[PATCH RFC v4 1/3] page_pool: fix timing for checking and disabling napi_local

[Yunsheng Lin <linyunsheng@huawei.com>]

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>
CC: Alexander Lobakin <aleksander.lobakin@intel.com>
Reviewed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
---
 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 f89cf93f6eb4..b3dae671eb26 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