Re: [PATCH 9/9] module: Reorder functions

[Kristen Carlson Accardi]

On Mon, 2020-04-20 at 14:01 +0200, Ard Biesheuvel wrote:
> On Wed, 15 Apr 2020 at 23:07, Kristen Carlson Accardi
> <kristen@xxxxxxxxxxxxxxx> wrote:
> > If a module has functions split out into separate text sections
> > (i.e. compiled with the -ffunction-sections flag), reorder the
> > functions to provide some code diversification to modules.
> > 
> 
> Is that the only prerequisite? I.e., is it sufficient for another
> architecture to add -ffunction-sections to the module CFLAGS to get
> this functionality? (assuming it defines CONFIG_FG_KASLR=y)

I think it would work for modules. I've not tested this of course. It
might not make sense for some architectures (like 32 bit), but it would
probably work.

> 
> > Signed-off-by: Kristen Carlson Accardi <kristen@xxxxxxxxxxxxxxx>
> > Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
> > ---
> >  kernel/module.c | 82
> > +++++++++++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 82 insertions(+)
> > 
> > diff --git a/kernel/module.c b/kernel/module.c
> > index 646f1e2330d2..e432ec5f6df4 100644
> > --- a/kernel/module.c
> > +++ b/kernel/module.c
> > @@ -53,6 +53,8 @@
> >  #include <linux/bsearch.h>
> >  #include <linux/dynamic_debug.h>
> >  #include <linux/audit.h>
> > +#include <linux/random.h>
> > +#include <asm/setup.h>
> >  #include <uapi/linux/module.h>
> >  #include "module-internal.h"
> > 
> > @@ -2370,6 +2372,83 @@ static long get_offset(struct module *mod,
> > unsigned int *size,
> >         return ret;
> >  }
> > 
> > +/*
> > + * shuffle_text_list()
> > + * Use a Fisher Yates algorithm to shuffle a list of text
> > sections.
> > + */
> > +static void shuffle_text_list(Elf_Shdr **list, int size)
> > +{
> > +       int i;
> > +       unsigned int j;
> > +       Elf_Shdr *temp;
> > +
> > +       for (i = size - 1; i > 0; i--) {
> > +               /*
> > +                * pick a random index from 0 to i
> > +                */
> > +               get_random_bytes(&j, sizeof(j));
> > +               j = j % (i + 1);
> > +
> > +               temp = list[i];
> > +               list[i] = list[j];
> > +               list[j] = temp;
> > +       }
> > +}
> > +
> > +/*
> > + * randomize_text()
> > + * Look through the core section looking for executable code
> > sections.
> > + * Store sections in an array and then shuffle the sections
> > + * to reorder the functions.
> > + */
> > +static void randomize_text(struct module *mod, struct load_info
> > *info)
> > +{
> > +       int i;
> > +       int num_text_sections = 0;
> > +       Elf_Shdr **text_list;
> > +       int size = 0;
> > +       int max_sections = info->hdr->e_shnum;
> > +       unsigned int sec = find_sec(info, ".text");
> > +
> > +       if (sec == 0)
> > +               return;
> > +
> > +       text_list = kmalloc_array(max_sections, sizeof(*text_list),
> > GFP_KERNEL);
> > +       if (text_list == NULL)
> > +               return;
> > +
> > +       for (i = 0; i < max_sections; i++) {
> > +               Elf_Shdr *shdr = &info->sechdrs[i];
> > +               const char *sname = info->secstrings + shdr-
> > >sh_name;
> > +
> > +               if (!(shdr->sh_flags & SHF_ALLOC) ||
> > +                   !(shdr->sh_flags & SHF_EXECINSTR) ||
> > +                   strstarts(sname, ".init"))
> > +                       continue;
> > +
> > +               text_list[num_text_sections] = shdr;
> > +               num_text_sections++;
> > +       }
> > +
> > +       shuffle_text_list(text_list, num_text_sections);
> > +
> > +       for (i = 0; i < num_text_sections; i++) {
> > +               Elf_Shdr *shdr = text_list[i];
> > +
> > +               /*
> > +                * get_offset has a section index for it's last
> > +                * argument, that is only used by
> > arch_mod_section_prepend(),
> > +                * which is only defined by parisc. Since this this
> > type
> > +                * of randomization isn't supported on parisc, we
> > can
> > +                * safely pass in zero as the last argument, as it
> > is
> > +                * ignored.
> > +                */
> > +               shdr->sh_entsize = get_offset(mod, &size, shdr, 0);
> > +       }
> > +
> > +       kfree(text_list);
> > +}
> > +
> >  /* Lay out the SHF_ALLOC sections in a way not dissimilar to how
> > ld
> >     might -- code, read-only data, read-write data, small
> > data.  Tally
> >     sizes, and place the offsets into sh_entsize fields: high bit
> > means it
> > @@ -2460,6 +2539,9 @@ static void layout_sections(struct module
> > *mod, struct load_info *info)
> >                         break;
> >                 }
> >         }
> > +
> > +       if (IS_ENABLED(CONFIG_FG_KASLR) && kaslr_enabled())
> 
> kaslr_enabled() only exists [as a function] on x86

CONFIG_FG_KASLR is dependant on x86_64. If people really think there is
value in having the module randomization not dependent on the kernel
randomization it can be changed to a different config option - but I am
not sure that there is a ton of value in the module randomization on
it's own.

> 
> 
> > +               randomize_text(mod, info);
> >  }
> > 
> >  static void set_license(struct module *mod, const char *license)
> > --
> > 2.20.1
> >