{"id":"CVE-2026-98811","kind":"vulnerability","aliases":["CVE-2026-98811"],"related":[],"title":"Linux kernel: failed tc filter adds leave an empty classifier linked","description":"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 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)","summary":"Two racing tc filter add requests on the same chain and priority can leave an empty classifier proto linked in the kernel after both fail, holding references to the chain, block and classifier module. The sources describe a resource leak rather than code execution or privilege escalation.","severity":"low","cvss":null,"epss":null,"exploited":false,"exploited_at":null,"products":[],"ecosystems":[],"tags":["infrastructure","linux-kernel","networking","tc"],"not_affected_if":["flower-not-enabled","no-untrusted-local-users"],"how_to_check":"- `uname -r` and compare with the fixed kernel in the NVD record or your distribution's advisory\n- `tc filter show dev <iface> ingress` and look for a bare `filter protocol ip pref 1 flower chain 0` header with no filter entries\n- Confirm whether `CONFIG_NET_CLS_FLOWER` is enabled in your kernel config","fix":"- Update the Linux kernel to a build that includes the cls_api fix (unknown version in the sources)\n- Reboot into the patched kernel, since the fix is in the running kernel only after restart\n- Until patched, avoid running concurrent tc filter add requests on the same chain and priority","status":"open","published_at":"2026-10-18T16:17:35+00:00","first_seen_at":"2026-10-23T21:57:16+00:00","triaged_at":"2026-10-25T05:14:54+00:00","updated_at":"2026-10-25T05:14:54+00:00","category":"infrastructure","confidence":0.6,"triage":{"at":"2026-10-25T05:14:54Z","model":"claude-haiku-5-5","output":{"fix":"- Update the Linux kernel to a build that includes the cls_api fix (unknown version in the sources)\n- Reboot into the patched kernel, since the fix is in the running kernel only after restart\n- Until patched, avoid running concurrent tc filter add requests on the same chain and priority","reach":"local","title":"Linux kernel: failed tc filter adds leave an empty classifier linked","impact":"low","summary":"Two racing tc filter add requests on the same chain and priority can leave an empty classifier proto linked in the kernel after both fail, holding references to the chain, block and classifier module. The sources describe a resource leak rather than code execution or privilege escalation.","category":"infrastructure","nicknames":[],"who_cares":["linux-kernel","networking","tc"],"confidence":0.6,"needs_auth":"yes","worst_case":"Kernel memory and references leak until chain or block teardown","analyst_note":"Check the NVD record and the upstream stable commit for the affected and fixed kernel versions and any stated security impact beyond the resource leak.","how_to_check":"- `uname -r` and compare with the fixed kernel in the NVD record or your distribution's advisory\n- `tc filter show dev <iface> ingress` and look for a bare `filter protocol ip pref 1 flower chain 0` header with no filter entries\n- Confirm whether `CONFIG_NET_CLS_FLOWER` is enabled in your kernel config","needs_analyst":true,"not_affected_if":["no-untrusted-local-users","flower-not-enabled"],"who_is_affected":"Systems where unprivileged or namespaced users can run tc filter commands with CAP_NET_ADMIN, which the sources say unshare -Urn is enough to do. Hosts that never run tc or use flower classifiers with untrusted local users are unlikely to matter. unknown for the exact kernel version