Patch "cgroup/cpuset: Prevent UAF in proc_cpuset_show()" has been added to the 6.1-stable tree

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This is a note to let you know that I've just added the patch titled

    cgroup/cpuset: Prevent UAF in proc_cpuset_show()

to the 6.1-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     cgroup-cpuset-prevent-uaf-in-proc_cpuset_show.patch
and it can be found in the queue-6.1 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit 314b62481329a070a908d35df285559f7c03186b
Author: Chen Ridong <chenridong@xxxxxxxxxx>
Date:   Fri Jun 28 01:36:04 2024 +0000

    cgroup/cpuset: Prevent UAF in proc_cpuset_show()
    
    [ Upstream commit 1be59c97c83ccd67a519d8a49486b3a8a73ca28a ]
    
    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 is kfree_rcu after commit d23b5c577715
    ("cgroup: Make operations on the cgroup root_list RCU safe"),
    css->cgroup won't be freed during the critical section.
    To call cgroup_path_ns_locked, css_set_lock is needed, so it is safe to
    replace task_get_css with task_css.
    
    [1] https://syzkaller.appspot.com/bug?extid=9b1ff7be974a403aa4cd
    
    Fixes: a79a908fd2b0 ("cgroup: introduce cgroup namespaces")
    Signed-off-by: Chen Ridong <chenridong@xxxxxxxxxx>
    Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 278248907791f..370a6bce20a80 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -21,6 +21,7 @@
  *  License.  See the file COPYING in the main directory of the Linux
  *  distribution for more details.
  */
+#include "cgroup-internal.h"
 
 #include <linux/cpu.h>
 #include <linux/cpumask.h>
@@ -4213,10 +4214,14 @@ int proc_cpuset_show(struct seq_file *m, struct pid_namespace *ns,
 	if (!buf)
 		goto out;
 
-	css = task_get_css(tsk, cpuset_cgrp_id);
-	retval = cgroup_path_ns(css->cgroup, buf, PATH_MAX,
-				current->nsproxy->cgroup_ns);
-	css_put(css);
+	rcu_read_lock();
+	spin_lock_irq(&css_set_lock);
+	css = task_css(tsk, cpuset_cgrp_id);
+	retval = cgroup_path_ns_locked(css->cgroup, buf, PATH_MAX,
+				       current->nsproxy->cgroup_ns);
+	spin_unlock_irq(&css_set_lock);
+	rcu_read_unlock();
+
 	if (retval == -E2BIG)
 		retval = -ENAMETOOLONG;
 	if (retval < 0)




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