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CVE-2026-98163
cgroup: Avoid iteration of dying tasks with zero refcount
Published 2026-09-26 (23 days ago) by the Linux kernel CNA. Subsystem kernel/cgroup. Debian urgency: unimportant.
What to do, per Debian release
Debian's own status for CVE-2026-98163 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 | 7.2.8-1 | Update the kernel and reboot. Fixed in 7.2.8-1. A running kernel keeps the old code until the machine restarts. |
| Debian unstablesid | fixed | 7.2.8-1 | Update the kernel and reboot. Fixed in 7.2.8-1. A running kernel keeps the old code until the machine restarts. |
The vulnerability
What the bug is
The commit 260fbcb92bbea ("cgroup: Move dying_tasks cleanup from cgroup_task_release() to cgroup_task_free()") extended the lifetime of tasks on the dying_tasks list. The iterators have provision to go through dying_tasks because of dying threadgroup leaders or explicit CSS_TASK_ITER_WITH_DEAD, however, it was expected that such tasks can obtain a new reference (that is possible before cgroup_task_release()/put_task_struct_rcu_user()). The tasks after cgroup_task_release() and before cgroup_task_free() are subject to race when they may or may not have ->usage count > 0. The race window is between css_task_iter_next() invocations when css_set_lock is released and we may arrive at a new ->task_pos. The iterator should not attempt to resurrect tasks whose ->usage count dropped to zero. (When that happens, __put_task_struct_rcu_cb() is already imminent and the returned task_struct would could be used after free.) As for the fix, we cannot simply check the signal->live count of a task on the dying list because that won't distinguish regular zombies waiting to be reaped from RCU remnant tasks that are going to be free'd. Therefore add an extra check to rule out ->usage==0 tasks from any iteration. The repeat: loop in css_task_iter_advance() doesn't consider ->usage count, so add a new loop to css_task_iter_next() to skip de-used tasks on the dying_list. Rough illustration of the possible race R (reader of cgroup.procs) T (thread) L (group leader) --------------------------------- -------------------------------- -------------------------------- L exits, signal->live > 0 cgroup_task_dead(L) css_set_skip_task_iters() // skips only cset->tasks list_add_tail(&L->cg_list, &cset->dying_tasks) css_task_iter_next() take css_set_lock css_task_iter_advance() leader && signal->live != 0 => it->task_pos = &L->cg_list release css_set_lock T exits --signal->live == 0 cgroup_task_dead(T) // css_set_lock release_task(T) cgroup_task_release(T) release_task(L) // zap_leader cgroup_task_release(L) put_task_struct_rcu_user(L) ...RCU... put_task_struct(L) L->usage = 0 /* L still on dying_tasks */ ...RCU... __put_task_struct(L) css_task_iter_next() // another iteration take css_set_lock it->task_pos = &L->cg_list get_task_struct(L) => addition on 0 drop css_set_lock cgroup_task_free(L) css_set_skip_task_iters() // dying skip comes too late free_task(L) cgroup_procs_show() task_pid_vnr(L)
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/cgroup/cgroup.c
- What this is
- CONTROL GROUP (CGROUP)MAINTAINERS section matched on kernel/cgroup/ · cgroups@vger.kernel.org · Maintained
Check it on the machine
grep -w CONFIG_CGROUPS /boot/config-$(uname -r)Reads the booted kernel's own configuration, on the machine and nowhere else. =y is built in, =m is a module, and an is not set line means the code is not there.
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
- Build switch
- CONFIG_CGROUPS
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 kbuild Makefiles Debian linux-config packages
Signals
- Published
- 2026-09-26 (23 days ago)source
- CVSS
- 7.8 high
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:Hsource recompute it - Attack vector
- local (AV:L)source
- EPSS
- 0.10%, higher than 1.0% of all CVEssource
- CISA KEV
- not listedthe KEV catalogue
- Debian urgency
- unimportantsource
- Subsystem
- kernel/cgroup
Upstream fixes
The versions the CNA records as introducing and fixing this, and the commit for each. source
| Introduced | Fixed in | Commit |
|---|---|---|
| 6.19 | 7.2.8 | 828938118d6c |
| 6.19 | 7.3 | 057dac23d329 |
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 bug is in css_task_iter_next() (kernel/cgroup/cgroup.c), reached by a local read() of a cgroup.procs/cgroup.threads seq file via cgroup_procs_start/next/show. No remote peer supplies any input to this path.
- AC:LAttack complexity, low
- The attacker controls both sides of the race: one thread repeatedly reads its own cgroup.procs while the attacker's own multithreaded process exits its group leader and then its last thread, dropping the leader's ->usage to zero while it stays on dying_tasks. The attacker can retry until the reader lands on that task.
- PR:LPrivileges required, low
- cgroup_file_mode() makes cgroup.procs S_IRUGO, so any unprivileged local user can read it. Creating and exiting threads in their own cgroup needs no capability, so a basic local account is enough.
- UI:NUser interaction, none
- The attacker triggers the race entirely with their own processes and their own reads of cgroup.procs. No other user or administrator has to do anything.
- S:UScope, unchanged
- The use-after-free corrupts a kernel task_struct within the same kernel security authority. It does not cross a VM, IOMMU or similar boundary.
- C:HConfidentiality impact, high
- get_task_struct() on a zero-refcount task makes the iterator keep a task_struct that free_task() releases. cgroup_procs_show() then dereferences it via task_pid_vnr(), so a reclaimed object's contents are read and reported back to userspace.
- I:HIntegrity impact, high
- The iterator later calls put_task_struct() on the freed task_struct, decrementing ->usage in memory that may have been reallocated. This use-after-free write on reclaimed slab memory can be groomed into a corruption primitive.
- A:HAvailability impact, high
- The refcount_t "addition on 0" warning, followed by dereferencing and releasing a freed task_struct in css_task_iter_next()/css_task_iter_end(), causes an oops, or a panic with panic_on_warn/KASAN, and an unprivileged user can repeat it.