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Pedro Tammela says:
====================
net_sched: hfsc: Address reentrant enqueue adding class to eltree twice
Savino says:
"We are writing to report that this recent patch
(141d34391a)
can be bypassed, and a UAF can still occur when HFSC is utilized with
NETEM.
The patch only checks the cl->cl_nactive field to determine whether
it is the first insertion or not, but this field is only
incremented by init_vf.
By using HFSC_RSC (which uses init_ed), it is possible to bypass the
check and insert the class twice in the eltree.
Under normal conditions, this would lead to an infinite loop in
hfsc_dequeue for the reasons we already explained in this report.
However, if TBF is added as root qdisc and it is configured with a
very low rate,
it can be utilized to prevent packets from being dequeued.
This behavior can be exploited to perform subsequent insertions in the
HFSC eltree and cause a UAF."
To fix both the UAF and the infinite loop, with netem as an hfsc child,
check explicitly in hfsc_enqueue whether the class is already in the eltree
whenever the HFSC_RSC flag is set.
Also add a TDC test to reproduce the UAF scenario.
====================
Link: https://patch.msgid.link/20250522181448.1439717-1-pctammela@mojatatu.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
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