Re: [RFC PATCH] net: Fix one page_pool page leak from skb_frag_unref

[Dragos Tatulea]

On Wed, 2024-04-24 at 19:56 +0300, Dragos Tatulea wrote:
> The patch referenced in the fixes tag introduced the skb_frag_unref API.
This is wrong actually. Only skb_frag_ref was introduced. Sorry for the
confusion.

> This API still has a dark corner: when skb->pp_recycled is false and a
> fragment being referenced is backed by a page_pool page, skb_frag_unref
> can leak the page out of the page_pool, like in the following example:
> 
>  BUG: Bad page state in process swapper/4  pfn:103423
>  page: refcount:0 mapcount:0 mapping:0000000000000000 index:0x103423000 pfn:0x103423
>  flags: 0x200000000000000(node=0|zone=2)
>  page_type: 0xffffffff()
>  raw: 0200000000000000 dead000000000040 ffff888106f38000 0000000000000000
>  raw: 0000000103423000 0000000000000041 00000000ffffffff 0000000000000000
>  page dumped because: page_pool leak
>  Modules linked in: act_mirred act_csum act_pedit act_gact cls_flower
>  act_ct nf_flow_table sch_ingress xt_conntrack xt_MASQUERADE
>  nf_conntrack_netlink nfnetlink xt_addrtype iptable_nat nf_nat
>  br_netfilter overlay rpcrdma rdma_ucm ib_iser libiscsi
>  scsi_transport_iscsi ib_umad rdma_cm ib_ipoib iw_cm ib_cm mlx5_ib
>  ib_uverbs ib_core zram zsmalloc mlx5_core fuse CPU: 4 PID: 0 Comm:
>  swapper/4 Not tainted 6.9.0-rc4+ #2
>  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.13.0-0-gf21b5a4aeb02-prebuilt.qemu.org 04/01/2014
>  Call Trace:
>   <IRQ>
>   dump_stack_lvl+0x53/0x70
>   bad_page+0x6f/0xf0
>   free_unref_page_prepare+0x271/0x420
>   free_unref_page+0x38/0x120
>   ___pskb_trim+0x261/0x390
>   skb_segment+0xf60/0x1040
>   tcp_gso_segment+0xe8/0x4e0
>   inet_gso_segment+0x155/0x3d0
>   skb_mac_gso_segment+0x99/0x100
>   __skb_gso_segment+0xb4/0x160
>   ? netif_skb_features+0x95/0x2f0
>   validate_xmit_skb+0x16c/0x330
>   validate_xmit_skb_list+0x4c/0x70
>   sch_direct_xmit+0x23e/0x350
>   __dev_queue_xmit+0x334/0xbc0
>   tcf_mirred_to_dev+0x2a5/0x3e0 [act_mirred]
>   tcf_mirred_act+0xd7/0x4b0 [act_mirred]
>   ? tcf_pedit_act+0x6f/0x540 [act_pedit]
>   tcf_action_exec+0x82/0x170
>   fl_classify+0x1ee/0x200 [cls_flower]
>   ? tcf_mirred_to_dev+0x2a5/0x3e0 [act_mirred]
>   ? mlx5e_completion_event+0x39/0x40 [mlx5_core]
>   ? mlx5_eq_comp_int+0x1d7/0x1f0 [mlx5_core]
>   tcf_classify+0x26a/0x470
>   tc_run+0xa2/0x120
>   ? handle_irq_event+0x53/0x80
>   ? kvm_clock_get_cycles+0x11/0x20
>   __netif_receive_skb_core.constprop.0+0x932/0xee0
>   __netif_receive_skb_list_core+0xfe/0x1f0
>   netif_receive_skb_list_internal+0x1b5/0x2b0
>   napi_gro_complete.constprop.0+0xee/0x120
>   dev_gro_receive+0x3f4/0x710
>   napi_gro_receive+0x7d/0x220
>   mlx5e_handle_rx_cqe_mpwrq+0x10d/0x1d0 [mlx5_core]
>   mlx5e_poll_rx_cq+0x8b/0x6f0 [mlx5_core]
>   mlx5e_napi_poll+0xdc/0x6c0 [mlx5_core]
