Re: [PATCH net-next v7 2/8] page_pool: fix timing for checking and disabling napi_local
From: Toke Høiland-Jørgensen <hidden>
Date: 2025-01-10 15:40:18
Also in:
lkml
Yunsheng Lin [off-list ref] writes:
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
. .
Have you actually observed this happen, or are you just speculating?
Because I don't think it can; deleting a NAPI instance already requires
observing an RCU grace period, cf netdevice.h:
/**
* __netif_napi_del - remove a NAPI context
* @napi: NAPI context
*
* Warning: caller must observe RCU grace period before freeing memory
* containing @napi. Drivers might want to call this helper to combine
* all the needed RCU grace periods into a single one.
*/
void __netif_napi_del(struct napi_struct *napi);
/**
* netif_napi_del - remove a NAPI context
* @napi: NAPI context
*
* netif_napi_del() removes a NAPI context from the network device NAPI list
*/
static inline void netif_napi_del(struct napi_struct *napi)
{
__netif_napi_del(napi);
synchronize_net();
}
quoted hunk ↗ jump to hunk
Use rcu mechanism to avoid the above problem. Note, the above was found during code reviewing on how to fix the problem in [1]. As the following 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. 1. https://lore.kernel.org/lkml/8067f204-1380-4d37-8ffd-007fc6f26738@kernel.org/T/ (local) Fixes: dd64b232deb8 ("page_pool: unlink from napi during destroy") Signed-off-by: Yunsheng Lin <redacted> 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 9733206d6406..1aa7b93bdcc8 100644 --- a/net/core/page_pool.c +++ b/net/core/page_pool.c@@ -799,6 +799,7 @@ __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()))@@ -814,9 +815,15 @@ 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();
This rcu_read_lock/unlock() pair is redundant in the context you mention above, since skb_defer_free_flush() is only ever called from softirq context (within local_bh_disable()), which already function as an RCU read lock.
quoted hunk ↗ jump to hunk
- return napi && READ_ONCE(napi->list_owner) == cpuid; + return napi_local; } void page_pool_put_unrefed_netmem(struct page_pool *pool, netmem_ref netmem,@@ -1165,6 +1172,12 @@ 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();
Most drivers call page_pool_destroy() in a loop for each RX queue, so now you're introducing a full synchronize_rcu() wait for each queue. That can delay tearing down the device significantly, so I don't think this is a good idea. -Toke