range.","user_interaction":"no","needs_special_setup":"yes","severity_if_unknown":"low"},"cost_usd":0.000735,"input_tokens":3947,"output_tokens":680,"severity_before":"unknown"},"analyst_status":null,"analyst_note":null,"search":"'-2026':13A,16A '-98811':14A,17A '0':397C '1':352C,394C 'action':356C 'add':6A,28B,96C,337C,345C 'admin':409C 'admit':216C 'adopt':294C 'api':83C,289C 'aris':275C 'attempt':125C,153C 'bare':389C 'block':54B,167C,178C 'callback':253C 'cannot':274C 'cap':407C 'chain':33B,53B,121C,161C,164C,176C,205C,396C 'chain/prio':101C 'chang':108C 'check':258C 'classifi':10A,40B,56B,105C,171C,254C 'close':273C 'cls':82C,288C,330C 'code':66B 'commit':285C 'concurr':342C 'condit':325C 'config':328C 'creat':232C 'cve':12A,15A 'd4e359b3608a':286C 'delet':123C,207C,214C,245C,251C 'describ':60B 'dev':338C,346C 'drop':144C,357C 'empti':9A,39B,86C,124C,155C,202C,208C,215C,219C,252C,320C,377C 'emptiness-g':201C 'empty-proto':319C 'entri':406C 'error':90C,142C,187C 'escal':70B 'even':248C 'execut':67B 'fail':3A,48B,116C,191C,359C,385C 'filter':5A,27B,95C,344C,364C,390C,401C,405C 'fix':290C 'fl':361C 'flower':331C,353C,378C,395C 'follow':76C,282C 'follow-up':281C 'gate':203C 'give':126C 'guard':272C 'handl':132C,368C 'header':398C 'hold':49B,162C,194C 'hw':362C 'idr':137C,371C 'infrastructur':18B 'ingress':340C,348C 'insert':298C,307C 'insert-rac':297C 'ip':350C,392C 'kernel':2A,21B,45B,74C 'leak':63B 'leav':7A,37B 'left':317C 'link':11A,42B,158C,381C 'linux':1A,20B,73C 'linux-kernel':19B 'live':222C,314C 'local':412C 'loop':375C 'loser':130C,140C,304C 'loss':300C 'mark':242C,259C 'modul':57B,172C 'namespac':411C 'namespace-loc':410C 'net':329C,408C 'net/sched':81C,287C 'network':22B 'never':226C 'one':256C 'owner':240C 'pair':334C 'path':91C,143C,188C 'place':324C 'pref':351C,393C 'prioriti':35B 'privileg':69B 'proto':41B,87C,112C,156C,184C,196C,211C,224C,229C,261C,295C,315C,321C 'protocol':349C,391C 'publish':366C 'qdisc':336C 'race':25B,93C,269C,299C,308C 'rather':64B 'reclaim':84C,182C,234C 'reclam':152C,199C 'recreat':327C 'refer':50B,148C,165C,168C,173C,197C 'repeat':372C 'replac':363C 'request':29B,97C,192C,231C,384C 'residu':322C 'resolv':80C 'resourc':62B 'rtnl':265C 'rtnl-serial':264C 'run':107C 'safe':247C 'second':151C 'serial':266C 'share':111C,223C 'show':402C 'skip':354C 'sourc':59B 'stay':157C,380C 'still':134C 'stop':302C 'suffic':416C 'sw':355C 'tc':4A,23B,26B,94C,335C,343C,400C 'tcf':120C,204C 'teardown':291C 'torn':180C 'tp':122C,206C,379C 'two':24B,92C,341C 'uncondit':235C,262C 'unlink':209C,227C,310C 'unlock':104C 'unshar':414C 'urn':415C 'veth':333C 'veth0':339C,347C 'via':413C 'visibl':386C 'vulner':77C 'window':270C 'winner':118C,312C 'without':149C,249C,255C 'y':332C","nicknames":[],"mitigations":[],"who_affected":"Systems where unprivileged or namespaced users can run tc filter commands with CAP_NET_ADMIN, which the sources say unshare -Urn is enough to do. Hosts that never run tc or use flower classifiers with untrusted local users are unlikely to matter. unknown for the exact kernel version range.","tier":"C","verified":{},"analyst_revisit_at":null,"first_public_at":"2026-10-18T16:17:35+00:00","kev":null,"cvss_vector":null,"cvss_by":null,"poc_count":0,"sources":[{"source":"nvd","external_id":"CVE-2026-98811","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-98811","severity":null,"published_at":"2026-10-18T16:17:35+00:00","modified_at":"2026-10-21T13:21:47+00:00","first_seen_at":"2026-10-23T21:57:16+00:00","fetched_at":"2026-10-23T22:28:56+00:00","note":null}],"documents":[],"notice":"Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai"}