>   __napi_poll+0x25/0x1b0
>   net_rx_action+0x2c1/0x330
>   __do_softirq+0xcb/0x28c
>   irq_exit_rcu+0x69/0x90
>   common_interrupt+0x85/0xa0
>   </IRQ>
>   <TASK>
>   asm_common_interrupt+0x26/0x40
>  RIP: 0010:default_idle+0x17/0x20
>  Code: 00 4d 29 c8 4c 01 c7 4c 29 c2 e9 76 ff ff ff cc cc cc cc f3 0f 1e
>  fa 8b 05 76 3f 0a 01 85 c0 7e 07 0f 00 2d 1d 74 41 00 fb f4 <fa> c3 0f
>  1f 80 00 00 00 00 f3 0f 1e fa 65 48 8b 35 04 b3 42 7e f0
>  RSP: 0018:ffff88810087bed8 EFLAGS: 00000246
>  RAX: 0000000000000000 RBX: ffff8881008415c0 RCX: 000000e116d359fb
>  RDX: 0000000000000000 RSI: ffffffff8223e1d1 RDI: 000000000003f214
>  RBP: 0000000000000004 R08: 000000000003f214 R09: 000000e116d359fb
>  R10: 000000e116d359fb R11: 000000000005dfee R12: 0000000000000004
>  R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
>   default_idle_call+0x3d/0xf0
>   do_idle+0x1ce/0x1e0
>   cpu_startup_entry+0x29/0x30
>   start_secondary+0x109/0x130
>   common_startup_64+0x129/0x138
>   </TASK>
> 
> How it happens:
> -> skb_segment
>   -> clone skb into nskb
>   -> call __pskb_trim(nskb)
>     -> call pskb_expand_head(nskb) because nskb is cloned
>       -> call skb_release_data(nskb) because nskb is cloned
>         -> nskb->pp_recycle = 0
>     -> skb_unref(nskb->frag[i], nskb)
>     	-> nskb->pp_recycle is false, frag is page_pool page
> 	-> Fragment is released via put_page(frag page), hence leaking
> 	   from the page_pool.
> 
> Something tells me that this is not be the only issue of this kind...
> 
> The patch itself contains a suggested fix for this specific bug: it
> forces the unref in ___pskb_trim to recycle to the page_pool. If the
> page is not a page_pool page, it will be dereferenced as a regular page.
> 
> The alternative would be to save the skb->pp_recycled flag before
> pskb_expand_head and use it later during the unref.
> 

One more interesting point is the comment in skb_release_data [1] and it's
commit message [2] from Ilias. Looking at the commit message 

[1] https://elixir.bootlin.com/linux/v6.9-rc5/source/net/core/skbuff.c#L1137
[2]
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=2cc3aeb5ecccec0d266813172fcd82b4b5fa5803

> Signed-off-by: Dragos Tatulea <dtatulea@xxxxxxxxxx>
> Co-developed-by: Jianbo Liu <jianbol@xxxxxxxxxx>
> Fixes: a580ea994fd3 ("net: mirror skb frag ref/unref helpers")
> Cc: Mina Almasry <almasrymina@xxxxxxxxxx>
> ---
>  net/core/skbuff.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 37c858dc11a6..ab75b4f876ce 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -2634,7 +2634,7 @@ int ___pskb_trim(struct sk_buff *skb, unsigned int len)
>  		skb_shinfo(skb)->nr_frags = i;
>  
>  		for (; i < nfrags; i++)
> -			skb_frag_unref(skb, i);
> +			__skb_frag_unref(&skb_shinfo(skb)->frags[i], true);
>  
>  		if (skb_has_frag_list(skb))
>  			skb_drop_fraglist(skb);