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

[Kristen Carlson Accardi]

On Wed, 2020-04-22 at 18:22 +0200, Ard Biesheuvel wrote:
> On Mon, 20 Apr 2020 at 19:59, Kristen Carlson Accardi
> <kristen@xxxxxxxxxxxxxxx> wrote:
> > On Mon, 2020-04-20 at 10:56 -0700, Kristen Carlson Accardi wrote:
> > > 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.
> 
> I think there is. The modules are a sizable attack surface, and made
> up of drivers for a large part, many of which are not as carefully
> reviewed as core code. Also, as I pointed out, the ELF loading trick
> from the decompressor is simply infeasible on arm64, given that there
> is no decompressor in the first place.

I can make a separate config option for modules so that you can enable
this separately from the main kernel text randomization. I'll make that
change for v2.