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CVE-2026-89521
sched/core: Handle pick_task() releasing the rq lock
Published 2026-09-11 (37 days ago) by the Linux kernel CNA. Subsystem kernel/sched. Debian urgency: unimportant.
What to do, per Debian release
Debian's own status for CVE-2026-89521 in every release this site tracks, and what each one means in practice. Every status below is the one the Debian Security Team publishes for this CVE. source
| Release | Status | Version | What this means |
|---|---|---|---|
| Debian 12bookworm | not affected | 6.1.190-1 | Nothing to do. bookworm never shipped the vulnerable code. |
| Debian 13trixie | not affected | 6.12.113-1 | Nothing to do. trixie never shipped the vulnerable code. |
| Debian 14forky | fixed | 7.2.6-1 | Update the kernel and reboot. Fixed in 7.2.6-1. A running kernel keeps the old code until the machine restarts. |
| Debian unstablesid | fixed | 7.2.6-1 | Update the kernel and reboot. Fixed in 7.2.6-1. A running kernel keeps the old code until the machine restarts. |
The vulnerability
What the bug is
Core scheduling's pick_next_task() breaks when a ->pick_task() implementation can release the rq lock. The selection state derived on entry is only valid while the lock is held continuously. Once a pick can drop the lock, an interleaving selection can invalidate all of it: the single-CPU fast path can commit an uncookied pick although the core went cookied during the release, and forceidle committed by the interleaving selection skews the restarted pass's accounting. Fix it by restarting the whole selection when a pick returns RETRY_TASK after releasing the lock: a single restart point above the state derivation replaces the per-loop restart labels, so a retry picks up state committed by interleaving selections and accounts and resets forceidle like a fresh selection would. need_sync and fi_before latch across retries. Clock validity can't be re-derived - there is no program-ordered way to tell whether the own and core rq clocks are still updated after the lock was released, as other lockers' pin cycles may or may not have invalidated them. When restarting, clear core_clock_updated so that the sibling loop re-updates the core rq, and update the own rq clock if invalidated.
Where it lives
The fix touches these files. If the subsystem is one you do not use, the practical exposure is lower, though the package is still the vulnerable one.
kernel/sched/core.c
- What this is
- SCHEDULERMAINTAINERS section matched on kernel/sched/ · linux-kernel@vger.kernel.org · Maintained
Show whether Debian builds this, per release
Debian builds this code. Every release below compiles the affected file into the kernels it ships.
- bookworm, trixie, forky, sidy
- y
- built into the kernel image on every flavour - only a patch removes it
From the configuration Debian publishes for every kernel flavour it builds. It covers only the architectures Debian ships a linux-config package for, and nothing about a kernel you built yourself. Debian's own status above stays authoritative about the source package.MAINTAINERS Debian linux-config packages
Signals
- Published
- 2026-09-11 (37 days ago)source
- CVSS
- 7.3 high
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:L/A:Hsource recompute it - Attack vector
- local (AV:L)source
- EPSS
- 0.15%, higher than 4.0% of all CVEssource
- CISA KEV
- not listedthe KEV catalogue
- Debian urgency
- unimportantsource
- Subsystem
- kernel/sched
Upstream fixes
The versions the CNA records as introducing and fixing this, and the commit for each. source
| Introduced | Fixed in | Commit |
|---|---|---|
| 6.19 | 7.2.4 | 88ed5a66467c |
| 6.19 | 7.3 | c10b216a072f |
Why the CNA scored it this way
Quoted from the kernel CNA's own scoring file, word for word. A highlighted metric carries the most severe value CVSS v3.1 defines for it, which is what pushes the score up. source
- AV:LAttack vector, local
- The flaw is in pick_next_task() in kernel/sched/core.c, reached only from the local __schedule() hotpath. An attacker triggers it with prctl(PR_SCHED_CORE) and their own threads; it is not reachable from network packet processing, ksmbd/nfsd, or other remote handlers.
- AC:LAttack complexity, low
- The attacker controls both sides of the race: they create cookied and uncookied tasks, pin them on both SMT siblings, and generate concurrent scheduling so pick_task_scx() drops the rq lock during core-wide selection. Success does not depend on an uncontrollable victim state or memory layout.
- PR:LPrivileges required, low
- prctl(PR_SCHED_CORE) is available to an unprivileged process (only ptrace_may_access on its own tasks). The lock-dropping pick_task() is sched_ext, which an attacker can load with CAP_BPF/CAP_PERFMON via a user namespace or BPF token, or which may already be loaded on the host.
- UI:NUser interaction, none
- The attacker enables core scheduling, runs their own threads on SMT siblings, and drives context switches themselves. No victim action such as mounting a filesystem or opening a file is required.
- S:UScope, unchanged
- Impact is confined to the host kernel scheduler (SMT cookie selection and forceidle/vruntime accounting). This is a same-host isolation failure, not a VM/guest-to-host escape or IOMMU/DMA boundary bypass.
- C:HConfidentiality impact, high
- After pick_task() drops the rq lock, the single-CPU fast path can commit an uncookied pick while an interleaving selection made the core cookied, so SMT siblings run mismatched cookies and bypass core scheduling's L1TF/MDS isolation, enabling cross-domain memory disclosure via SMT side channels.
- I:LIntegrity impact, low
- The misplaced RETRY_TASK restart skips forceidle reset and can apply invalidated rq clocks to vruntime and forceidle accounting, which is limited corruption of scheduler metadata rather than an arbitrary kernel write or control-flow hijack.
- A:HAvailability impact, high
- Invalidated rq clocks and skewed forceidle accounting after a lock-dropping pick can apply a huge bogus vruntime delta or incorrectly force-idle SMT siblings, stranding runnable tasks in a hang-class scheduling DoS on the core.