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

Ard Biesheuvel
2020-04-22T16:23:15Z

On Mon, 20 Apr 2020 at 19:59, Kristen Carlson Accardi
<kristen@linux.intel.com> 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@linux.intel.com> 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@linux.intel.com>
> > > > Reviewed-by: Kees Cook <keescook@chromium.org>
> > > > ---
> > > > 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 should have added - thank you for pointing this out, I will fix it in
> the next version. I was on a tangent about whether you should even use
> this without the main kernel randomization :).
>

Yeah. I was just pointing out that this breaks the build for all other
architectures :-)