{"id":"CVE-2025-21897","state":"PUBLISHED","title":"sched_ext: Fix pick_task_scx() picking non-queued tasks when it's called without balance()","description":"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.","publishedAt":"2025-04-01T15:26:49.705Z","updatedAt":"2026-05-11T21:08:38.332Z","severity":null,"assigner":"Linux","products":["Linux Linux"],"affected":[{"vendor":"Linux","product":"Linux","defaultStatus":"unaffected","versions":[{"version":"a6250aa251eacaf3ebfcfe152a96a727fd483ecd","status":"affected","lessThan":"5324c459f90d16b0c43a78b494c598915d782b7a","versionType":"git"},{"version":"a6250aa251eacaf3ebfcfe152a96a727fd483ecd","status":"affected","lessThan":"de60a31cb0bcacfaf9487546eac5e70e0a9c66d7","versionType":"git"},{"version":"a6250aa251eacaf3ebfcfe152a96a727fd483ecd","status":"affected","lessThan":"8fef0a3b17bb258130a4fcbcb5addf94b25e9ec5","versionType":"git"}],"repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","programFiles":["kernel/sched/ext.c"]},{"vendor":"Linux","product":"Linux","defaultStatus":"affected","versions":[{"version":"6.12","status":"affected"},{"version":"0","status":"unaffected","lessThan":"6.12","versionType":"semver"},{"version":"6.12.18","status":"unaffected","lessThanOrEqual":"6.12.*","versionType":"semver"},{"version":"6.13.6","status":"unaffected","lessThanOrEqual":"6.13.*","versionType":"semver"},{"version":"6.14","status":"unaffected","lessThanOrEqual":"*","versionType":"original_commit_for_fix"}],"repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","programFiles":["kernel/sched/ext.c"]}],"references":["https://git.kernel.org/stable/c/5324c459f90d16b0c43a78b494c598915d782b7a","https://git.kernel.org/stable/c/de60a31cb0bcacfaf9487546eac5e70e0a9c66d7","https://git.kernel.org/stable/c/8fef0a3b17bb258130a4fcbcb5addf94b25e9ec5"],"weaknesses":[],"ingestedAt":"2026-10-29T20:08:39.238Z","source":{"repository":"CVEProject/cvelistV5","path":"cves/2025/21xxx/CVE-2025-21897.json","commit":"556c9d3a8cd17e218bb8e452ba7cd619a384ff47"},"raw":{"dataType":"CVE_RECORD","dataVersion":"5.2","cveMetadata":{"cveId":"CVE-2025-21897","state":"PUBLISHED","datePublished":"2025-04-01T15:26:49.705Z","dateUpdated":"2026-05-11T21:08:38.332Z","assignerShortName":"Linux","assignerOrgId":"416baaa9-dc9f-4396-8d5f-8c081fb06d67","dateReserved":"2024-12-29T08:45:45.783Z"},"containers":{"cna":{"title":"sched_ext: Fix pick_task_scx() picking non-queued tasks when it's called without balance()","descriptions":[{"lang":"en","value":"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."}],"affected":[{"vendor":"Linux","product":"Linux","defaultStatus":"unaffected","versions":[{"version":"a6250aa251eacaf3ebfcfe152a96a727fd483ecd","status":"affected","lessThan":"5324c459f90d16b0c43a78b494c598915d782b7a","versionType":"git"},{"version":"a6250aa251eacaf3ebfcfe152a96a727fd483ecd","status":"affected","lessThan":"de60a31cb0bcacfaf9487546eac5e70e0a9c66d7","versionType":"git"},{"version":"a6250aa251eacaf3ebfcfe152a96a727fd483ecd","status":"affected","lessThan":"8fef0a3b17bb258130a4fcbcb5addf94b25e9ec5","versionType":"git"}],"repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","programFiles":["kernel/sched/ext.c"]},{"vendor":"Linux","product":"Linux","defaultStatus":"affected","versions":[{"version":"6.12","status":"affected"},{"version":"0","status":"unaffected","lessThan":"6.12","versionType":"semver"},{"version":"6.12.18","status":"unaffected","lessThanOrEqual":"6.12.*","versionType":"semver"},{"version":"6.13.6","status":"unaffected","lessThanOrEqual":"6.13.*","versionType":"semver"},{"version":"6.14","status":"unaffected","lessThanOrEqual":"*","versionType":"original_commit_for_fix"}],"repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","programFiles":["kernel/sched/ext.c"]}],"references":[{"url":"https://git.kernel.org/stable/c/5324c459f90d16b0c43a78b494c598915d782b7a"},{"url":"https://git.kernel.org/stable/c/de60a31cb0bcacfaf9487546eac5e70e0a9c66d7"},{"url":"https://git.kernel.org/stable/c/8fef0a3b17bb258130a4fcbcb5addf94b25e9ec5"}],"providerMetadata":{"shortName":"Linux","orgId":"416baaa9-dc9f-4396-8d5f-8c081fb06d67","dateUpdated":"2026-05-11T21:08:38.332Z"},"cpeApplicability":[{"nodes":[{"operator":"OR","negate":false,"cpeMatch":[{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"6.12","versionEndExcluding":"6.12.18"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"6.12","versionEndExcluding":"6.13.6"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"6.12","versionEndExcluding":"6.14"}]}]}],"x_generator":{"engine":"bippy-1.2.0"}}}}}