Re: [PATCH v2 0/5] Use ELF_ET_DYN_BASE only for PIE

[Kees Cook]

On Sat, Jun 24, 2017 at 2:11 AM, Russell King - ARM Linux
<linux@xxxxxxxxxxxxxxx> wrote:
> On Fri, Jun 23, 2017 at 01:59:55PM -0700, Kees Cook wrote:
>> This is v2 (to refresh the 5 patches in -mm) for moving ELF_ET_DYN_BASE
>> safely lower. Changes are clarifications in the commit logs (suggested
>> by mpe), a compat think-o fix for arm64 (thanks to Ard), and to add
>> Rik and mpe's Acks.
>>
>> Quoting patch 1/5:
>>
>> The ELF_ET_DYN_BASE position was originally intended to keep loaders
>> away from ET_EXEC binaries. (For example, running "/lib/ld-linux.so.2
>> /bin/cat" might cause the subsequent load of /bin/cat into where the
>> loader had been loaded.) With the advent of PIE (ET_DYN binaries with
>> an INTERP Program Header), ELF_ET_DYN_BASE continued to be used since
>> the kernel was only looking at ET_DYN. However, since ELF_ET_DYN_BASE
>> is traditionally set at the top 1/3rd of the TASK_SIZE, a substantial
>> portion of the address space is unused.
>
> With existing kernels on ARM:
>
> 00010000-00017000 r-xp 00000000 08:01 270810     /bin/cat
> 00026000-00027000 r--p 00006000 08:01 270810     /bin/cat
> 00027000-00028000 rw-p 00007000 08:01 270810     /bin/cat
> 7f661000-7f679000 r-xp 00000000 08:01 281659     /lib/arm-linux-gnueabihf/ld-2.23.so
> 7f688000-7f689000 r--p 00017000 08:01 281659     /lib/arm-linux-gnueabihf/ld-2.23.so
> 7f689000-7f68a000 rw-p 00018000 08:01 281659     /lib/arm-linux-gnueabihf/ld-2.23.so
>
> If the loader is loaded at 4MB, this means the size of an ET_EXEC
> program is limited to less than 4MB - and distros aren't yet
> building everything as PIE on ARM.

The loader isn't loaded at 4MB; that's what patch 1 changes: loaders
are moved into the mmap region so they will not collide with either
ET_EXEC nor PIE (ET_DYN-with-INTERP).

(After this patch, the name "ELF_ET_DYN_BASE" becomes a bit misleading...)

-Kees

-- 
Kees Cook
Pixel Security