Bug 2366848 (CVE-2025-37890)

Summary: CVE-2025-37890 kernel: net_sched: hfsc: Fix a UAF vulnerability in class with netem as child qdisc
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security DevOps Team <prodsec-dev>
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Priority: medium    
Version: unspecifiedCC: dfreiber, drow, jburrell, vkumar
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OS: Linux   
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Description OSIDB Bzimport 2025-05-16 14:01:18 UTC 
In the Linux kernel, the following vulnerability has been resolved:

net_sched: hfsc: Fix a UAF vulnerability in class with netem as child qdisc

As described in Gerrard's report [1], we have a UAF case when an hfsc class
has a netem child qdisc. The crux of the issue is that hfsc is assuming
that checking for cl->qdisc->q.qlen == 0 guarantees that it hasn't inserted
the class in the vttree or eltree (which is not true for the netem
duplicate case).

This patch checks the n_active class variable to make sure that the code
won't insert the class in the vttree or eltree twice, catering for the
reentrant case.

[1] https://lore.kernel.org/netdev/CAHcdcOm+03OD2j6R0=YHKqmy=VgJ8xEOKuP6c7mSgnp-TEJJbw@mail.gmail.com/
Comment 1 Avinash Hanwate 2025-05-19 03:35:45 UTC 
Upstream advisory:
https://lore.kernel.org/linux-cve-announce/2025051617-CVE-2025-37890-437b@gregkh/T