Re: [PATCH -next] cgroup: fix uaf when proc_cpuset_show

Waiman Long <longman@xxxxxxxxxx> · Mon, 24 Jun 2024 22:40:25 -0400

On 6/24/24 21:46, chenridong wrote:

On 2024/6/25 7:59, Waiman Long wrote:
On 6/23/24 22:59, chenridong wrote:

On 2024/6/22 23:05, Waiman Long wrote:

On 6/22/24 07:38, Chen Ridong wrote:
We found a refcount UAF bug as follows:

BUG: KASAN: use-after-free in cgroup_path_ns+0x112/0x150
Read of size 8 at addr ffff8882a4b242b8 by task atop/19903

CPU: 27 PID: 19903 Comm: atop Kdump: loaded Tainted: GF
Call Trace:
  dump_stack+0x7d/0xa7
  print_address_description.constprop.0+0x19/0x170
  ? cgroup_path_ns+0x112/0x150
  __kasan_report.cold+0x6c/0x84
  ? print_unreferenced+0x390/0x3b0
  ? cgroup_path_ns+0x112/0x150
  kasan_report+0x3a/0x50
  cgroup_path_ns+0x112/0x150
  proc_cpuset_show+0x164/0x530
  proc_single_show+0x10f/0x1c0
  seq_read_iter+0x405/0x1020
  ? aa_path_link+0x2e0/0x2e0
  seq_read+0x324/0x500
  ? seq_read_iter+0x1020/0x1020
  ? common_file_perm+0x2a1/0x4a0
  ? fsnotify_unmount_inodes+0x380/0x380
  ? bpf_lsm_file_permission_wrapper+0xa/0x30
  ? security_file_permission+0x53/0x460
  vfs_read+0x122/0x420
  ksys_read+0xed/0x1c0
  ? __ia32_sys_pwrite64+0x1e0/0x1e0
  ? __audit_syscall_exit+0x741/0xa70
  do_syscall_64+0x33/0x40
  entry_SYSCALL_64_after_hwframe+0x67/0xcc

This is also reported by: 
https://syzkaller.appspot.com/bug?extid=9b1ff7be974a403aa4cd

This can be reproduced by the following methods:
1.add an mdelay(1000) before acquiring the cgroup_lock In the
  cgroup_path_ns function.
2.$cat /proc/<pid>/cpuset   repeatly.
3.$mount -t cgroup -o cpuset cpuset /sys/fs/cgroup/cpuset/
$umount /sys/fs/cgroup/cpuset/   repeatly.

The race that cause this bug can be shown as below:

(umount)        |    (cat /proc/<pid>/cpuset)
css_release        |    proc_cpuset_show
css_release_work_fn    |    css = task_get_css(tsk, cpuset_cgrp_id);
css_free_rwork_fn    |    cgroup_path_ns(css->cgroup, ...);
cgroup_destroy_root    |    mutex_lock(&cgroup_mutex);
rebind_subsystems    |
cgroup_free_root     |
            |    // cgrp was freed, UAF
            |    cgroup_path_ns_locked(cgrp,..);

When the cpuset is initialized, the root node top_cpuset.css.cgrp
will point to &cgrp_dfl_root.cgrp. In cgroup v1, the mount 
operation will
allocate cgroup_root, and top_cpuset.css.cgrp will point to the 
allocated
&cgroup_root.cgrp. When the umount operation is executed,
top_cpuset.css.cgrp will be rebound to &cgrp_dfl_root.cgrp.

The problem is that when rebinding to cgrp_dfl_root, there are cases
where the cgroup_root allocated by setting up the root for cgroup v1
is cached. This could lead to a Use-After-Free (UAF) if it is
subsequently freed. The descendant cgroups of cgroup v1 can only be
freed after the css is released. However, the css of the root will 
never
be released, yet the cgroup_root should be freed when it is 
unmounted.
This means that obtaining a reference to the css of the root does
not guarantee that css.cgrp->root will not be freed.

To solve this issue, we have added a cgroup reference count in
the proc_cpuset_show function to ensure that css.cgrp->root will not
be freed prematurely. This is a temporary solution. Let's see if 
anyone
has a better solution.

