Re: [PATCH net-next v3 2/3] page_pool: fix timing for checking and disabling napi_local

[Xuan Zhuo <xuanzhuo@linux.alibaba.com>]

On Tue, 22 Oct 2024 11:22:12 +0800, Yunsheng Lin <linyunsheng@huawei.com> wrote:
> page_pool page may be freed from skb_defer_free_flush() in
> softirq context without binding to any specific napi, it
> may cause use-after-free problem due to the below time window,
> as below, CPU1 may still access napi->list_owner after CPU0
> free the napi memory:
>
>             CPU 0                           CPU1
>       page_pool_destroy()          skb_defer_free_flush()
>              .                               .
>              .                napi = READ_ONCE(pool->p.napi);
>              .                               .
> page_pool_disable_direct_recycling()         .
>    driver free napi memory                   .
>              .                               .
>              .       napi && READ_ONCE(napi->list_owner) == cpuid
>              .                               .
>
> Use rcu mechanism to avoid the above problem.
>
> Note, the above was found during code reviewing on how to fix
> the problem in [1].
>
> 1. https://lore.kernel.org/lkml/8067f204-1380-4d37-8ffd-007fc6f26738@kernel.org/T/
>
> Fixes: dd64b232deb8 ("page_pool: unlink from napi during destroy")
> Signed-off-by: Yunsheng Lin <linyunsheng@huawei.com>


Reviewed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>

> CC: Alexander Lobakin <aleksander.lobakin@intel.com>
> ---
> As the IOMMU fix patch depends on synchronize_rcu() added in this
> patch and the time window is so small that it doesn't seem to be
> an urgent fix, so target the net-next as the IOMMU fix patch does.
> ---
>  net/core/page_pool.c | 15 ++++++++++++++-
>  1 file changed, 14 insertions(+), 1 deletion(-)
>
> diff --git a/net/core/page_pool.c b/net/core/page_pool.c
> index a813d30d2135..dd497f5c927d 100644
> --- a/net/core/page_pool.c
> +++ b/net/core/page_pool.c
> @@ -795,6 +795,7 @@ __page_pool_put_page(struct page_pool *pool, netmem_ref netmem,
>  static bool page_pool_napi_local(const struct page_pool *pool)
>  {
>  	const struct napi_struct *napi;
> +	bool napi_local;
>  	u32 cpuid;
>
>  	if (unlikely(!in_softirq()))
> @@ -810,9 +811,15 @@ static bool page_pool_napi_local(const struct page_pool *pool)
>  	if (READ_ONCE(pool->cpuid) == cpuid)
>  		return true;
>
> +	/* Synchronizated with page_pool_destory() to avoid use-after-free
> +	 * for 'napi'.
> +	 */
> +	rcu_read_lock();
>  	napi = READ_ONCE(pool->p.napi);
> +	napi_local = napi && READ_ONCE(napi->list_owner) == cpuid;
> +	rcu_read_unlock();
>
> -	return napi && READ_ONCE(napi->list_owner) == cpuid;
> +	return napi_local;
>  }
>
>  void page_pool_put_unrefed_netmem(struct page_pool *pool, netmem_ref netmem,
> @@ -1126,6 +1133,12 @@ void page_pool_destroy(struct page_pool *pool)
>  	if (!page_pool_release(pool))
>  		return;
>
> +	/* Paired with rcu lock in page_pool_napi_local() to enable clearing
> +	 * of pool->p.napi in page_pool_disable_direct_recycling() is seen
> +	 * before returning to driver to free the napi instance.
> +	 */
> +	synchronize_rcu();
> +
>  	page_pool_detached(pool);
>  	pool->defer_start = jiffies;
>  	pool->defer_warn  = jiffies + DEFER_WARN_INTERVAL;
> --
> 2.33.0
>
>