Re: [PATCH V2] cgroup/cpuset: Prevent UAF in proc_cpuset_show()

[chenridong <chenridong@huawei.com>]

On 2024/6/26 11:17, Waiman Long wrote:
>
> On 6/25/24 23:05, Chen Ridong wrote:
>> An UAF can happen when /proc/cpuset is read as reported in [1].
>>
>> 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.
>>
>> Fix this problem by using rcu_read_lock in proc_cpuset_show().
>> As cgroup root_list is already RCU-safe, css->cgroup is safe.
>> This is similar to commit 9067d90006df ("cgroup: Eliminate the
>> need for cgroup_mutex in proc_cgroup_show()")
>>
>> [1] https://syzkaller.appspot.com/bug?extid=9b1ff7be974a403aa4cd
>>
>> Fixes: a79a908fd2b0 ("cgroup: introduce cgroup namespaces")
>> Signed-off-by: Chen Ridong <chenridong@huawei.com>
>> ---
>>   include/linux/cgroup.h |  3 +++
>>   kernel/cgroup/cpuset.c | 11 +++++++++--
>>   2 files changed, 12 insertions(+), 2 deletions(-)
>>
>> diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
>> index 2150ca60394b..bae7b54957fc 100644
>> --- a/include/linux/cgroup.h
>> +++ b/include/linux/cgroup.h
>> @@ -786,6 +786,9 @@ struct cgroup_namespace *copy_cgroup_ns(unsigned 
>> long flags,
>>   int cgroup_path_ns(struct cgroup *cgrp, char *buf, size_t buflen,
>>              struct cgroup_namespace *ns);
>>   +int cgroup_path_ns_locked(struct cgroup *cgrp, char *buf, size_t 
>> buflen,
>> +              struct cgroup_namespace *ns);
>
> The function prototype for cgroup_path_ns_locked() is available in 
> "kernel/cgroup/cgroup-internal.h". You just need to include 
> "cgroup-internal.h" in cpuset.c instead of exposed this internal API 
> to the world.
>
>> +
>>   #else /* !CONFIG_CGROUPS */
>>     static inline void free_cgroup_ns(struct cgroup_namespace *ns) { }
>> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
>> index c12b9fdb22a4..e57762f613d6 100644
>> --- a/kernel/cgroup/cpuset.c
>> +++ b/kernel/cgroup/cpuset.c
>> @@ -5052,8 +5052,15 @@ int proc_cpuset_show(struct seq_file *m, 
>> struct pid_namespace *ns,
>>           goto out;
>>         css = task_get_css(tsk, cpuset_cgrp_id);
>> -    retval = cgroup_path_ns(css->cgroup, buf, PATH_MAX,
>> -                current->nsproxy->cgroup_ns);
>> +    rcu_read_lock();
>> +    spin_lock_irq(&css_set_lock);
>> +    /* In case the root has already been unmounted. */
>> +    if (css->cgroup)
>> +        retval = cgroup_path_ns_locked(css->cgroup, buf, PATH_MAX,
>> +            current->nsproxy->cgroup_ns);
>
> Could you properly align the wrapped cgroup_ns argument?
>
> Cheers,
> Longman
>
>> +
>> +    spin_unlock_irq(&css_set_lock);
>> +    rcu_read_unlock();
>>       css_put(css);
>>       if (retval == -E2BIG)
>>           retval = -ENAMETOOLONG;
>
>
Thank you, i will do that in V3。

Regards,
Ridong