Re: [PATCH v2 8/9] kallsyms: Hide layout

[Kees Cook <keescook@chromium.org>]

On Thu, May 21, 2020 at 09:56:39AM -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@linux.intel.com>
> Reviewed-by: Tony Luck <tony.luck@intel.com>
> Tested-by: Tony Luck <tony.luck@intel.com>
> ---
>  kernel/kallsyms.c | 138 +++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 137 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/kallsyms.c b/kernel/kallsyms.c
> index 16c8c605f4b0..558963b275ec 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 +666,121 @@ int kallsyms_show_value(void)
>  	}
>  }
>  
> +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)
> +{
> +	unsigned long sym_pos;
> +	struct kallsyms_iter_list *node, *last;
> +	struct list_head *head = (struct list_head *)data;
> +
> +	node = kmalloc(sizeof(*node), GFP_KERNEL);
> +	if (!node)
> +		return -ENOMEM;
> +
> +	if (list_empty(head)) {
> +		sym_pos = 0;
> +		memset(node, 0, sizeof(*node));
> +		reset_iter(&node->iter, 0);
> +		node->iter.show_value = kallsyms_show_value();
> +	} else {
> +		last = list_first_entry(head, struct kallsyms_iter_list, next);
> +		memcpy(node, last, sizeof(*node));
> +		sym_pos = last->iter.pos;
> +	}
> +
> +	INIT_LIST_HEAD(&node->next);
> +	list_add(&node->next, head);
> +
> +	/*
> +	 * update_iter returns false when at end of file
> +	 * which in this case we don't care about and can
> +	 * safely ignore. update_iter() will increment
> +	 * the value of iter->pos, for ksymbol_core.
> +	 */
> +	if (sym_pos >= kallsyms_num_syms)
> +		sym_pos++;
> +
> +	(void)update_iter(&node->iter, sym_pos);
> +
> +	return 0;
> +}
> +
> +static int kallsyms_sorted_open(struct inode *inode, struct file *file)
> +{
> +	int ret;
> +	struct list_head *list;
> +
> +	list = __seq_open_private(file, &kallsyms_sorted_op, sizeof(*list));
> +	if (!list)
> +		return -ENOMEM;
> +
> +	INIT_LIST_HEAD(list);
> +
> +	ret = kallsyms_on_each_symbol(get_all_symbol_name, list);
> +	if (ret != 0)
> +		return ret;
> +
> +	list_sort(NULL, list, kallsyms_list_cmp);
> +
> +	return 0;
> +}
> +
>  static int kallsyms_open(struct inode *inode, struct file *file)
>  {
>  	/*
> @@ -704,9 +825,24 @@ static const struct proc_ops kallsyms_proc_ops = {
>  	.proc_release	= seq_release_private,
>  };
>  
> +static const struct proc_ops kallsyms_sorted_proc_ops = {
> +	.proc_open = kallsyms_sorted_open,
> +	.proc_read = seq_read,
> +	.proc_lseek = seq_lseek,
> +	.proc_release = seq_release_private,
> +};
> +
>  static int __init kallsyms_init(void)
>  {
> -	proc_create("kallsyms", 0444, NULL, &kallsyms_proc_ops);
> +	/*
> +	 * When fine grained kaslr is enabled, we need to
> +	 * print out the symbols sorted by name rather than by
> +	 * by address, because this reveals the randomization order.
> +	 */
> +	if (!IS_ENABLED(CONFIG_FG_KASLR))
> +		proc_create("kallsyms", 0444, NULL, &kallsyms_proc_ops);
> +	else
> +		proc_create("kallsyms", 0444, NULL, &kallsyms_sorted_proc_ops);
>  	return 0;

Since this is compile-time selected, instead of the separate name and
test here, how about just redefine kallsyms_open initializer instead?

#ifdef CONFIG_FG_KASLR
...sorting routines...
static int kallsyms_open(struct inode *inode, struct file *file)
{ ... sorted version ... }
#else
static int kallsyms_open(struct inode *inode, struct file *file)
{ ... normal version ... }
#endif

(And then just move the comment to the sorted open version.)

-- 
Kees Cook