[SRU][N][PATCH v2 0/1] CVE-2025-37797

[Ian Whitfield]

[Impact]

net_sched: hfsc: Fix a UAF vulnerability in class handling

This patch fixes a Use-After-Free vulnerability in the HFSC qdisc class
handling. The issue occurs due to a time-of-check/time-of-use condition
in hfsc_change_class() when working with certain child qdiscs like netem
or codel.

The vulnerability works as follows:
1. hfsc_change_class() checks if a class has packets (q.qlen != 0)
2. It then calls qdisc_peek_len(), which for certain qdiscs (e.g.,
   codel, netem) might drop packets and empty the queue
3. The code continues assuming the queue is still non-empty, adding
   the class to vttree
4. This breaks HFSC scheduler assumptions that only non-empty classes
   are in vttree
5. Later, when the class is destroyed, this can lead to a Use-After-Free

The fix adds a second queue length check after qdisc_peek_len() to verify
the queue wasn't emptied.

[Backport]

Cherry pick applied cleanly.

[Fix]

Plucky:   fixed via stable updates
Oracular: EOL
Noble:    cherry pick
Jammy:    fixed via stable updates
Focal:    sent to ESM ML
Bionic:   sent to ESM ML
Xenial:   not affected
Trusty:   not affected

[Test Case]

Compile and boot tested.

[Where problems could occur]

This fix affects those who use a Hierarchical Fair Service Curve (HFSC) network
scheduler queue discipline (qdisc) with certain child qdiscs. An issue with this
fix would be visible to the user as a use-after-free which could read private
information or crash the kernel.

v2: Updated targets for this patchset

Cong Wang (1):
  net_sched: hfsc: Fix a UAF vulnerability in class handling

 net/sched/sch_hfsc.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

-- 
2.43.0