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Use After Free Affecting kernel-devel package, versions <0:5.14.0-427.28.1.el9_4

Severity

 Recommended
medium

Based on AlmaLinux security rating.

Threat Intelligence

EPSS
0.05% (19th percentile)

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  • Snyk IDSNYK-ALMALINUX9-KERNELDEVEL-8327085
  • published4 Nov 2024
  • disclosed31 Jul 2024
Report a new vulnerability Found a mistake?

Introduced: 31 Jul 2024

CVE-2024-26737  (opens in a new tab)
CWE-416  (opens in a new tab)

How to fix?

Upgrade AlmaLinux:9 kernel-devel to version 0:5.14.0-427.28.1.el9_4 or higher.
This issue was patched in ALSA-2024:4928.

NVD Description

Note: Versions mentioned in the description apply only to the upstream kernel-devel package and not the kernel-devel package as distributed by AlmaLinux. See How to fix? for AlmaLinux:9 relevant fixed versions and status.

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

bpf: Fix racing between bpf_timer_cancel_and_free and bpf_timer_cancel

The following race is possible between bpf_timer_cancel_and_free and bpf_timer_cancel. It will lead a UAF on the timer->timer.

bpf_timer_cancel(); spin_lock(); t = timer->time; spin_unlock();

                bpf_timer_cancel_and_free();
                    spin_lock();
                    t = timer-&gt;timer;
                    timer-&gt;timer = NULL;
                    spin_unlock();
                    hrtimer_cancel(&amp;t-&gt;timer);
                    kfree(t);


/* UAF on t */
hrtimer_cancel(&amp;t-&gt;timer);

In bpf_timer_cancel_and_free, this patch frees the timer->timer after a rcu grace period. This requires a rcu_head addition to the "struct bpf_hrtimer". Another kfree(t) happens in bpf_timer_init, this does not need a kfree_rcu because it is still under the spin_lock and timer->timer has not been visible by others yet.

In bpf_timer_cancel, rcu_read_lock() is added because this helper can be used in a non rcu critical section context (e.g. from a sleepable bpf prog). Other timer->timer usages in helpers.c have been audited, bpf_timer_cancel() is the only place where timer->timer is used outside of the spin_lock.

Another solution considered is to mark a t->flag in bpf_timer_cancel and clear it after hrtimer_cancel() is done. In bpf_timer_cancel_and_free, it busy waits for the flag to be cleared before kfree(t). This patch goes with a straight forward solution and frees timer->timer after a rcu grace period.

CVSS Base Scores

version 3.1