[PATCH v5 0/6] sched/debug: Introduce per-CPU debugfs files

[Aaron Tomlin <atomlin@atomlin.com>]

Hi Peter, Juri, Ingo, Vincent,

This patch series addresses a few pre-existing memory safety and list
traversal concurrency issues in scheduler debugfs handlers, and introduces
per-CPU debugfs files under /sys/kernel/debug/sched/cpu/cpu<N>/debug.

Patch 1 introduces a new prerequisite patch that annotates struct rq's rd
(root_domain) pointer with __rcu in kernel/sched/sched.h and updates
lockless readers across the core scheduler to use rcu_dereference(),
ensuring Sparse compliance and proper memory barriers on weakly ordered
architectures.

Patch 2 fixes a use-after-free in print_dl_rq() where cpu_rq(cpu)->rd is
dereferenced locklessly to display deadline bandwidth statistics. During
CPU hot-unplug or cgroup cpuset repartitioning events,
partition_sched_domains() calls rq_attach_root() to detach the CPU from its
root_domain and schedules free_rootdomain() via call_rcu(). Without an RCU
read lock, an RCU grace period can resolve concurrently while debugfs reads
the file, allowing free_rootdomain() to execute kfree() and causing a UAF
when reading dl_bw->bw. This patch adds rcu_assign_pointer() on the writer
side in rq_attach_root() and uses guard(rcu)() with rcu_dereference() in
print_dl_rq().

Patch 3 fixes a potential use-after-free in print_cpu() where rq->curr is
dereferenced locklessly to output the running task's PID. If the task exits
concurrently and its reference count drops to zero, put_task_struct()
schedules __put_task_struct_rcu_cb() via call_rcu(). Without holding an RCU
read lock, an RCU grace period can elapse concurrently and free the task
structure via free_task(), leading to a use-after-free race condition. This
patch protects rq->curr access using rcu_dereference() inside an RCU
read-side critical section.

Patch 4 fixes both a time-of-check to time-of-use race condition and a
potential use-after-free in sched_show_numa(), where p->mm is checked
locklessly and then passed to P(mm->numa_scan_seq). If the task exits
concurrently via exit_mm(p), current->mm is set to NULL under task_lock(p)
before mmput() is called to free the struct mm_struct. Wrapping the p->mm
check and dereference in task_lock(p) eliminates both hazards.

Patch 5 fixes an RCU traversal violation in print_cfs_stats() where
rq->leaf_cfs_rq_list is traversed locklessly using
for_each_leaf_cfs_rq_safe(), which expands to list_for_each_entry_safe().
Although leaf_cfs_rq_list is modified using list_add_rcu(),
list_for_each_entry_safe() lacks READ_ONCE() and pre-fetches the next
pointer without memory barriers. Furthermore, because cfs_rq nodes are
re-linked on enqueue/dequeue without waiting for RCU grace periods,
concurrent list churn can cause backward jumps or infinite loops. This
patch introduces for_each_leaf_cfs_rq_rcu(), bounds traversal with a
circuit-breaker ceiling, and emits an explicit truncation notice if the
ceiling is reached.

Patch 6 introduces per-CPU debugfs entries under
/sys/kernel/debug/sched/cpu/cpu<N>/debug, allowing targeted inspection of
an individual CPU's runqueue on demand. If the target CPU is currently
offline, reading its file returns -ENODEV.

Changes since v4:

 - Added a new prerequisite patch to annotate struct rq's rd field with
   __rcu and updated lockless readers to use
   rcu_dereference()/rcu_dereference_sched()

 - Updated print_dl_rq() to use guard(rcu)() and rcu_dereference() on
   rq->rd (Daniel Vacek and K Prateek Nayak)

 - Replaced READ_ONCE(p->mm) with task_lock(p)/task_unlock(p) in
   sched_show_numa() to prevent use-after-free against concurrent exit_mm()
   and mmput()

 - Updated print_cfs_stats() to use guard(rcu)()

 - Increased SCHED_DEBUG_MAX_ITER from 1024 to 4096 and added an explicit
   truncation notice

 - Moved SEQ_printf() and SEQ_printf_task_group_path() to
   kernel/sched/sched.h, replaced strcpy() with strscpy(), and used
   IS_ENABLED(CONFIG_FAIR_GROUP_SCHED) with a typed static inline fallback
   stub

 - Corrected the "Fixes:" commit tag in Patch 5 to 039ae8bcf7a5 ("sched/fair:
   Fix O(nr_cgroups) in the load balancing path")

 - Linked to v4: https://lore.kernel.org/lkml/20260810015812.428999-1-atomlin@atomlin.com/

Changes since v3:

 - Updated Patch 1 to use rcu_dereference(rq->curr) instead of READ_ONCE()
   to preserve __rcu

 - Added missing writer-side RCU publication barrier (rcu_assign_pointer())
   in rq_attach_root() for Patch 2

 - Added Patch 3 to fix a TOCTOU condition in sched_show_numa() using
   READ_ONCE(p->mm)

 - Added a safety iteration ceiling in print_cfs_stats() for Patch 4 to
   prevent unbounded list iteration and RCU stalls under heavy
   leaf_cfs_rq_list churn

 - Linked to v3: https://lore.kernel.org/lkml/20260808235522.380038-1-atomlin@atomlin.com/

Changes since v2:

 - Protected lockless rq->curr dereferencing in print_cpu() with
   rcu_read_lock() and READ_ONCE()

 - Protected lockless rq->rd dereferencing in print_dl_rq() against CPU
   hot-unplug and cgroup cpuset repartitioning races

 - Introduced for_each_leaf_cfs_rq_rcu() using list_for_each_entry_rcu()
   for lockless leaf_cfs_rq_list iteration

 - Linked to v2: https://lore.kernel.org/lkml/20260728205238.18447-1-atomlin@atomlin.com/

Changes since v1:

 - Reframed commit message motivation around targeted interactive
   debugging on large SMP topologies (Peter Zijlstra and Zhan Xusheng)

 - Gated sched_debug_cpu_show() with a cpu_online(cpu) check
   returning -ENODEV when target CPU is offline (Zhan Xusheng)

 - Linked to v1: https://lore.kernel.org/lkml/20260728020309.6169-1-atomlin@atomlin.com/

Aaron Tomlin (6):
  sched: Annotate rq->rd with __rcu and update lockless readers
  sched/debug: Protect lockless rq->rd access in print_dl_rq()
  sched/debug: Protect lockless rq->curr access in print_cpu()
  sched/debug: Protect p->mm access in sched_show_numa()
  sched/fair: Use list_for_each_entry_rcu() in print_cfs_stats()
  sched/debug: Introduce per-CPU debugfs files

 kernel/sched/core.c     | 16 ++++---
 kernel/sched/deadline.c |  8 ++--
 kernel/sched/debug.c    | 92 ++++++++++++++++++++++++-----------------
 kernel/sched/fair.c     | 62 +++++++++++++++++++--------
 kernel/sched/sched.h    | 53 +++++++++++++++++++++++-
 kernel/sched/topology.c |  2 +-
 6 files changed, 165 insertions(+), 68 deletions(-)

-- 
2.55.0