Signed-off-by: Chen Ridong <chenridong@xxxxxxxxxx>
---
  kernel/cgroup/cpuset.c | 20 ++++++++++++++++++++
  1 file changed, 20 insertions(+)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index c12b9fdb22a4..782eaf807173 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -5045,6 +5045,7 @@ int proc_cpuset_show(struct seq_file *m, 
struct pid_namespace *ns,
      char *buf;
      struct cgroup_subsys_state *css;
      int retval;
+    struct cgroup *root_cgroup = NULL;
        retval = -ENOMEM;
      buf = kmalloc(PATH_MAX, GFP_KERNEL);
@@ -5052,9 +5053,28 @@ int proc_cpuset_show(struct seq_file *m, 
struct pid_namespace *ns,
          goto out;
        css = task_get_css(tsk, cpuset_cgrp_id);
+    rcu_read_lock();
+    /*
+     * When the cpuset subsystem is mounted on the legacy hierarchy,
+     * the top_cpuset.css->cgroup does not hold a reference count of
+     * cgroup_root.cgroup. This makes accessing css->cgroup very
+     * dangerous because when the cpuset subsystem is remounted 
to the
+     * default hierarchy, the cgroup_root.cgroup that css->cgroup 
points
+     * to will be released, leading to a UAF issue. To avoid this 
problem,
+     * get the reference count of top_cpuset.css->cgroup first.
+     *
+     * This is ugly!!
+     */
+    if (css == &top_cpuset.css) {
+        cgroup_get(css->cgroup);
+        root_cgroup = css->cgroup;
+    }
+    rcu_read_unlock();
      retval = cgroup_path_ns(css->cgroup, buf, PATH_MAX,
                  current->nsproxy->cgroup_ns);
      css_put(css);
+    if (root_cgroup)
+        cgroup_put(root_cgroup);
      if (retval == -E2BIG)
          retval = -ENAMETOOLONG;
      if (retval < 0)

Thanks for reporting this UAF bug. Could you try the attached patch 
to see if it can fix the issue?


+/*
+ * With a cgroup v1 mount, root_css.cgroup can be freed. We need to 
take a
+ * reference to it to avoid UAF as proc_cpuset_show() may access 
the content
+ * of this cgroup.
+ */
 static void cpuset_bind(struct cgroup_subsys_state *root_css)
 {
+    static struct cgroup *v1_cgroup_root;
+
     mutex_lock(&cpuset_mutex);
+    if (v1_cgroup_root) {
+        cgroup_put(v1_cgroup_root);
+        v1_cgroup_root = NULL;
+    }
     spin_lock_irq(&callback_lock);

     if (is_in_v2_mode()) {
@@ -4159,6 +4170,10 @@ static void cpuset_bind(struct 
cgroup_subsys_state *root_css)
     }

     spin_unlock_irq(&callback_lock);
+    if (!cgroup_subsys_on_dfl(cpuset_cgrp_subsys)) {
+        v1_cgroup_root = root_css->cgroup;
+        cgroup_get(v1_cgroup_root);
+    }
     mutex_unlock(&cpuset_mutex);
 }

Thanks for your suggestion. If we take a reference at rebind(call 
->bind()) function, cgroup_root allocated when setting up root for 
cgroup v1 can never be released, because the reference count will 
never be reduced to zero.

We have already tried similar methods to fix this issue, however 
doing so causes another issue as mentioned previously.

You are right. Taking the reference in cpuset_bind() will prevent 
cgroup_destroy_root() from being called. I had overlooked that.

Now I have an even simpler fix. Could you try the attached v2 patch 
to verify if that can fix the problem?

Thanks,
Longman

Thanks you for your reply, v2 patch will lead to  ABBA deadlock.

(cat /proc/<pid>/cpuset)                  | (rebind_subsystems)
                                                          | 
lockdep_assert_held(&cgroup_mutex);
mutex_lock(&cpuset_mutex);            |
cgroup_path_ns                                 |    ->bind()
                                                          | 
mutex_lock(&cpuset_mutex);
mutex_lock(&cgroup_mutex);           |

Bummer.

Another alternative is to create a cgroup_path_ns() variant that accepts 
a "struct cgroup **pcgrp" and retrieve the actual cgroup pointer inside 
its critical section. That should also fix the UAF.

Cheers,
Longman








