CVE-2025-37890

In the Linux kernel, the following vulnerability has been resolved:<br /> <br /> net_sched: hfsc: Fix a UAF vulnerability in class with netem as child qdisc<br /> <br /> As described in Gerrard&amp;#39;s report [1], we have a UAF case when an hfsc class<br /> has a netem child qdisc. The crux of the issue is that hfsc is assuming<br /> that checking for cl-&gt;qdisc-&gt;q.qlen == 0 guarantees that it hasn&amp;#39;t inserted<br /> the class in the vttree or eltree (which is not true for the netem<br /> duplicate case).<br /> <br /> This patch checks the n_active class variable to make sure that the code<br /> won&amp;#39;t insert the class in the vttree or eltree twice, catering for the<br /> reentrant case.<br /> <br /> [1] https://lore.kernel.org/netdev/CAHcdcOm+03OD2j6R0=YHKqmy=VgJ8xEOKuP6c7mSgnp-TEJJbw@mail.gmail.com/