Re: [PATCH v2] mm: mmap: allow for the maximum number of bits for randomizing mmap_base by default

[Rafael Aquini <aquini@redhat.com>]

On Tue, Jun 11, 2024 at 07:43:12PM -0700, Andrew Morton wrote:
> On Tue, 11 Jun 2024 21:35:24 -0400 Rafael Aquini <aquini@redhat.com> wrote:
> 
> > > Shouldn't we make this the default (at least for 32-bit) so the
> > > regressed kernels are fixed simply by applying this patch?
> > > 
> > 
> > That is a fair take, indeed. I guess we could do something like
> > 
> >  config FORCE_MAX_MMAP_RND_BITS
> >         bool "Force maximum number of bits to use for ASLR of mmap base address"
> > -       default n
> > +       default y if !64BIT
> 
> "something like" != "exhaustively tested" ;)
> 
> I'll toss that in there, but please do let me know when it is suitable
> for an upstream merge.
>

The follow-up patch does work as intended:

  [raquini@optiplex-lnx linux]$ make ARCH=i386 defconfig
  *** Default configuration is based on 'i386_defconfig'
  #
  # configuration written to .config
  #
  [raquini@optiplex-lnx linux]$ grep MMAP_RND_BITS .config
  CONFIG_ARCH_MMAP_RND_BITS_MIN=8
  CONFIG_ARCH_MMAP_RND_BITS_MAX=16
  CONFIG_HAVE_ARCH_MMAP_RND_BITS=y
  # CONFIG_FORCE_MAX_MMAP_RND_BITS is not set
  CONFIG_ARCH_MMAP_RND_BITS=8
  
  [raquini@optiplex-lnx linux]$ patch -Np1 < patch.diff 
  patching file arch/Kconfig
  [raquini@optiplex-lnx linux]$ make ARCH=i386 defconfig
  *** Default configuration is based on 'i386_defconfig'
  #
  # configuration written to .config
  #
  [raquini@optiplex-lnx linux]$ grep MMAP_RND_BITS .config
  CONFIG_ARCH_MMAP_RND_BITS_MIN=8
  CONFIG_ARCH_MMAP_RND_BITS_MAX=16
  CONFIG_HAVE_ARCH_MMAP_RND_BITS=y
  CONFIG_FORCE_MAX_MMAP_RND_BITS=y
  CONFIG_ARCH_MMAP_RND_BITS=16

 
> I also added cc:stable.
>

Thank you, Andrew.