Date: Thu, 25 Apr 2024 08:17:28 +0000
From: Dragos Tatulea <dtatulea@...dia.com>
To: "almasrymina@...gle.com" <almasrymina@...gle.com>
CC: "davem@...emloft.net" <davem@...emloft.net>, "pabeni@...hat.com"
	<pabeni@...hat.com>, "ilias.apalodimas@...aro.org"
	<ilias.apalodimas@...aro.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "jacob.e.keller@...el.com"
	<jacob.e.keller@...el.com>, "netdev@...r.kernel.org"
	<netdev@...r.kernel.org>, Jianbo Liu <jianbol@...dia.com>,
	"edumazet@...gle.com" <edumazet@...gle.com>, "kuba@...nel.org"
	<kuba@...nel.org>
Subject: Re: [RFC PATCH] net: Fix one page_pool page leak from skb_frag_unref

On Wed, 2024-04-24 at 15:08 -0700, Mina Almasry wrote:
> On Wed, Apr 24, 2024 at 10:25 AM Dragos Tatulea <dtatulea@...dia.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.
> 
Right. It was a conversation starter :).

> 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.
> 
It is still possible for skb_frag_ref() to grab a regular page ref for a
page_pool page: for example in skb_segment() when a nskb is created (not cloned,
so pp_recycle is off) and frags with page_pool pages are added to
it. I was seeing this in my test as well.

If the intention is to always use page_pool ref/unref for page_pool pages, then
the recycle flag check doesn't belong skb_frag_ref/unref().

>  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 don't think this is a good way forward. For example, skb->pp_recycle is used
as a hint in skb_gro_receive to avoid coalescing skbs with different pp_recycle
flag states. This could interfere with that.

> I'm unable to repro this issue. Do you need to do anything special? Or
> is 1 flow that hits skb_segment() good enough?
> 
I don't have a very easy repro. But the test is a forwarding one.

> 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?
> 
I can try it out, it might work. But again, I fear that simply reverting this
change can have trigger other unexpected behaviours. Also, this doesn't handle
the behaviour for skb_frag_ref mentioned above.

>  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.
This is the safest bet.

So, to recap, I see 2 possibilities:

1) Revert a580ea994fd3 ("net: mirror skb frag ref/unref helpers"): safe, but it
will probably have to come back in one way or another.
2) Drop the recycle checks from skb_frag_ref/unref: this enforces the rule of
always referencing/dereferencing pages based on their type (page_pool or
normal). 

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

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