Re: [PATCH 6/8] mm: Allow arch specific arch_randomize_brk() with CONFIG_ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT

On 11/22/21 12:47, Christophe Leroy wrote:

Le 22/11/2021 à 12:22, Alex Ghiti a écrit :

quoted

Hi Christophe,

Le 22/11/2021 à 09:48, Christophe Leroy a écrit :

quoted

Commit e7142bf5d231 ("arm64, mm: make randomization selected by
generic topdown mmap layout") introduced a default version of
arch_randomize_brk() provided when
CONFIG_ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT is selected.

powerpc could select CONFIG_ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT
but needs to provide its own arch_randomize_brk().

In order to allow that, don't make
CONFIG_ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT select
CONFIG_ARCH_HAS_ELF_RANDOMIZE. Instead, ensure that
selecting CONFIG_ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT and
selecting CONFIG_ARCH_HAS_ELF_RANDOMIZE has the same effect.

This feels weird to me since if CONFIG_ARCH_HAS_ELF_RANDOMIZE is used somewhere else at some point, it is not natural to add CONFIG_ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT: can't we use a __weak function or a new CONFIG_ARCH_HAS_RANDOMIZE_BRK?

Yes I also found things a bit weird.

CONFIG_ARCH_HAS_RANDOMIZE_BRK could be an idea but how different would it be from CONFIG_ARCH_HAS_ELF_RANDOMIZE ? In fact I find it weird that CONFIG_ARCH_HAS_ELF_RANDOMIZE is selected by CONFIG_ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT and not by the arch itself.

IIRC, this was a request from Kees Cook who wanted to enforce this security measure.

On the other hand CONFIG_ARCH_HAS_ELF_RANDOMIZE also handles arch_mmap_rnd() and here we are talking about arch_randomize_brk() only.

In the begining I was thinking about adding a CONFIG_ARCH_WANT_DEFAULT_RANDOMIZE_BRK, but it was meaning adding it to the few other arches selecting CONFIG_ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT.

So I think I will go for the __weak function option.

Ok, thanks.

Alex

Thanks
Christophe