Re: [PATCH] io_uring/napi: fix list use-after-free on PREEMPT_RT

[Steven Rostedt <rostedt@goodmis.org>]

On Thu, 29 Oct 2026 15:52:44 +0530
Anvik Loitongbam <permissiontogallop@gmail.com> wrote:

> Commit 45b3941d09d1 ("io_uring/napi: fix io_napi_entry RCU accesses")
> converted the NAPI list teardown to list_for_each_entry() and
> kfree_rcu(), with a comment claiming that list_for_each_entry_safe()
> is not necessary because kfree_rcu() defers the free to the next
> quiescent state.

Not to mention, depending on the internals of list_del_rcu() and
kfree_rcu() is just not good design, regardless that it only breaks under
PREEMPT_RT.

> 
> This assumption only holds while the surrounding spinlock section is
> non-preemptible. On PREEMPT_RT, spinlock_t is an rtmutex and the task
> can be preempted while holding ctx->napi_lock. The raw lock inside
> kfree_rcu() only defers the preemption until it is released, and the
> release still happens before the iterator advances. Once the task is
> switched out, the context switch is an RCU quiescent state, if the
> preemption lasts long enough for the batch to be flushed and a grace
> period to complete, the callback frees the entry, and the iterator
> then reads e->list.next from freed memory. The same pattern exists
> in io_napi_free(). I did try to produce a uaf on an rt kernel
> build, but it did not work.
> 
> All list mutators are serialized by ctx->napi_lock, so the entry after
> the current one cannot be freed before its own iteration. Caching it
> first with list_for_each_entry_safe() in both loops might be a more
> safer and correct approach.
> 
> Fixes: 45b3941d09d1 ("io_uring/napi: fix io_napi_entry RCU accesses")
> Cc: stable@vger.kernel.org
> Signed-off-by: Anvik Loitongbam <permissiontogallop@gmail.com>

Reviewed-by: Steven Rostedt <rostedt@goodmis.org>

-- Steve

> ---
>  io_uring/napi.c | 14 ++++----------
>  1 file changed, 4 insertions(+), 10 deletions(-)
> 
> diff --git a/io_uring/napi.c b/io_uring/napi.c
> index 7e698720cd51..d299c89cfc3d 100644
> --- a/io_uring/napi.c
> +++ b/io_uring/napi.c
> @@ -110,16 +110,10 @@ static int __io_napi_del_id(struct io_ring_ctx *ctx, unsigned int napi_id)
>  
>  static void __io_napi_remove_stale(struct io_ring_ctx *ctx)
>  {
> -	struct io_napi_entry *e;
> +	struct io_napi_entry *e, *next;
>  
>  	guard(spinlock)(&ctx->napi_lock);
> -	/*
> -	 * list_for_each_entry_safe() is not required as long as:
> -	 * 1. list_del_rcu() does not reset the deleted node next pointer
> -	 * 2. kfree_rcu() delays the memory freeing until the next quiescent
> -	 *    state
> -	 */
> -	list_for_each_entry(e, &ctx->napi_list, list) {
> +	list_for_each_entry_safe(e, next, &ctx->napi_list, list) {
>  		if (time_after(jiffies, READ_ONCE(e->timeout))) {
>  			list_del_rcu(&e->list);
>  			hash_del_rcu(&e->node);
> @@ -264,10 +258,10 @@ void io_napi_init(struct io_ring_ctx *ctx)
>   */
>  void io_napi_free(struct io_ring_ctx *ctx)
>  {
> -	struct io_napi_entry *e;
> +	struct io_napi_entry *e, *next;
>  
>  	guard(spinlock)(&ctx->napi_lock);
> -	list_for_each_entry(e, &ctx->napi_list, list) {
> +	list_for_each_entry_safe(e, next, &ctx->napi_list, list) {
>  		hash_del_rcu(&e->node);
>  		kfree_rcu(e, rcu);
>  	}