{"id":"CVE-2025-21897","summary":"sched_ext: Fix pick_task_scx() picking non-queued tasks when it's called without balance()","details":"In the Linux kernel, the following vulnerability has been resolved:\n\nsched_ext: Fix pick_task_scx() picking non-queued tasks when it's called without balance()\n\na6250aa251ea (\"sched_ext: Handle cases where pick_task_scx() is called\nwithout preceding balance_scx()\") added a workaround to handle the cases\nwhere pick_task_scx() is called without prececing balance_scx() which is due\nto a fair class bug where pick_taks_fair() may return NULL after a true\nreturn from balance_fair().\n\nThe workaround detects when pick_task_scx() is called without preceding\nbalance_scx() and emulates SCX_RQ_BAL_KEEP and triggers kicking to avoid\nstalling. Unfortunately, the workaround code was testing whether @prev was\non SCX to decide whether to keep the task running. This is incorrect as the\ntask may be on SCX but no longer runnable.\n\nThis could lead to a non-runnable task to be returned from pick_task_scx()\nwhich cause interesting confusions and failures. e.g. A common failure mode\nis the task ending up with (!on_rq && on_cpu) state which can cause\npotential wakers to busy loop, which can easily lead to deadlocks.\n\nFix it by testing whether @prev has SCX_TASK_QUEUED set. This makes\n@prev_on_scx only used in one place. Open code the usage and improve the\ncomment while at it.","modified":"2026-04-02T12:45:16.602777Z","published":"2025-04-01T15:26:49.705Z","database_specific":{"cna_assigner":"Linux","osv_generated_from":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2025/21xxx/CVE-2025-21897.json"},"references":[{"type":"WEB","url":"https://git.kernel.org/stable/c/5324c459f90d16b0c43a78b494c598915d782b7a"},{"type":"WEB","url":"https://git.kernel.org/stable/c/8fef0a3b17bb258130a4fcbcb5addf94b25e9ec5"},{"type":"WEB","url":"https://git.kernel.org/stable/c/de60a31cb0bcacfaf9487546eac5e70e0a9c66d7"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2025/21xxx/CVE-2025-21897.json"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2025-21897"},{"type":"PACKAGE","url":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git"}],"affected":[{"ranges":[{"type":"GIT","repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","events":[{"introduced":"a6250aa251eacaf3ebfcfe152a96a727fd483ecd"},{"fixed":"5324c459f90d16b0c43a78b494c598915d782b7a"},{"fixed":"de60a31cb0bcacfaf9487546eac5e70e0a9c66d7"},{"fixed":"8fef0a3b17bb258130a4fcbcb5addf94b25e9ec5"}]}],"database_specific":{"source":"https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2025-21897.json"}}],"schema_version":"1.7.5"}