[PATCH 6.12.y 1/2] padata: Fix pd UAF once and for all

Bin Lan
2026-04-25T08:14:54Z

From: Herbert Xu <herbert@gondor.apana.org.au>

[ Upstream commit 71203f68c7749609d7fc8ae6ad054bdedeb24f91 ]

There is a race condition/UAF in padata_reorder that goes back
to the initial commit. A reference count is taken at the start
of the process in padata_do_parallel, and released at the end in
padata_serial_worker.

This reference count is (and only is) required for padata_replace
to function correctly. If padata_replace is never called then
there is no issue.

In the function padata_reorder which serves as the core of padata,
as soon as padata is added to queue->serial.list, and the associated
spin lock released, that padata may be processed and the reference
count on pd would go away.

Fix this by getting the next padata before the squeue->serial lock
is released.

In order to make this possible, simplify padata_reorder by only
calling it once the next padata arrives.

Fixes: 16295bec6398 ("padata: Generic parallelization/serialization interface")
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
[ Adjust context of padata_find_next(). Replace
cpumask_next_wrap(cpu, pd->cpumask.pcpu) with
cpumask_next_wrap(cpu, pd->cpumask.pcpu, -1, false) in padata_reorder() in
v6.12 according to dc5bb9b769c9 ("cpumask: deprecate cpumask_next_wrap()") and
f954a2d37637 ("padata: switch padata_find_next() to using cpumask_next_wrap()")
. ]
Signed-off-by: Bin Lan <lanbincn@139.com>
---
include/linux/padata.h | 3 -
kernel/padata.c | 136 +++++++++++------------------------------
2 files changed, 37 insertions(+), 102 deletions(-)

diff --git a/include/linux/padata.h b/include/linux/padata.h
index 0146daf34430..b486c7359de2 100644
--- a/include/linux/padata.h
+++ b/include/linux/padata.h
@@ -91,7 +91,6 @@ struct padata_cpumask {
* @cpu: Next CPU to be processed.
* @cpumask: The cpumasks in use for parallel and serial workers.
* @reorder_work: work struct for reordering.
- * @lock: Reorder lock.
*/
struct parallel_data {
struct padata_shell *ps;
@@ -102,8 +101,6 @@ struct parallel_data {
unsigned int processed;
int cpu;
struct padata_cpumask cpumask;
- struct work_struct reorder_work;
- spinlock_t ____cacheline_aligned lock;
};

/**
diff --git a/kernel/padata.c b/kernel/padata.c
index c3810f5bd715..e61bdc248551 100644
--- a/kernel/padata.c
+++ b/kernel/padata.c
@@ -261,20 +261,17 @@ EXPORT_SYMBOL(padata_do_parallel);
* be parallel processed by another cpu and is not yet present in
* the cpu's reorder queue.
*/
-static struct padata_priv *padata_find_next(struct parallel_data *pd,
- bool remove_object)
+static struct padata_priv *padata_find_next(struct parallel_data *pd, int cpu,
+ unsigned int processed)
{
struct padata_priv *padata;
struct padata_list *reorder;
- int cpu = pd->cpu;

reorder = per_cpu_ptr(pd->reorder_list, cpu);

spin_lock(&reorder->lock);
- if (list_empty(&reorder->list)) {
- spin_unlock(&reorder->lock);
- return NULL;
- }
+ if (list_empty(&reorder->list))
+ goto notfound;

padata = list_entry(reorder->list.next, struct padata_priv, list);

@@ -282,101 +279,52 @@ static struct padata_priv *padata_find_next(struct parallel_data *pd,
* Checks the rare case where two or more parallel jobs have hashed to
* the same CPU and one of the later ones finishes first.
*/
- if (padata->seq_nr != pd->processed) {
- spin_unlock(&reorder->lock);
- return NULL;
- }
-
- if (remove_object) {
- list_del_init(&padata->list);
- ++pd->processed;
- /* When sequence wraps around, reset to the first CPU. */
- if (unlikely(pd->processed == 0))
- pd->cpu = cpumask_first(pd->cpumask.pcpu);
- else
- pd->cpu = cpumask_next_wrap(cpu, pd->cpumask.pcpu, -1, false);
- }
+ if (padata->seq_nr != processed)
+ goto notfound;

+ list_del_init(&padata->list);
spin_unlock(&reorder->lock);
return padata;
+
+notfound:
+ pd->processed = processed;
+ pd->cpu = cpu;
+ spin_unlock(&reorder->lock);
+ return NULL;
}

-static void padata_reorder(struct parallel_data *pd)
+static void padata_reorder(struct padata_priv *padata)
{
+ struct parallel_data *pd = padata->pd;
struct padata_instance *pinst = pd->ps->pinst;
- int cb_cpu;
- struct padata_priv *padata;
- struct padata_serial_queue *squeue;
- struct padata_list *reorder;
+ unsigned int processed;
+ int cpu;

