[v10,03/15] kallsyms: randomize /proc/kallsyms output order

Commit Message

Alexander Lobakin Feb. 9, 2022, 6:57 p.m. UTC

From: Kristen Carlson Accardi <[e-mail]>

For now, /proc/kallsyms printing code replaces the actual addresses
with zeros for !CAP_SYSLOG accesses, but still emits the symbols
sorted by address.
This makes it easier for an attacker to determine the address of a
desired function if the address of some other symbol is already
known. It is especially critical for FG-KASLR as the text layout
with it is random each boot and sorted /proc/kallsyms renders the
entire feature useless.

Make /proc/kallsyms display symbols in a random order to completely
hide the kernel layout. Determine the full number of symbols prior
to traversing through them and then shuffle the indexes, so the
printing function will pick them randomly.

Alexander Lobakin:

Don't depend FG-KASLR and always do that for unprivileged accesses
as suggested by several folks. Rename show_value to show_layout to
reflect the new functionality.
Also, introduce and use a generic shuffle_array() macro which
shuffles an arbitrary array using a Fisher-Yates algorithm.

Signed-off-by: Kristen Carlson Accardi <[e-mail]>
Reviewed-by: Tony Luck <[e-mail]>
Tested-by: Tony Luck <[e-mail]>
Reported-by: kernel test robot <[e-mail]> # swap.cocci
Suggested-by: Ard Biesheuvel <[e-mail]> # always do that
Suggested-by: Josh Poimboeuf <[e-mail]> # always do that
Suggested-by: Peter Zijlstra <[e-mail]> # always do that, macro
Co-developed-by: Alexander Lobakin <[e-mail]>
Signed-off-by: Alexander Lobakin <[e-mail]>
---
 include/linux/random.h | 16 +++++++
 kernel/kallsyms.c      | 94 +++++++++++++++++++++++++++++++++++-------
 2 files changed, 95 insertions(+), 15 deletions(-)

Patch

diff --git a/include/linux/random.h b/include/linux/random.h
index c45b2693e51f..596b27423150 100644
--- a/include/linux/random.h
+++ b/include/linux/random.h
@@ -110,6 +110,22 @@  declare_get_random_var_wait(long)
 
 unsigned long randomize_page(unsigned long start, unsigned long range);
 
+/**
+ * shuffle_array - use a Fisher-Yates algorithm to shuffle an array.
+ * @arr: pointer to the array
+ * @nents: the number of elements in the array
+ */
+#define shuffle_array(arr, nents) ({				\
+	typeof(&(arr)[0]) __arr = &(arr)[0];			\
+	size_t __i;						\
+								\
+	for (__i = (nents) - 1; __i > 0; __i--) {		\
+		size_t __j = get_random_long() % (__i + 1);	\
+								\
+		swap(__arr[__i], __arr[__j]);			\
+	}							\
+})
+
 /*
  * This is designed to be standalone for just prandom
  * users, but for now we include it from <linux/random.h>
diff --git a/kernel/kallsyms.c b/kernel/kallsyms.c
index 951c93216fc4..d991afa9b5bf 100644
--- a/kernel/kallsyms.c
+++ b/kernel/kallsyms.c
@@ -575,13 +575,15 @@  struct kallsym_iter {
 	loff_t pos_mod_end;
 	loff_t pos_ftrace_mod_end;
 	loff_t pos_bpf_end;
+	loff_t pos_end;
 	unsigned long value;
 	unsigned int nameoff; /* If iterating in core kernel symbols. */
 	char type;
 	char name[KSYM_NAME_LEN];
 	char module_name[MODULE_NAME_LEN];
 	int exported;
-	int show_value;
+	int show_layout:1;
+	loff_t shuffled_pos[];
 };
 
 int __weak arch_get_kallsym(unsigned int symnum, unsigned long *value,
@@ -661,11 +663,19 @@  static int get_ksymbol_bpf(struct kallsym_iter *iter)
  */
 static int get_ksymbol_kprobe(struct kallsym_iter *iter)
 {
+	int ret;
+
 	strlcpy(iter->module_name, "__builtin__kprobes", MODULE_NAME_LEN);
 	iter->exported = 0;
-	return kprobe_get_kallsym(iter->pos - iter->pos_bpf_end,
-				  &iter->value, &iter->type,
-				  iter->name) < 0 ? 0 : 1;
+	ret = kprobe_get_kallsym(iter->pos - iter->pos_bpf_end,
+				 &iter->value, &iter->type,
+				 iter->name);
+	if (ret < 0) {
+		iter->pos_end = iter->pos;
+		return 0;
+	}
+
+	return 1;
 }
 
