{"id":"CVE-2026-98990","kind":"vulnerability","aliases":["CVE-2026-98990"],"related":[],"title":"Linux kernel: busy submounts can be unmounted, breaking in-use mounts","description":"In the Linux kernel, the following vulnerability has been resolved:\n\nnamespace: check a submount for references right before unmounting it\n\nshrink_submounts() and mark_mounts_for_expiry() first collect all the\nmounts they are allowed to unmount and then unmount them.\n\nWhether a mount is busy is decided by propagate_mount_busy() on the\ntree. But unmounting the first mount changes the tree that the second\none propagates into. propagate_umount()\nmoves a surviving overmount off a copy it unmounts and mounts it where\nthe copy was mounted. If that's where the propagated copy of the second\nvictim is looked up the overmount becomes a candidate of the second\numount_tree(). It's childless and so trim_one() commits it without\nlooking at its reference count.\n\nSo a synchronous umount pulls out a busy mount that is in use somewhere\nelse and marks it MNT_SYNC_UMOUNT while it has users:\n\n  umount2(/tmp/plshrink/p1, 0) = 0 errno 0\n  cwd is (unreachable)\n\nThe same two umounts requested one after the other fail with EBUSY.\nThis needs a mount with MNT_SHRINKABLE, i.e., automounted submounts of\nNFS, AFS and CIFS or the tracefs mount below debugfs. Bind mounts\ninherit the flag.\n\nCheck each mount right before it is unmounted under the same hold of\nnamespace_sem and mount_lock as the umount itself. Make sure that the\nalgorithm stays linear.","summary":"A race in the Linux kernel's mount handling lets a synchronous umount remove a mount that is still in use elsewhere, because the busy check runs before an earlier unmount changes the mount tree. The sources do not say what an attacker gains beyond disrupting mounts such as NFS, AFS, CIFS or tracefs automounts.","severity":"unknown","cvss":null,"epss":0.00209,"exploited":false,"exploited_at":null,"products":[],"ecosystems":[],"tags":["afs","cifs","infrastructure","linux-kernel","nfs","tracefs"],"not_affected_if":["no-automounted-submounts","no-tracefs"],"how_to_check":"- Run `uname -r` and compare against the fixed kernel version in the vendor advisory (unknown from these sources).\n- Check whether automounted NFS, AFS, CIFS or tracefs submounts are in use with `findmnt -o TARGET,FSTYPE,PROPAGATION`.","fix":"- Update the Linux kernel to a release that includes the upstream commit titled 'namespace: check a submount for references right before unmounting it' (version unknown from these sources).","status":"open","published_at":"2026-10-23T13:17:20+00:00","first_seen_at":"2026-10-23T21:57:33+00:00","triaged_at":"2026-10-25T04:56:07+00:00","updated_at":"2026-10-25T12:38:50+00:00","category":"infrastructure","confidence":0.5,"triage":{"at":"2026-10-25T04:56:07Z","model":"claude-haiku-5-5","output":{"fix":"- Update the Linux kernel to a release that includes the upstream commit titled 'namespace: check a submount for references right before unmounting it' (version unknown from these sources).","reach":"local","title":"Linux kernel: busy submounts can be unmounted, breaking in-use mounts","impact":"low","summary":"A race in the Linux kernel's mount handling lets a synchronous umount remove a mount that is still in use elsewhere, because the busy check runs before an earlier unmount changes the mount tree. The sources do not say what an attacker gains beyond disrupting mounts such as NFS, AFS, CIFS or tracefs automounts.","category":"infrastructure","nicknames":[],"who_cares":["linux-kernel","nfs","cifs","afs","tracefs"],"confidence":0.5,"needs_auth":"unknown","worst_case":"A busy mount is forcibly detached, breaking processes that still use it","analyst_note":"Find the fixed kernel version and stable backport commit at https://nvd.nist.gov/vuln/detail/CVE-2026-98990 and the linked kernel.org commit.","how_to_check":"- Run `uname -r` and compare against the fixed kernel version in the vendor advisory (unknown from these sources).