__blob=publicationFile&v=2
>
> It looks like they ran checksec from
> http://github.com/slimm609/checksec.sh on the root file system of the
> devices and came up with these results. The numbers for the Linksys
> wrt32x look very similar to current OpenWrt master, even for MIPS CPUs.
>
> I attached two outputs of checksec to this mail from the lantiq target
> with a MIP24Kc CPU. One with master and the current default
> configuration and one with master + activated ASLR configuration option.
>
> You can generate these yourself like this:
> ../checksec.sh/checksec -d build_dir/target-mips_24kc_musl/root-lantiq/
This is not accurate for some of the busybox symlinks. Some of them
are linked to "busybox", giving the proper result. Others to
/bin/busybox which gives a result for the local machine. Maybe should
be fixed.
>
> ASLR increases the image size by about 2.8%:
> Without ASLR:   5.386.965 bytes
> With ASLR:      5.540.565 bytes
>
> This is caused by increased user space binary size, see for example
> busybox binary which is 7% bigger:
> Without ASLR:   425.532 bytes
> With ASLR:      457.336 bytes
>
> The fortified function count does not work with fortify-headers, but
> only with glibc. With glibc some function calls are getting replaced
> with calls to *_chk functions which are taking extra arguments, this is
> done by some glibc header magic. For musl libc OpenWrt uses
> fortify-headers which overwrites the original functions and inlined some
> extra security checks into the calling application. The result should be
> similar, so I assume that we have at least in most places similar
> security for the glibc fortified functions.
> I checked this by compiling an test application and checked the
> assembler code, it contained some extra size checks.
>
> It looks like the detection does not work correctly for kernel modules.
>
> Currently RELRO is not activated for the following libraries:
>         root-lantiq/usr/lib/libbz2.so.1.0
>         root-lantiq/usr/lib/libbz2.so.1.0.6
>         root-lantiq/lib/libgcc_s.so.1
> this looks like a bug.
>
> For libgcc_s.so.1 also NX is disabled, which is not good.
>
> Some binaries do not use a stack canary, I assume that these binaries
For ramips, it seems to be most binaries for me. Maybe something wrong
with the build settings...
> just do not have an array on the stack which could be exploited. The
> compiler adds stack canaries only to functions which the compiler thinks
> need it.
>
> ASLR is deactivated for root-lantiq/sbin/vdsl_cpe_control, because this
> application does not link any more when ASLR is activated, this is a bug
> in the package build system.
>
> Hauke
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