[PATCH v8 07/14] kallsyms: Hide layout

Alexander Lobakin
2021-12-02T22:33:26Z

From: Kristen Carlson Accardi <kristen@linux.intel.com>

This patch makes /proc/kallsyms display in a random order, 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>
Reported-by: kernel test robot <lkp@intel.com> # swap.cocci
Signed-off-by: Alexander Lobakin <alexandr.lobakin@intel.com>
---
kernel/kallsyms.c | 158 +++++++++++++++++++++++++++++++++++++++++++++-
1 file changed, 157 insertions(+), 1 deletion(-)

diff --git a/kernel/kallsyms.c b/kernel/kallsyms.c
index 3011bc33a5ba..ff9d8b651966 100644
--- a/kernel/kallsyms.c
+++ b/kernel/kallsyms.c
@@ -583,6 +583,12 @@ struct kallsym_iter {
int show_value;
};

+struct kallsyms_shuffled_iter {
+ struct kallsym_iter iter;
+ loff_t total_syms;
+ loff_t shuffled_index[];
+};
+
int __weak arch_get_kallsym(unsigned int symnum, unsigned long *value,
char *type, char *name)
{
@@ -830,7 +836,7 @@ bool kallsyms_show_value(const struct cred *cred)
}
}

-static int kallsyms_open(struct inode *inode, struct file *file)
+static int __kallsyms_open(struct inode *inode, struct file *file)
{
/*
* We keep iterator in m->private, since normal case is to
@@ -851,6 +857,156 @@ static int kallsyms_open(struct inode *inode, struct file *file)
return 0;
}

+/*
+ * When function granular kaslr is enabled, we need to print out the symbols
+ * at random so we don't reveal the new layout.
+ */
+#ifdef CONFIG_FG_KASLR
+static int update_random_pos(struct kallsyms_shuffled_iter *s_iter,
+ loff_t pos, loff_t *new_pos)
+{
+ loff_t new;
+
+ if (pos >= s_iter->total_syms)
+ return 0;
+
+ new = s_iter->shuffled_index[pos];
+
+ /*
+ * normally this would be done as part of update_iter, however,
+ * we want to avoid triggering this in the event that new is
+ * zero since we don't want to blow away our pos end indicators.
+ */
+ if (new == 0) {
+ s_iter->iter.name[0] = '\0';
+ s_iter->iter.nameoff = get_symbol_offset(new);
+ s_iter->iter.pos = new;
+ }
+
+ *new_pos = new;
+ return 1;
+}
+
+static void *shuffled_start(struct seq_file *m, loff_t *pos)
+{
+ struct kallsyms_shuffled_iter *s_iter = m->private;
+ loff_t new_pos;
+
+ if (!update_random_pos(s_iter, *pos, &new_pos))
+ return NULL;
+
+ return s_start(m, &new_pos);
+}
+
+static void *shuffled_next(struct seq_file *m, void *p, loff_t *pos)
+{
+ struct kallsyms_shuffled_iter *s_iter = m->private;
+ loff_t new_pos;
+
+ (*pos)++;
+
+ if (!update_random_pos(s_iter, *pos, &new_pos))
+ return NULL;
+
+ if (!update_iter(m->private, new_pos))
+ return NULL;
+
+ return p;
+}
+
+/*
+ * shuffle_index_list()
+ * Use a Fisher Yates algorithm to shuffle a list of text sections.
+ */
+static void shuffle_index_list(loff_t *indexes, loff_t size)
+{
+ u32 i, j;
+
+ for (i = size - 1; i > 0; i--) {
+ /* pick a random index from 0 to i */
+ j = get_random_u32() % (i + 1);
+
+ swap(indexes[i], indexes[j]);
+ }
+}
+
+static const struct seq_operations kallsyms_shuffled_op = {
+ .start = shuffled_start,
+ .next = shuffled_next,
+ .stop = s_stop,
+ .show = s_show
+};
+
+static int kallsyms_random_open(struct inode *inode, struct file *file)
+{
+ loff_t pos;
+ struct kallsyms_shuffled_iter *shuffled_iter;
+ struct kallsym_iter iter;
+ bool show_value;
+
+ /*
+ * If privileged, go ahead and use the normal algorithm for
+ * displaying symbols
+ */
+ show_value = kallsyms_show_value(file->f_cred);
+ if (show_value)
+ return __kallsyms_open(inode, file);
+
+ /*
+ * we need to figure out how many extra symbols there are
+ * to print out past kallsyms_num_syms
+ */
+ pos = kallsyms_num_syms;
+ reset_iter(&iter, 0);
+ do {
+ if (!update_iter(&iter, pos))
+ break;
+ pos++;
+ } while (1);
+
+ /*
+ * add storage space for an array of loff_t equal to the size
+ * of the total number of symbols we need to print
+ */
+ shuffled_iter = __seq_open_private(file, &kallsyms_shuffled_op,
+ sizeof(*shuffled_iter) +
+ (sizeof(pos) * pos));
+ if (!shuffled_iter)
+ return -ENOMEM;
+
+ reset_iter(&shuffled_iter->iter, 0);
+ shuffled_iter->iter.show_value = show_value;
+ shuffled_iter->total_syms = pos;
+
+ /*
+ * the existing update_iter algorithm requires that we
+ * are either moving along increasing pos sequentially,
+ * or that these values are correct. Since these values
+ * were discovered above, initialize our new iter so we
+ * can use update_iter non-sequentially.
+ */
+ shuffled_iter->iter.pos_arch_end = iter.pos_arch_end;
+ shuffled_iter->iter.pos_mod_end = iter.pos_mod_end;
+ shuffled_iter->iter.pos_ftrace_mod_end = iter.pos_ftrace_mod_end;
+
+ /*
+ * initialize the array with all possible pos values, then
+ * shuffle the array so that the values will display in a random
+ * order.
+ */
+ for (pos = 0; pos < shuffled_iter->total_syms; pos++)
+ shuffled_iter->shuffled_index[pos] = pos;
+
+ shuffle_index_list(shuffled_iter->shuffled_index, shuffled_iter->total_syms);
+
+ return 0;
+}
+
+#define kallsyms_open kallsyms_random_open
+#else
+#define kallsyms_open __kallsyms_open
+#endif /* !CONFIG_FG_KASLR */
+
#ifdef CONFIG_KGDB_KDB
const char *kdb_walk_kallsyms(loff_t *pos)
{
--
2.33.1