\n- Check whether automounted NFS, AFS, CIFS or tracefs submounts are in use with `findmnt -o TARGET,FSTYPE,PROPAGATION`.","needs_analyst":true,"not_affected_if":["no-automounted-submounts","no-tracefs"],"who_is_affected":"Systems running a Linux kernel without this fix that use automounted submounts (NFS, AFS, CIFS) or the tracefs mount beneath debugfs, since bind mounts inherit the flag. The sources do not say who can trigger the umount, so the exposure of multi-user hosts is unknown.","user_interaction":"unknown","needs_special_setup":"yes","severity_if_unknown":"unknown"},"cost_usd":0.000708,"input_tokens":3700,"output_tokens":675,"severity_before":"unknown"},"analyst_status":null,"analyst_note":null,"search":"'-2026':14A,17A '-98990':15A,18A '/tmp/plshrink/p1':235C '0':236C,237C,239C 'af':19B,77B,267C 'algorithm':307C 'allow':116C 'attack':69B 'automount':81B,263C 'becom':186C 'beyond':71B 'bind':276C 'break':8A 'busi':3A,51B,127C,133C,216C 'candid':188C 'chang':58B,142C 'check':52B,93C,281C 'childless':196C 'cif':20B,78B,269C 'collect':110C 'commit':201C 'copi':159C,167C,176C 'count':208C 'cve':13A,16A 'cwd':240C 'debugf':275C 'decid':129C 'disrupt':72B 'earlier':56B 'ebusi':254C 'els':223C 'elsewher':48B 'errno':238C 'expiri':108C 'fail':252C 'first':109C,140C 'flag':280C 'follow':87C 'gain':70B 'handl':35B 'hold':292C 'i.e':262C 'in-us':9A 'infrastructur':21B 'inherit':278C 'kernel':2A,24B,32B,85C 'let':36B 'linear':309C 'linux':1A,23B,31B,84C 'linux-kernel':22B 'lock':298C 'look':182C,204C 'make':303C 'mark':105C,225C 'mnt':227C,260C 'mount':12A,34B,42B,60B,73B,106C,113C,125C,132C,141C,163C,169C,217C,258C,273C,277C,283C,297C 'move':153C 'namespac':92C,294C 'need':256C 'nfs':25B,76B,266C 'one':148C,200C,248C 'overmount':156C,185C 'propag':131C,149C,151C,175C 'pull':213C 'race':28B 'refer':97C,207C 'remov':40B 'request':247C 'resolv':91C 'right':98C,284C 'run':53B 'say':66B 'second':147C,179C,191C 'sem':295C 'shrink':102C 'shrinkabl':261C 'somewher':222C 'sourc':63B 'stay':308C 'still':45B 'submount':4A,95C,103C,264C 'sure':304C 'surviv':155C 'sync':228C 'synchron':38B,211C 'tracef':26B,80B,272C 'tree':61B,136C,144C,193C 'trim':199C 'two':245C 'umount':39B,152C,192C,212C,229C,246C,301C 'umount2':234C 'unmount':7A,57B,100C,118C,121C,138C,161C,288C 'unreach':242C 'use':11A,47B,221C 'user':233C 'victim':180C 'vulner':88C 'whether':123C 'without':203C","nicknames":[],"mitigations":[],"who_affected":"Systems running a Linux kernel without this fix that use automounted submounts (NFS, AFS, CIFS) or the tracefs mount beneath debugfs, since bind mounts inherit the flag. The sources do not say who can trigger the umount, so the exposure of multi-user hosts is unknown.","tier":"C","verified":{},"analyst_revisit_at":null,"first_public_at":"2026-10-23T13:17:20+00:00","kev":null,"cvss_vector":null,"cvss_by":null,"poc_count":0,"sources":[{"source":"nvd","external_id":"CVE-2026-98990","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-98990","severity":null,"published_at":"2026-10-23T13:17:20+00:00","modified_at":"2026-10-23T13:17:20+00:00","first_seen_at":"2026-10-23T21:57:33+00:00","fetched_at":"2026-10-23T22:29:03+00:00","note":null},{"source":"epss","external_id":"CVE-2026-98990","url":"https://api.first.org/data/v1/epss?cve=CVE-2026-98990","severity":null,"published_at":null,"modified_at":null,"first_seen_at":"2026-10-25T12:38:50+00:00","fetched_at":"2026-10-25T12:38:50+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"}