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

Mina Almasry
2024-04-24T22:08:58Z

On Wed, Apr 24, 2024 at 10:25 AM Dragos Tatulea <dtatulea@nvidia.com> wrote:
>
> 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
>

Sorry for the bug. I don't think this is the right fix to be honest
though. As you mention this is only 1 such instance of a general
underlying issue.

The underlying issue is that before the fixes commit, skb_frag_ref()
grabbed a regular page ref. After the fixes commit, skb_frag_ref()
grabs a page-pool ref. The unref path always dropped a regular page
ref, thanks to this commit as you point out:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=2cc3aeb5ecccec0d266813172fcd82b4b5fa5803

AFAICT the correct fix is to actually revert commit 2cc3aeb5eccc
("skbuff: Fix a potential race while recycling page_pool packets").
The reason is that now that skb_frag_ref() can grab page-pool refs, we
don't need to make sure there is only 1 SKB that triggers the recycle
path anymore. All the skb and its clones can obtain page-pool refs,
and in the unref path we drop the page-pool refs. page_pool_put_page()
detects correctly that the last page-pool ref is put and recycles the
page only then.

I'm unable to repro this issue. Do you need to do anything special? Or
is 1 flow that hits skb_segment() good enough?

Reverting commit 2cc3aeb5eccc ("skbuff: Fix a potential race while
recycling page_pool packets") shows no regressions for me. Is it
possible to try that out? If that doesn't work, I think I prefer
reverting a580ea994fd3 ("net: mirror skb frag ref/unref helpers")
rather than merging this fix to make sure we removed the underlying
cause of the issue.

> [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@nvidia.com>
> > Co-developed-by: Jianbo Liu <jianbol@nvidia.com>
> > Fixes: a580ea994fd3 ("net: mirror skb frag ref/unref helpers")
> > Cc: Mina Almasry <almasrymina@google.com>
> > ---
> > 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);
>

--
Thanks,
Mina