Re: [PATCH net] skbuff: disable coalescing for page_pool recycling

[Yunsheng Lin <linyunsheng@huawei.com>]

+cc Alexander Duyck

On 2022/3/25 1:29, Jean-Philippe Brucker wrote:
> Fix a use-after-free when using page_pool with page fragments. We
> encountered this problem during normal RX in the hns3 driver:
> 
> (1) Initially we have three descriptors in the RX queue. The first one
>     allocates PAGE1 through page_pool, and the other two allocate one
>     half of PAGE2 each. Page references look like this:
> 
>                 RX_BD1 _______ PAGE1
>                 RX_BD2 _______ PAGE2
>                 RX_BD3 _________/
> 
> (2) Handle RX on the first descriptor. Allocate SKB1, eventually added
>     to the receive queue by tcp_queue_rcv().
> 
> (3) Handle RX on the second descriptor. Allocate SKB2 and pass it to
>     netif_receive_skb():
> 
>     netif_receive_skb(SKB2)
>       ip_rcv(SKB2)
>         SKB3 = skb_clone(SKB2)
> 
>     SKB2 and SKB3 share a reference to PAGE2 through
>     skb_shinfo()->dataref. The other ref to PAGE2 is still held by
>     RX_BD3:
> 
>                       SKB2 ---+- PAGE2
>                       SKB3 __/   /
>                 RX_BD3 _________/
> 
>  (3b) Now while handling TCP, coalesce SKB3 with SKB1:
> 
>       tcp_v4_rcv(SKB3)
>         tcp_try_coalesce(to=SKB1, from=SKB3)    // succeeds
>         kfree_skb_partial(SKB3)
>           skb_release_data(SKB3)                // drops one dataref
> 
>                       SKB1 _____ PAGE1
>                            \____
>                       SKB2 _____ PAGE2
>                                  /
>                 RX_BD3 _________/
> 
>     The problem is here: both SKB1 and SKB2 point to PAGE2 but SKB1 does
>     not actually hold a reference to PAGE2.

it seems the SKB1 *does* hold a reference to PAGE2 by calling __skb_frag_ref(),
which increments the page->_refcount instead of incrementing pp_frag_count,
as skb_cloned(SKB3) is true and __skb_frag_ref() does not handle page pool
case:

https://elixir.bootlin.com/linux/v5.17-rc1/source/net/core/skbuff.c#L5308

 Without coalescing, when
>     releasing both SKB2 and SKB3, a single reference to PAGE2 would be
>     dropped. Now when releasing SKB1 and SKB2, two references to PAGE2
>     will be dropped, resulting in underflow.
> 
>  (3c) Drop SKB2:
> 
>       af_packet_rcv(SKB2)
>         consume_skb(SKB2)
>           skb_release_data(SKB2)                // drops second dataref
>             page_pool_return_skb_page(PAGE2)    // drops one pp_frag_count
> 
>                       SKB1 _____ PAGE1
>                            \____
>                                  PAGE2
>                                  /
>                 RX_BD3 _________/
> 
> (4) Userspace calls recvmsg()
>     Copies SKB1 and releases it. Since SKB3 was coalesced with SKB1, we
>     release the SKB3 page as well:
> 
>     tcp_eat_recv_skb(SKB1)
>       skb_release_data(SKB1)
>         page_pool_return_skb_page(PAGE1)
>         page_pool_return_skb_page(PAGE2)        // drops second pp_frag_count
> 
> (5) PAGE2 is freed, but the third RX descriptor was still using it!
>     In our case this causes IOMMU faults, but it would silently corrupt
>     memory if the IOMMU was disabled.
> 
> A proper implementation would probably take another reference from the
> page_pool at step (3b), but that seems too complicated for a fix. Keep
> it simple for now, prevent coalescing for page_pool users.
> 
> Fixes: 53e0961da1c7 ("page_pool: add frag page recycling support in page pool")
> Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
> ---
> 
> The Fixes tag says frag page recycling but I'm not sure, does it not
> also affect full page recycling?  Coalescing is one case, are there
> other places where we move page fragments between skbuffs?  I'm still
> too unfamiliar with this code to figure it out.
> 
> Previously discussed here:
> https://lore.kernel.org/netdev/YfFbDivUPbpWjh%2Fm@myrica/
> ---
>  net/core/skbuff.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 10bde7c6db44..431f7ce88049 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -5276,11 +5276,11 @@ bool skb_try_coalesce(struct sk_buff *to, struct sk_buff *from,
>  	if (skb_cloned(to))
>  		return false;
>  
> -	/* The page pool signature of struct page will eventually figure out
> -	 * which pages can be recycled or not but for now let's prohibit slab
> -	 * allocated and page_pool allocated SKBs from being coalesced.
> +	/* Prohibit adding page_pool allocated SKBs, because that would require
> +	 * transferring the page fragment reference. For now let's also prohibit
> +	 * slab allocated and page_pool allocated SKBs from being coalesced.
>  	 */
> -	if (to->pp_recycle != from->pp_recycle)
> +	if (to->pp_recycle || from->pp_recycle)
>  		return false;
>  
>  	if (len <= skb_tailroom(to)) {
>