[PATCH v9 03/15] kallsyms: Hide layout

Alexander Lobakin
2021-12-23T00:24:08Z

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.

alobakin:
Don't depend FG-KASLR and always do that for unpriviledged accesses
as suggested by several folks.
Also, introduce and use a shuffle_array() macro which shuffles an
array using Fisher-Yates. We'll make use of it several more times
later on.

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
Suggested-by: Ard Biesheuvel <ardb@kernel.org> # always do that
Suggested-by: Josh Poimboeuf <jpoimboe@redhat.com> # always do that
Suggested-by: Peter Zijlstra <peterz@infradead.org> # always do that, macro
Co-developed-by: Alexander Lobakin <alexandr.lobakin@intel.com>
Signed-off-by: Alexander Lobakin <alexandr.lobakin@intel.com>
---
include/linux/random.h | 16 ++++++++
kernel/kallsyms.c | 93 ++++++++++++++++++++++++++++++++++--------
2 files changed, 93 insertions(+), 16 deletions(-)

diff --git a/include/linux/random.h b/include/linux/random.h
index f45b8be3e3c4..c859a698089c 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 3011bc33a5ba..5d41b993113f 100644
--- a/kernel/kallsyms.c
+++ b/kernel/kallsyms.c
@@ -574,13 +574,15 @@ 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;
+ bool show_layout;
+ loff_t shuffled_pos[];
};

int __weak arch_get_kallsym(unsigned int symnum, unsigned long *value,
@@ -660,11 +662,19 @@ 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. */
@@ -687,11 +697,12 @@ 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 (new_pos == 0) {
+ if (iter->show_layout && 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;
}
}

@@ -720,13 +731,23 @@ 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);

@@ -769,7 +790,7 @@ 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;
@@ -806,9 +827,10 @@ 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).
@@ -838,16 +860,54 @@ 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)
+ goto open;
+
+ for (pos = kallsyms_num_syms; update_iter_mod(&fake, pos); pos++)
+ ;
+
+ size = struct_size(iter, shuffled_pos, fake.pos_end + 1);
+
+open:
+ 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;
}

@@ -858,6 +918,7 @@ 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) {
--
2.33.1