{"id":"CVE-2026-99065","kind":"vulnerability","aliases":["CVE-2026-99065"],"related":[],"title":"Linux kernel: cpuset CPU hotplug race can cause a NULL pointer dereference","description":"In the Linux kernel, the following vulnerability has been resolved:\n\ncgroup/cpuset: Handle cpu hotplug race in guarantee_active_cpus()\n\nWith commit 2125c0034c5d (\"cgroup/cpuset: Make cpuset hotplug processing\nsynchronous\"), the cpuset hotplug operation becomes synchronous. That\ncommit also removes the code that handles race between cpuset_hotplug_work\nand cpu hotplug notifier with the assumption that race is now gone.\nLater commit 7a0aabd9ce69 (\"cgroup/cpuset: Always use cpu_active_mask\")\nupdates the cpuset code to always use cpu_active_mask instead of\ncpu_online_mask in some places including guarantee_online_cpus() which\nis renamed to guarantee_active_cpus() in that commit.\n\nIn the case of CPU offline operation, cpu_active_mask is updated\nfirst in sched_cpu_deactivate() to remove the offline CPU before\ncpuset_handle_hotplug() is called to update the effective_cpus of the\naffected cpusets. The cpu_online_mask is updated after that near the end\nof the offline operation to remove the offline CPU. As a result, the race\ncomes back and the top cpuset may not have any active CPU leading to NULL\npointer dereference during the race window when guarantee_active_cpus()\nis called after cpu_active_mask is updated to remove the CPU to be torn\ndown but before cpuset_handle_hotplug() is able to properly update the\neffective_cpus of the top cpuset.\n\nFix this by adding back the NULL cs check to avoid this problem.","summary":"During a CPU offline operation, a race in the cpuset code can leave the top cpuset with no active CPU, and guarantee_active_cpus() then dereferences a NULL pointer. The sources do not describe what an attacker can gain beyond a kernel crash.","severity":"unknown","cvss":null,"epss":null,"exploited":false,"exploited_at":null,"products":[],"ecosystems":[],"tags":["cgroups","cpuset","linux-kernel","runtime"],"not_affected_if":["kernel-without-cpuset"],"how_to_check":"- unknown","fix":"- Apply the upstream Linux kernel fix that adds back the NULL cs check in guarantee_active_cpus() (cgroup/cpuset), and check your distribution's kernel advisory for the fixed package version.","status":"open","published_at":"2026-10-23T13:17:30+00:00","first_seen_at":"2026-10-23T21:57:34+00:00","triaged_at":"2026-10-25T04:53:51+00:00","updated_at":"2026-10-25T04:53:51+00:00","category":"runtime","confidence":0.5,"triage":{"at":"2026-10-25T04:53:51Z","model":"claude-haiku-5-5","output":{"fix":"- Apply the upstream Linux kernel fix that adds back the NULL cs check in guarantee_active_cpus() (cgroup/cpuset), and check your distribution's kernel advisory for the fixed package version.","reach":"local","title":"Linux kernel: cpuset CPU hotplug race can cause a NULL pointer dereference","impact":"low","summary":"During a CPU offline operation, a race in the cpuset code can leave the top cpuset with no active CPU, and guarantee_active_cpus() then dereferences a NULL pointer. The sources do not describe what an attacker can gain beyond a kernel crash.","category":"runtime","nicknames":[],"who_cares":["linux-kernel","cgroups","cpuset"],"confidence":0.5,"needs_auth":"unknown","worst_case":"Kernel NULL pointer dereference causing a crash or denial of service","analyst_note":"Check the NVD entry and the upstream kernel stable commits for affected and fixed kernel versions, and whether a local user can trigger the race.","how_to_check":"- unknown","needs_analyst":true,"not_affected_if":["kernel-without-cpuset"],"who_is_affected":"Systems running a Linux kernel with cgroup cpuset support that take CPUs offline at runtime, where the race window can be hit. The sources do not name specific distributions or kernel versions. unknown","user_interaction":"no","needs_special_setup":"unknown","severity_if_unknown":"unknown"},"cost_usd":0.000633,"input_tokens":3757,"output_tokens":514,"severity_before":"unknown"},"analyst_status":null,"analyst_note":null,"search":"'-2026':14A,17A '-99065':15A,18A '2125c0034c5d':89C '7a0aabd9ce69':129C 'abl':277C 'activ':43B,47B,85C,134C,144C,163C,176C,240C,253C,259C 'add':291C 'affect':203C 'also':104C 'alway':131C,141C 'assumpt':121C 'attack':61B 'avoid':298C 'back':231C,292C 'becom':100C 'beyond':64B 'call':195C,256C 'case':170C 'caus':8A 'cgroup':19B 'cgroup/cpuset':78C,90C,130C 'check':296C 'code':35B,107C,139C 'come':230C 'commit':88C,103C,128C,167C 'cpu':4A,27B,44B,80C,116C,133C,143C,148C,172C,175C,183C,189C,206C,224C,241C,258C,266C 'cpus':48B,86C,157C,164C,200C,254C,283C 'cpuset':3A,20B,34B,40B,92C,97C,112C,138C,191C,204C,235C,273C,287C 'crash':67B 'cs':295C 'cve':13A,16A 'deactiv':184C 'derefer':12A,50B,246C 'describ':58B 'effect':199C,282C 'end':215C 'first':180C 'fix':288C 'follow':73C 'gain':63B 'gone':126C 'guarante':46B,84C,155C,162C,252C 'handl':79C,109C,192C,274C 'hotplug':5A,81C,93C,98C,113C,117C,193C,275C 'includ':154C 'instead':146C 'kernel':2A,23B,66B,71C 'later':127C 'lead':242C 'leav':37B 'linux':1A,22B,70C 'linux-kernel':21B 'make':91C 'mask':135C,145C,150C,177C,208C,260C 'may':236C 'near':213C 'notifi':118C 'null':10A,52B,244C,294C 'offlin':28B,173C,188C,218C,223C 'onlin':149C,156C,207C 'oper':29B,99C,174C,219C 'place':153C 'pointer':11A,53B,245C 'problem':300C 'process':94C 'proper':279C 'race':6A,31B,82C,110C,123C,229C,249C 'remov':105C,186C,221C,264C 'renam':160C 'resolv':77C 'result':227C 'runtim':24B 'sched':182C 'sourc':55B 'synchron':95C,101C 'top':39B,234C,286C 'torn':269C 'updat':136C,179C,197C,210C,262C,280C 'use':132C,142C 'vulner':74C 'window':250C 'work':114C","nicknames":[],"mitigations":[],"who_affected":"Systems running a Linux kernel with cgroup cpuset support that take CPUs offline at runtime, where the race window can be hit. The sources do not name specific distributions or kernel versions. unknown","tier":"C","verified":{},"analyst_revisit_at":null,"first_public_at":"2026-10-23T13:17:30+00:00","kev":null,"cvss_vector":null,"cvss_by":null,"poc_count":0,"sources":[{"source":"nvd","external_id":"CVE-2026-99065","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-99065","severity":null,"published_at":"2026-10-23T13:17:30+00:00","modified_at":"2026-10-23T13:17:30+00:00","first_seen_at":"2026-10-23T21:57:34+00:00","fetched_at":"2026-10-23T22:29:03+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"}