🛡️ CVE-2021-47488 — linux

In the Linux kernel, the following vulnerability has been resolved:

cgroup: Fix memory leak caused by missing cgroup_bpf_offline

When enabling CONFIG_CGROUP_BPF, kmemleak can be observed by running

the command as below:

$mount -t cgroup -o none,name=foo cgroup cgroup/

$umount cgroup/

unreferenced object 0xc3585c40 (size 64):

comm "mount", pid 425, jiffies 4294959825 (age 31.990s)

hex dump (first 32 bytes):

01 00 00 80 84 8c 28 c0 00 00 00 00 00 00 00 00 ......(.........

00 00 00 00 00 00 00 00 6c 43 a0 c3 00 00 00 00 ........lC......

backtrace:

[<e95a2f9e>] cgroup_bpf_inherit+0x44/0x24c

[<1f03679c>] cgroup_setup_root+0x174/0x37c

[<ed4b0ac5>] cgroup1_get_tree+0x2c0/0x4a0

[<f85b12fd>] vfs_get_tree+0x24/0x108

[<f55aec5c>] path_mount+0x384/0x988

[<e2d5e9cd>] do_mount+0x64/0x9c

[<208c9cfe>] sys_mount+0xfc/0x1f4

[<06dd06e0>] ret_fast_syscall+0x0/0x48

[<a8308cb3>] 0xbeb4daa8

This is because that since the commit 2b0d3d3e4fcf ("percpu_ref: reduce

memory footprint of percpu_ref in fast path") root_cgrp->bpf.refcnt.data

is allocated by the function percpu_ref_init in cgroup_bpf_inherit which

is called by cgroup_setup_root when mounting, but not freed along with

root_cgrp when umounting. Adding cgroup_bpf_offline which calls

percpu_ref_kill to cgroup_kill_sb can free root_cgrp->bpf.refcnt.data in

umount path.

This patch also fixes the commit 4bfc0bb2c60e ("bpf: decouple the lifetime

of cgroup_bpf from cgroup itself"). A cgroup_bpf_offline is needed to do a

cleanup that frees the resources which are allocated by cgroup_bpf_inherit

in cgroup_setup_root.

And inside cgroup_bpf_offline, cgroup_get() is at the beginning and

cgroup_put is at the end of cgroup_bpf_release which is called by

cgroup_bpf_offline. So cgroup_bpf_offline can keep the balance of

cgroup's refcount.