[OpenWrt-Devel] [PATCH] build: Activate ASLR PIE by default

[Hauke Mehrtens]

On 2/13/19 11:51 PM, Felix Fietkau wrote:
> On 2019-02-13 23:15, Hauke Mehrtens wrote:
>> This will build all executable as Position Independent Executables (PIE)
>> by default. PIE executable can make full use of Address Space Layout
>> Randomization (ASLR) because all sections can be placed at random
>> offsets of the executed program. This makes it harder to exploit bugs
>> in our binaries.
>>
>> This will increase the size of executable, libraries are already build
>> position independent and their size will not change.
>>
>> This increases the size of the resulting images by about 3% on MIPS BE.
>> I tested this with the default configuration for the lantiq xrx200
>> target.
>>
>> The size of the initramfs binaries increased by 2.88%:
>> Without PIE:
>> 5.303.716 openwrt-lantiq-xrx200-bt_homehub-v5a-initramfs-kernel.bin
>> With PIE:
>> 5.456.339 openwrt-lantiq-xrx200-bt_homehub-v5a-initramfs-kernel.bin
>>
>> With PIE activated the executable are getting bigger, here are some
>> examples from the lantiq mips_24kc target:
>>
>> Without PIE:
>> 112.309 /bin/opkg
>> 299.061 /bin/busybox
>> 456.549 /usr/sbin/wpad
>>
>> With PIE:
>> 142.496 /bin/opkg       (26.87% increase)
>> 388.404 /bin/busybox    (29.87% increase)
>> 580.128 /usr/sbin/wpad  (27.06% increase)
>>
>> With PIE activated the sections of the binaries are loaded to
>> different offsets for each program instance like shown here:
>>
>> root at OpenWrt:/# cat /proc/self/maps
>> 555c4000-55622000 r-xp 00000000 00:02 1030       /bin/busybox
>> 55631000-55632000 r-xp 0005d000 00:02 1030       /bin/busybox
>> 55632000-55633000 rwxp 0005e000 00:02 1030       /bin/busybox
>> 55633000-55634000 rwxp 00000000 00:00 0
>> 77ee2000-77f04000 r-xp 00000000 00:02 331        /lib/libgcc_s.so.1
>> 77f04000-77f05000 r-xp 00012000 00:02 331        /lib/libgcc_s.so.1
>> 77f05000-77f06000 rwxp 00013000 00:02 331        /lib/libgcc_s.so.1
>> 77f06000-77f9a000 r-xp 00000000 00:02 329        /lib/libc.so
>> 77fa9000-77fab000 rwxp 00093000 00:02 329        /lib/libc.so
>> 77fab000-77fad000 rwxp 00000000 00:00 0
>> 7fb26000-7fb47000 rw-p 00000000 00:00 0          [stack]
>> 7fefb000-7fefc000 r-xp 00000000 00:00 0
>> 7ff0a000-7ff0b000 r--p 00000000 00:00 0          [vvar]
>> 7ff0b000-7ff0c000 r-xp 00000000 00:00 0          [vdso]
>> root at OpenWrt:/# cat /proc/self/maps
>> 5561d000-5567b000 r-xp 00000000 00:02 1030       /bin/busybox
>> 5568a000-5568b000 r-xp 0005d000 00:02 1030       /bin/busybox
>> 5568b000-5568c000 rwxp 0005e000 00:02 1030       /bin/busybox
>> 5568c000-5568d000 rwxp 00000000 00:00 0
>> 77e8e000-77eb0000 r-xp 00000000 00:02 331        /lib/libgcc_s.so.1
>> 77eb0000-77eb1000 r-xp 00012000 00:02 331        /lib/libgcc_s.so.1
>> 77eb1000-77eb2000 rwxp 00013000 00:02 331        /lib/libgcc_s.so.1
>> 77eb2000-77f46000 r-xp 00000000 00:02 329        /lib/libc.so
>> 77f55000-77f57000 rwxp 00093000 00:02 329        /lib/libc.so
>> 77f57000-77f59000 rwxp 00000000 00:00 0
>> 7fd1c000-7fd3d000 rw-p 00000000 00:00 0          [stack]
>> 7fefb000-7fefc000 r-xp 00000000 00:00 0
>> 7ff60000-7ff61000 r--p 00000000 00:00 0          [vvar]
>> 7ff61000-7ff62000 r-xp 00000000 00:00 0          [vdso]
>> root at OpenWrt:/#
>>
>> Signed-off-by: Hauke Mehrtens <hauke at hauke-m.de>
>> ---
>>
>> I would like to get some comments if we should activate PIE by default.
>> The advantage is that it will be harder to exploit OpenWrt, but on the 
>> other hand the binaries are getting bigger. We could also restrict this 
>> to some CPU types, but as targets share the binaries it is not really 
>> possible to do this based on the target.
>>
>> I am not sure if this should go into the next release or wait for later.
>>
>> This could also break some packages, as it is possible to activate PIE 
>> by default for some time many bugs are already fixed, but probably not 
>> all of them.
> I think this is a lot of extra bloat. Maybe we can add a restricted PIE
> mode where packages can opt-in individually?

So we should probably make it a chose with 3 options:
1. No PIE
2. Use PIE for exposed binaries
3. Use PIE for all binaries

Then we need something in addition to the existing PKG_ASLR_PIE we
already have to deactivate it.

Do we want a generic name like this:
PKG_CRITICAL
or something specific to PIE:
PKG_ASLR_PIE_PREFERED

Hauke

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