linux-kernel - Re: [PATCH v3 09/10] kallsyms: Hide layout

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Date:   Wed, 24 Jun 2020 00:20:57 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Kristen Carlson Accardi <kristen@...ux.intel.com>
Cc:     tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        arjan@...ux.intel.com, x86@...nel.org,
        linux-kernel@...r.kernel.org, kernel-hardening@...ts.openwall.com,
        rick.p.edgecombe@...el.com, Tony Luck <tony.luck@...el.com>
Subject: Re: [PATCH v3 09/10] kallsyms: Hide layout

On Tue, Jun 23, 2020 at 10:23:26AM -0700, Kristen Carlson Accardi wrote:
> This patch makes /proc/kallsyms display alphabetically by symbol
> name rather than sorted by address in order to hide the newly
> randomized address layout.
> 
> Signed-off-by: Kristen Carlson Accardi <kristen@...ux.intel.com>
> Reviewed-by: Tony Luck <tony.luck@...el.com>
> Tested-by: Tony Luck <tony.luck@...el.com>
> ---
>  kernel/kallsyms.c | 128 ++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 128 insertions(+)
> 
> diff --git a/kernel/kallsyms.c b/kernel/kallsyms.c
> index 16c8c605f4b0..df2b20e1b7f2 100644
> --- a/kernel/kallsyms.c
> +++ b/kernel/kallsyms.c
> @@ -25,6 +25,7 @@
>  #include <linux/filter.h>
>  #include <linux/ftrace.h>
>  #include <linux/compiler.h>
> +#include <linux/list_sort.h>
>  
>  /*
>   * These will be re-linked against their real values
> @@ -446,6 +447,11 @@ struct kallsym_iter {
>  	int show_value;
>  };
>  
> +struct kallsyms_iter_list {
> +	struct kallsym_iter iter;
> +	struct list_head next;
> +};
> +
>  int __weak arch_get_kallsym(unsigned int symnum, unsigned long *value,
>  			    char *type, char *name)
>  {
> @@ -660,6 +666,127 @@ int kallsyms_show_value(void)
>  	}
>  }

The #ifdef can be moved up to here:

#ifdef CONFIG_FG_KASLR

Otherwise without CONFIG_FG_KASLR, I get:

kernel/kallsyms.c:714:12: warning: ‘kallsyms_list_cmp’ defined but not used [-Wunused-function]
  714 | static int kallsyms_list_cmp(void *priv, struct list_head *a,
      |            ^~~~~~~~~~~~~~~~~


>  
> +static int sorted_show(struct seq_file *m, void *p)
> +{
> +	struct list_head *list = m->private;
> +	struct kallsyms_iter_list *iter;
> +	int rc;
> +
> +	if (list_empty(list))
> +		return 0;
> +
> +	iter = list_first_entry(list, struct kallsyms_iter_list, next);
> +
> +	m->private = iter;
> +	rc = s_show(m, p);
> +	m->private = list;
> +
> +	list_del(&iter->next);
> +	kfree(iter);
> +
> +	return rc;
> +}
> +
> +static void *sorted_start(struct seq_file *m, loff_t *pos)
> +{
> +	return m->private;
> +}
> +
> +static void *sorted_next(struct seq_file *m, void *p, loff_t *pos)
> +{
> +	struct list_head *list = m->private;
> +
> +	(*pos)++;
> +
> +	if (list_empty(list))
> +		return NULL;
> +
> +	return p;
> +}
> +
> +static const struct seq_operations kallsyms_sorted_op = {
> +	.start = sorted_start,
> +	.next = sorted_next,
> +	.stop = s_stop,
> +	.show = sorted_show
> +};
> +
> +static int kallsyms_list_cmp(void *priv, struct list_head *a,
> +			     struct list_head *b)
> +{
> +	struct kallsyms_iter_list *iter_a, *iter_b;
> +
> +	iter_a = list_entry(a, struct kallsyms_iter_list, next);
> +	iter_b = list_entry(b, struct kallsyms_iter_list, next);
> +
> +	return strcmp(iter_a->iter.name, iter_b->iter.name);
> +}
> +
> +int get_all_symbol_name(void *data, const char *name, struct module *mod,
> +			unsigned long addr)

This can be static too.

Otherwise, looks good!

Reviewed-by: Kees Cook <keescook@...omium.org>

-- 
Kees Cook