In the Linux kernel, the following vulnerability has been resolved:
net/sched: cls_api: reclaim an empty proto on the error path
Two racing tc filter add requests on the same chain/prio of an
unlocked classifier both run change() on the shared proto and both
can fail: the winner's tcf_chain_tp_delete_empty() attempt gives up
because the loser's handle is still in the idr, and the loser's
error path drops only its own reference without a second reclamation
attempt. The empty proto stays linked in the chain, holding the
chain reference, a block reference and the classifier module
reference until the chain or block is torn down.
Reclaim the proto on the error path of any failed request that holds
a proto reference. The reclamation is emptiness-gated:
tcf_chain_tp_delete_empty() unlinks the proto only when
delete_empty() admits it is empty, so a live shared proto is never
unlinked. A proto the request created is reclaimed unconditionally -
it is the only owner, so marking it for deletion is safe even without
a delete_empty callback. Classifiers without one (the check marks the
proto unconditionally) are rtnl-serialized, so the raced window this
guard closes cannot arise for them.
This is a follow-up to commit d4e359b3608a ("net/sched: cls_api: fix
teardown of an adopted proto on insert-race loss"), which stopped the
loser of the insert race from unlinking the winner's live proto but
left the empty-proto residual in place.
Conditions to recreate:
- CONFIG_NET_CLS_FLOWER=y; veth pair
- tc qdisc add dev veth0 ingress
- two concurrent `tc filter add dev veth0 ingress protocol ip pref 1
flower skip_sw ... action drop` (both fail in fl_hw_replace_filter
after publishing their handle in the idr); repeat in a loop
- an empty flower tp stays linked after both requests fail; visible
as a bare `filter protocol ip pref 1 flower chain 0` header in
`tc filter show` with no filter entries
- CAP_NET_ADMIN (namespace-local via unshare -Urn suffices)
net/sched: cls_api: reclaim an empty proto on the error path
Two racing tc filter add requests on the same chain/prio of an
unlocked classifier both run change() on the shared proto and both
can fail: the winner's tcf_chain_tp_delete_empty() attempt gives up
because the loser's handle is still in the idr, and the loser's
error path drops only its own reference without a second reclamation
attempt. The empty proto stays linked in the chain, holding the
chain reference, a block reference and the classifier module
reference until the chain or block is torn down.
Reclaim the proto on the error path of any failed request that holds
a proto reference. The reclamation is emptiness-gated:
tcf_chain_tp_delete_empty() unlinks the proto only when
delete_empty() admits it is empty, so a live shared proto is never
unlinked. A proto the request created is reclaimed unconditionally -
it is the only owner, so marking it for deletion is safe even without
a delete_empty callback. Classifiers without one (the check marks the
proto unconditionally) are rtnl-serialized, so the raced window this
guard closes cannot arise for them.
This is a follow-up to commit d4e359b3608a ("net/sched: cls_api: fix
teardown of an adopted proto on insert-race loss"), which stopped the
loser of the insert race from unlinking the winner's live proto but
left the empty-proto residual in place.
Conditions to recreate:
- CONFIG_NET_CLS_FLOWER=y; veth pair
- tc qdisc add dev veth0 ingress
- two concurrent `tc filter add dev veth0 ingress protocol ip pref 1
flower skip_sw ... action drop` (both fail in fl_hw_replace_filter
after publishing their handle in the idr); repeat in a loop
- an empty flower tp stays linked after both requests fail; visible
as a bare `filter protocol ip pref 1 flower chain 0` header in
`tc filter show` with no filter entries
- CAP_NET_ADMIN (namespace-local via unshare -Urn suffices)
Default status is the baseline for the product, each version can override it (e.g. patched versions marked unaffected).
| Vendor | Product | Default status | Versions | |||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Linux | Linux | unaffected |
|
|||||||||||||||||||||
| Linux | Linux | affected |
|
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Fixes
Solution
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Workaround
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References
History
Sun, 18 Oct 2026 20:45:00 +0000
| Type | Values Removed | Values Added |
|---|---|---|
| Weaknesses | CWE-362 CWE-754 |
Sun, 18 Oct 2026 16:00:00 +0000
| Type | Values Removed | Values Added |
|---|---|---|
| Description | In the Linux kernel, the following vulnerability has been resolved: net/sched: cls_api: reclaim an empty proto on the error path Two racing tc filter add requests on the same chain/prio of an unlocked classifier both run change() on the shared proto and both can fail: the winner's tcf_chain_tp_delete_empty() attempt gives up because the loser's handle is still in the idr, and the loser's error path drops only its own reference without a second reclamation attempt. The empty proto stays linked in the chain, holding the chain reference, a block reference and the classifier module reference until the chain or block is torn down. Reclaim the proto on the error path of any failed request that holds a proto reference. The reclamation is emptiness-gated: tcf_chain_tp_delete_empty() unlinks the proto only when delete_empty() admits it is empty, so a live shared proto is never unlinked. A proto the request created is reclaimed unconditionally - it is the only owner, so marking it for deletion is safe even without a delete_empty callback. Classifiers without one (the check marks the proto unconditionally) are rtnl-serialized, so the raced window this guard closes cannot arise for them. This is a follow-up to commit d4e359b3608a ("net/sched: cls_api: fix teardown of an adopted proto on insert-race loss"), which stopped the loser of the insert race from unlinking the winner's live proto but left the empty-proto residual in place. Conditions to recreate: - CONFIG_NET_CLS_FLOWER=y; veth pair - tc qdisc add dev veth0 ingress - two concurrent `tc filter add dev veth0 ingress protocol ip pref 1 flower skip_sw ... action drop` (both fail in fl_hw_replace_filter after publishing their handle in the idr); repeat in a loop - an empty flower tp stays linked after both requests fail; visible as a bare `filter protocol ip pref 1 flower chain 0` header in `tc filter show` with no filter entries - CAP_NET_ADMIN (namespace-local via unshare -Urn suffices) | |
| Title | net/sched: cls_api: reclaim an empty proto on the error path | |
| First Time appeared |
Linux
Linux linux Kernel |
|
| CPEs | cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:* | |
| Vendors & Products |
Linux
Linux linux Kernel |
|
| References |
|
Projects
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MITRE
Status: PUBLISHED
Assigner: Linux
Published:
Updated: 2026-10-18T15:48:57.588Z
Reserved: 2026-09-25T10:25:14.371Z
Link: CVE-2026-98811
Vulnrichment
No data.
NVD
Status : Received
Published: 2026-10-18T16:17:35.200
Modified: 2026-10-18T16:17:35.200
Link: CVE-2026-98811
Redhat
No data.
OpenCVE Enrichment
Updated: 2026-10-18T20:30:17Z