{"id":"CVE-2026-98811","title":"net/sched: cls_api: reclaim an empty proto on the error path","summary":"In the Linux kernel, the following vulnerability has been resolved:\n\nnet/sched: cls_api: reclaim an empty proto on the error path\n\nTwo racing tc filter add requests on the same chain/prio of an\nunlocked classifier both run change() on th…","severity":"none","vendor":"Linux","product":"Linux","affected":["Linux >= 8b64678e0af8f4d62a40149baedebe78503a5255 < 315e4fe65cf02ef052de5e6298c3088e448b7517","Linux >= 8b64678e0af8f4d62a40149baedebe78503a5255 < ebc9611ab154c0cd9f1320e2a73f5e936d7535a9","Linux >= 8b64678e0af8f4d62a40149baedebe78503a5255 < 570141625d212fbbcffb9665cb45c0f04610e61a","Linux >= 8b64678e0af8f4d62a40149baedebe78503a5255 < 0bb8eab29b55045281a4963d4557f2776cdf8cfa","Linux 5.1"],"published":"2026-10-18","updated":"2026-10-18","sourceUpdated":"2026-10-18T15:48:57.588Z","source":"CVEORG","sourceUrl":"https://www.cve.org/CVERecord?id=CVE-2026-98811","references":[{"url":"https://git.kernel.org/stable/c/315e4fe65cf02ef052de5e6298c3088e448b7517"},{"url":"https://git.kernel.org/stable/c/ebc9611ab154c0cd9f1320e2a73f5e936d7535a9"},{"url":"https://git.kernel.org/stable/c/570141625d212fbbcffb9665cb45c0f04610e61a"},{"url":"https://git.kernel.org/stable/c/0bb8eab29b55045281a4963d4557f2776cdf8cfa"}],"tags":["cve.org"],"ingestedAt":"2026-10-18T16:47:32.478Z","slug":"CVE-2026-98811","body":"## Overview\n\nIn the Linux kernel, the following vulnerability has been resolved:\n\nnet/sched: cls_api: reclaim an empty proto on the error path\n\nTwo racing tc filter add requests on the same chain/prio of an\nunlocked classifier both run change() on the shared proto and both\ncan fail: the winner's tcf_chain_tp_delete_empty() attempt gives up\nbecause the loser's handle is still in the idr, and the loser's\nerror path drops only its own reference without a second reclamation\nattempt. The empty proto stays linked in the chain, holding the\nchain reference, a block reference and the classifier module\nreference until the chain or block is torn down.\n\nReclaim the proto on the error path of any failed request that holds\na proto reference. The reclamation is emptiness-gated:\ntcf_chain_tp_delete_empty() unlinks the proto only when\ndelete_empty() admits it is empty, so a live shared proto is never\nunlinked. A proto the request created is reclaimed unconditionally -\nit is the only owner, so marking it for deletion is safe even without\na delete_empty callback. Classifiers without one (the check marks the\nproto unconditionally) are rtnl-serialized, so the raced window this\nguard closes cannot arise for them.\n\nThis is a follow-up to commit d4e359b3608a (\"net/sched: cls_api: fix\nteardown of an adopted proto on insert-race loss\"), which stopped the\nloser of the insert race from unlinking the winner's live proto but\nleft the empty-proto residual in place.\n\nConditions to recreate:\n- CONFIG_NET_CLS_FLOWER=y; veth pair\n- tc qdisc add dev veth0 ingress\n- two concurrent `tc filter add dev veth0 ingress protocol ip pref 1\n  flower skip_sw ... action drop` (both fail in fl_hw_replace_filter\n  after publishing their handle in the idr); repeat in a loop\n- an empty flower tp stays linked after both requests fail; visible\n  as a bare `filter protocol ip pref 1 flower chain 0` header in\n  `tc filter show` with no filter entries\n- CAP_NET_ADMIN (namespace-local via unshare -Urn suffices)\n\n## Affected\n\n- `Linux >= 8b64678e0af8f4d62a40149baedebe78503a5255 < 315e4fe65cf02ef052de5e6298c3088e448b7517`\n- `Linux >= 8b64678e0af8f4d62a40149baedebe78503a5255 < ebc9611ab154c0cd9f1320e2a73f5e936d7535a9`\n- `Linux >= 8b64678e0af8f4d62a40149baedebe78503a5255 < 570141625d212fbbcffb9665cb45c0f04610e61a`\n- `Linux >= 8b64678e0af8f4d62a40149baedebe78503a5255 < 0bb8eab29b55045281a4963d4557f2776cdf8cfa`\n- `Linux 5.1`\n\n## Remediation\n\nRefer to the linked advisories for vendor-supplied fixes and affected version ranges.","depth":"sunlit","depthScore":3,"depthScoreParts":{"impact":2.8,"likelihood":0,"exploitation":0,"ransomware":0},"changes":[]}