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

[Russell King - ARM Linux]

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.

-- 
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