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Date: Sat, 22 Jun 2024 11:05:31 -0400
From: Waiman Long <longman@...hat.com>
To: Chen Ridong <chenridong@...wei.com>, tj@...nel.org,
lizefan.x@...edance.com, hannes@...xchg.org
Cc: bpf@...r.kernel.org, cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH -next] cgroup: fix uaf when proc_cpuset_show
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@...wei.com>
> ---
> 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?
Cheers,
Longman
View attachment "0001-cgroup-cpuset-Prevent-UAF-in-proc_cpuset_show.patch" of type "text/x-patch" (3357 bytes)
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