CVE-2021-47557

/// Technical description

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

net/sched: sch_ets: don't peek at classes beyond 'nbands'

when the number of DRR classes decreases, the round-robin active list can

contain elements that have already been freed in ets_qdisc_change(). As a

consequence, it's possible to see a NULL dereference crash, caused by the

attempt to call cl->qdisc->ops->peek(cl->qdisc) when cl->qdisc is NULL:

BUG: kernel NULL pointer dereference, address: 0000000000000018

#PF: supervisor read access in kernel mode

#PF: error_code(0x0000) - not-present page

PGD 0 P4D 0

Oops: 0000 [#1] PREEMPT SMP NOPTI

CPU: 1 PID: 910 Comm: mausezahn Not tainted 5.16.0-rc1+ #475

Hardware name: Red Hat KVM, BIOS 1.11.1-4.module+el8.1.0+4066+0f1aadab 04/01/2014

RIP: 0010:ets_qdisc_dequeue+0x129/0x2c0 [sch_ets]

Code: c5 01 41 39 ad e4 02 00 00 0f 87 18 ff ff ff 49 8b 85 c0 02 00 00 49 39 c4 0f 84 ba 00 00 00 49 8b ad c0 02 00 00 48 8b 7d 10 <48> 8b 47 18 48 8b 40 38 0f ae e8 ff d0 48 89

/// Remediation

Check the references section for vendor advisories, patches, and mitigation guidance. If immediate patching is not possible, review the CVSS vector to understand the attack surface and apply compensating controls such as network segmentation or access restrictions.

/// References