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SubjectRe: [PATCH 1/2] module: Prepare for addition of new ro_after_init sections
On Tue, Apr 9, 2019 at 6:14 PM Joel Fernandes (Google)
<joel@joelfernandes.org> wrote:
>
> For the purposes of hardening modules by adding sections to
> ro_after_init sections, prepare for addition of new ro_after_init
> entries which we do in future patches. Create a table to which new
> entries could be added later. This makes it less error prone and reduce
> code duplication.
>
> Cc: paulmck@linux.vnet.ibm.com
> Cc: rostedt@goodmis.org
> Cc: mathieu.desnoyers@efficios.com
> Cc: rcu@vger.kernel.org
> Cc: kernel-hardening@lists.openwall.com
> Cc: kernel-team@android.com
> Suggested-by: keescook@chromium.org
> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
>
> ---
> kernel/module.c | 42 ++++++++++++++++++++++++------------------
> 1 file changed, 24 insertions(+), 18 deletions(-)
>
> diff --git a/kernel/module.c b/kernel/module.c
> index 524da609c884..f9221381d076 100644
> --- a/kernel/module.c
> +++ b/kernel/module.c
> @@ -3300,11 +3300,28 @@ static bool blacklisted(const char *module_name)
> }
> core_param(module_blacklist, module_blacklist, charp, 0400);
>
> +/*
> + * Mark ro_after_init section with SHF_RO_AFTER_INIT so that
> + * layout_sections() can put it in the right place.
> + * Note: ro_after_init sections also have SHF_{WRITE,ALLOC} set.
> + */
> +static char *ro_after_init_sections[] = {

static const char * const ... Need to make sure the table and its
strings can't be changed. :)

> + ".data..ro_after_init",
> +
> + /*
> + * __jump_table structures are never modified, with the exception of
> + * entries that refer to code in the __init section, which are
> + * annotated as such at module load time.
> + */
> + "__jump_table",
> + NULL

Since this table is known at build time, you don't need a NULL
terminator, you can use ARRAY_SIZE() instead.

> +};
> +
> static struct module *layout_and_allocate(struct load_info *info, int flags)
> {
> struct module *mod;
> unsigned int ndx;
> - int err;
> + int err, i;
>
> err = check_modinfo(info->mod, info, flags);
> if (err)
> @@ -3319,23 +3336,12 @@ static struct module *layout_and_allocate(struct load_info *info, int flags)
> /* We will do a special allocation for per-cpu sections later. */
> info->sechdrs[info->index.pcpu].sh_flags &= ~(unsigned long)SHF_ALLOC;
>
> - /*
> - * Mark ro_after_init section with SHF_RO_AFTER_INIT so that
> - * layout_sections() can put it in the right place.
> - * Note: ro_after_init sections also have SHF_{WRITE,ALLOC} set.
> - */
> - ndx = find_sec(info, ".data..ro_after_init");
> - if (ndx)
> - info->sechdrs[ndx].sh_flags |= SHF_RO_AFTER_INIT;
> - /*
> - * Mark the __jump_table section as ro_after_init as well: these data
> - * structures are never modified, with the exception of entries that
> - * refer to code in the __init section, which are annotated as such
> - * at module load time.
> - */
> - ndx = find_sec(info, "__jump_table");
> - if (ndx)
> - info->sechdrs[ndx].sh_flags |= SHF_RO_AFTER_INIT;
> + /* Set sh_flags for read-only after init sections */
> + for (i = 0; ro_after_init_sections[i]; i++) {

i.e. for (i = 0; i < ARRAY_SIZE(ro_after_init_sections); i++)

> + ndx = find_sec(info, ro_after_init_sections[i]);
> + if (ndx)
> + info->sechdrs[ndx].sh_flags |= SHF_RO_AFTER_INIT;
> + }
>
> /* Determine total sizes, and put offsets in sh_entsize. For now
> this is done generically; there doesn't appear to be any

Otherwise, yeah, looks good to me.

--
Kees Cook

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