 /* Returns space to next name. */
@@ -688,11 +698,16 @@  static void reset_iter(struct kallsym_iter *iter, loff_t new_pos)
 	iter->name[0] = '\0';
 	iter->nameoff = get_symbol_offset(new_pos);
 	iter->pos = new_pos;
+
+	if (!iter->show_layout)
+		return;
+
 	if (new_pos == 0) {
 		iter->pos_arch_end = 0;
 		iter->pos_mod_end = 0;
 		iter->pos_ftrace_mod_end = 0;
 		iter->pos_bpf_end = 0;
+		iter->pos_end = 0;
 	}
 }
 
@@ -721,13 +736,23 @@  static int update_iter_mod(struct kallsym_iter *iter, loff_t pos)
 	    get_ksymbol_bpf(iter))
 		return 1;
 
-	return get_ksymbol_kprobe(iter);
+	if ((!iter->pos_end || iter->pos_end > pos) &&
+	    get_ksymbol_kprobe(iter))
+		return 1;
+
+	return 0;
 }
 
 /* Returns false if pos at or past end of file. */
 static int update_iter(struct kallsym_iter *iter, loff_t pos)
 {
-	/* Module symbols can be accessed randomly. */
+	if (!iter->show_layout) {
+		if (pos > iter->pos_end)
+			return 0;
+
+		pos = iter->shuffled_pos[pos];
+	}
+
 	if (pos >= kallsyms_num_syms)
 		return update_iter_mod(iter, pos);
 
@@ -770,7 +795,7 @@  static int s_show(struct seq_file *m, void *p)
 	if (!iter->name[0])
 		return 0;
 
-	value = iter->show_value ? (void *)iter->value : NULL;
+	value = iter->show_layout ? (void *)iter->value : NULL;
 
 	if (iter->module_name[0]) {
 		char type;
@@ -807,9 +832,10 @@  static inline int kallsyms_for_perf(void)
 }
 
 /*
- * We show kallsyms information even to normal users if we've enabled
- * kernel profiling and are explicitly not paranoid (so kptr_restrict
- * is clear, and sysctl_perf_event_paranoid isn't set).
+ * We show kallsyms information and display them sorted by address even
+ * to normal users if we've enabled kernel profiling and are explicitly
+ * not paranoid (so kptr_restrict is clear, and sysctl_perf_event_paranoid
+ * isn't set).
  *
  * Otherwise, require CAP_SYSLOG (assuming kptr_restrict isn't set to
  * block even that).
@@ -839,16 +865,53 @@  static int kallsyms_open(struct inode *inode, struct file *file)
 	 * using get_symbol_offset for every symbol.
 	 */
 	struct kallsym_iter *iter;
-	iter = __seq_open_private(file, &kallsyms_op, sizeof(*iter));
-	if (!iter)
-		return -ENOMEM;
-	reset_iter(iter, 0);
+	/*
+	 * This fake iter is needed for the cases with unprivileged
+	 * access. We need to know the exact number of symbols to
+	 * randomize the display layout.
+	 */
+	struct kallsym_iter fake;
+	size_t size = sizeof(*iter);
+	loff_t pos;
+
+	fake.show_layout = true;
+	reset_iter(&fake, 0);
 
 	/*
 	 * Instead of checking this on every s_show() call, cache
 	 * the result here at open time.
 	 */
-	iter->show_value = kallsyms_show_value(file->f_cred);
+	fake.show_layout = kallsyms_show_value(file->f_cred);
+	if (!fake.show_layout) {
+		pos = kallsyms_num_syms;
+		while (update_iter_mod(&fake, pos))
+			pos++;
+
+		size = struct_size(iter, shuffled_pos, fake.pos_end + 1);
+	}
+
+	iter = __seq_open_private(file, &kallsyms_op, size);
+	if (!iter)
+		return -ENOMEM;
+
+	iter->show_layout = fake.show_layout;
+	reset_iter(iter, 0);
+
+	if (iter->show_layout)
+		return 0;
+
+	/* Copy the bounds since they were already discovered above */
+	iter->pos_arch_end = fake.pos_arch_end;
+	iter->pos_mod_end = fake.pos_mod_end;
+	iter->pos_ftrace_mod_end = fake.pos_ftrace_mod_end;
+	iter->pos_bpf_end = fake.pos_bpf_end;
+	iter->pos_end = fake.pos_end;
+
+	for (pos = 0; pos <= iter->pos_end; pos++)
+		iter->shuffled_pos[pos] = pos;
+
+	shuffle_array(iter->shuffled_pos, iter->pos_end + 1);
+
 	return 0;
 }
 
@@ -859,6 +922,7 @@  const char *kdb_walk_kallsyms(loff_t *pos)
 	if (*pos == 0) {
 		memset(&kdb_walk_kallsyms_iter, 0,
 		       sizeof(kdb_walk_kallsyms_iter));
+		kdb_walk_kallsyms_iter.show_layout = true;
 		reset_iter(&kdb_walk_kallsyms_iter, 0);
 	}
 	while (1) {