- /*
- * We need to ensure that only one cpu can work on dequeueing of
- * the reorder queue the time. Calculating in which percpu reorder
- * queue the next object will arrive takes some time. A spinlock
- * would be highly contended. Also it is not clear in which order
- * the objects arrive to the reorder queues. So a cpu could wait to
- * get the lock just to notice that there is nothing to do at the
- * moment. Therefore we use a trylock and let the holder of the lock
- * care for all the objects enqueued during the holdtime of the lock.
- */
- if (!spin_trylock_bh(&pd->lock))
- return;
+ processed = pd->processed;
+ cpu = pd->cpu;

- while (1) {
- padata = padata_find_next(pd, true);
+ do {
+ struct padata_serial_queue *squeue;
+ int cb_cpu;

- /*
- * If the next object that needs serialization is parallel
- * processed by another cpu and is still on it's way to the
- * cpu's reorder queue, nothing to do for now.
- */
- if (!padata)
- break;
+ cpu = cpumask_next_wrap(cpu, pd->cpumask.pcpu, -1, false);
+ processed++;

cb_cpu = padata->cb_cpu;
squeue = per_cpu_ptr(pd->squeue, cb_cpu);

spin_lock(&squeue->serial.lock);
list_add_tail(&padata->list, &squeue->serial.list);
- spin_unlock(&squeue->serial.lock);
-
queue_work_on(cb_cpu, pinst->serial_wq, &squeue->work);
- }

- spin_unlock_bh(&pd->lock);
-
- /*
- * The next object that needs serialization might have arrived to
- * the reorder queues in the meantime.
- *
- * Ensure reorder queue is read after pd->lock is dropped so we see
- * new objects from another task in padata_do_serial. Pairs with
- * smp_mb in padata_do_serial.
- */
- smp_mb();
-
- reorder = per_cpu_ptr(pd->reorder_list, pd->cpu);
- if (!list_empty(&reorder->list) && padata_find_next(pd, false)) {
/*
- * Other context(eg. the padata_serial_worker) can finish the request.
- * To avoid UAF issue, add pd ref here, and put pd ref after reorder_work finish.
+ * If the next object that needs serialization is parallel
+ * processed by another cpu and is still on it's way to the
+ * cpu's reorder queue, end the loop.
*/
- padata_get_pd(pd);
- if (!queue_work(pinst->serial_wq, &pd->reorder_work))
- padata_put_pd(pd);
- }
-}
-
-static void invoke_padata_reorder(struct work_struct *work)
-{
- struct parallel_data *pd;
-
- local_bh_disable();
- pd = container_of(work, struct parallel_data, reorder_work);
- padata_reorder(pd);
- local_bh_enable();
- /* Pairs with putting the reorder_work in the serial_wq */
- padata_put_pd(pd);
+ padata = padata_find_next(pd, cpu, processed);
+ spin_unlock(&squeue->serial.lock);
+ } while (padata);
}

static void padata_serial_worker(struct work_struct *serial_work)
@@ -427,6 +375,7 @@ void padata_do_serial(struct padata_priv *padata)
struct padata_list *reorder = per_cpu_ptr(pd->reorder_list, hashed_cpu);
struct padata_priv *cur;
struct list_head *pos;
+ bool gotit = true;

spin_lock(&reorder->lock);
/* Sort in ascending order of sequence number. */
@@ -436,17 +385,14 @@ void padata_do_serial(struct padata_priv *padata)
if ((signed int)(cur->seq_nr - padata->seq_nr) < 0)
break;
}
- list_add(&padata->list, pos);
+ if (padata->seq_nr != pd->processed) {
+ gotit = false;
+ list_add(&padata->list, pos);
+ }
spin_unlock(&reorder->lock);

- /*
- * Ensure the addition to the reorder list is ordered correctly
- * with the trylock of pd->lock in padata_reorder. Pairs with smp_mb
- * in padata_reorder.
- */
- smp_mb();
-
- padata_reorder(pd);
+ if (gotit)
+ padata_reorder(padata);
}
EXPORT_SYMBOL(padata_do_serial);

@@ -643,9 +589,7 @@ static struct parallel_data *padata_alloc_pd(struct padata_shell *ps)
padata_init_squeues(pd);
pd->seq_nr = -1;
refcount_set(&pd->refcnt, 1);
- spin_lock_init(&pd->lock);
pd->cpu = cpumask_first(pd->cpumask.pcpu);
- INIT_WORK(&pd->reorder_work, invoke_padata_reorder);

return pd;

@@ -1155,12 +1099,6 @@ void padata_free_shell(struct padata_shell *ps)
if (!ps)
return;

- /*
- * Wait for all _do_serial calls to finish to avoid touching
- * freed pd's and ps's.
- */
- synchronize_rcu();
-
mutex_lock(&ps->pinst->lock);
list_del(&ps->list);
pd = rcu_dereference_protected(ps->pd, 1);
--
2.43.0