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)
