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

[Stefan Bader]

On 14.06.25 02:41, Ian Whitfield wrote:
> [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(-)
> 
Acked-by: Stefan Bader <stefan.bader at canonical.com>
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