[PATCH 8/9] kallsyms: hide layout

Kristen Carlson Accardi
2020-04-15T21:06:57Z

To support finer grained kaslr (fgkaslr), we need to hide our sorted
list of symbols, since this will give away our new layout.
This patch makes /proc/kallsyms only visible to priviledged users.

Signed-off-by: Kristen Carlson Accardi <kristen@linux.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..861972b6a879 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;
}
device_initcall(kallsyms_init);
--
2.20.1