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Date:   Wed, 6 Jul 2022 21:13:16 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     Nikolay Aleksandrov <razor@...ckwall.org>,
        Hans Schultz <schultz.hans@...il.com>
Cc:     davem@...emloft.net, kuba@...nel.org, netdev@...r.kernel.org,
        Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Paolo Abeni <pabeni@...hat.com>, Jiri Pirko <jiri@...nulli.us>,
        Ivan Vecera <ivecera@...hat.com>,
        Roopa Prabhu <roopa@...dia.com>, Shuah Khan <shuah@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Ido Schimmel <idosch@...dia.com>, linux-kernel@...r.kernel.org,
        bridge@...ts.linux-foundation.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH V3 net-next 1/4] net: bridge: add fdb flag to extent
 locked port feature

Hi Nikolay,

On Wed, May 25, 2022 at 01:18:49PM +0300, Nikolay Aleksandrov wrote:
> >>>>>> Hi Hans,
> >>>>>> So this approach has a fundamental problem, f->dst is changed without any synchronization
> >>>>>> you cannot rely on it and thus you cannot account for these entries properly. We must be very
> >>>>>> careful if we try to add any new synchronization not to affect performance as well.
> >>>>>> More below...
> >>>>>>
> >>>>>>> @@ -319,6 +326,9 @@ static void fdb_delete(struct net_bridge *br, struct net_bridge_fdb_entry *f,
> >>>>>>>  	if (test_bit(BR_FDB_STATIC, &f->flags))
> >>>>>>>  		fdb_del_hw_addr(br, f->key.addr.addr);
> >>>>>>>  
> >>>>>>> +	if (test_bit(BR_FDB_ENTRY_LOCKED, &f->flags) && !test_bit(BR_FDB_OFFLOADED, &f->flags))
> >>>>>>> +		atomic_dec(&f->dst->locked_entry_cnt);
> >>>>>>
> >>>>>> Sorry but you cannot do this for multiple reasons:
> >>>>>>  - f->dst can be NULL
> >>>>>>  - f->dst changes without any synchronization
> >>>>>>  - there is no synchronization between fdb's flags and its ->dst
> >>>>>>
> >>>>>> Cheers,
> >>>>>>  Nik
> >>>>>
> >>>>> Hi Nik,
> >>>>>
> >>>>> if a port is decoupled from the bridge, the locked entries would of
> >>>>> course be invalid, so maybe if adding and removing a port is accounted
> >>>>> for wrt locked entries and the count of locked entries, would that not
> >>>>> work?
> >>>>>
> >>>>> Best,
> >>>>> Hans
> >>>>
> >>>> Hi Hans,
> >>>> Unfortunately you need the correct amount of locked entries per-port if you want
> >>>> to limit their number per-port, instead of globally. So you need a
> >>>> consistent
> >>>
> >>> Hi Nik,
> >>> the used dst is a port structure, so it is per-port and not globally.
> >>>
> >>> Best,
> >>> Hans
> >>>
> >>
> >> Yeah, I know. :) That's why I wrote it, if the limit is not a feature requirement I'd suggest
> >> dropping it altogether, it can be enforced externally (e.g. from user-space) if needed.
> >>
> >> By the way just fyi net-next is closed right now due to merge window. And one more
> >> thing please include a short log of changes between versions when you send a new one.
> >> I had to go look for v2 to find out what changed.
> >>
> > 
> > Okay, I will drop the limit in the bridge module, which is an easy thing
> > to do. :) (It is mostly there to ensure against DOS attacks if someone
> > bombards a locked port with random mac addresses.)
> > I have a similar limitation in the driver, which should then probably be
> > dropped too?
> > 
> 
> That is up to you/driver, I'd try looking for similar problems in other switch drivers
> and check how those were handled. There are people in the CC above that can
> directly answer that. :)

Not sure whom you're referring to?

In fact I was pretty sure that I didn't see any OOM protection in the
source code of the Linux bridge driver itself either, so I wanted to
check that for myself, so I wrote a small "killswitch" program that's
supposed to, well, kill a switch. It took me a while to find a few free
hours to do the test, sorry for that.

https://github.com/vladimiroltean/killswitch/blob/master/src/killswitch.c

Sure enough, I can kill a Marvell Armada 3720 device with 1GB of RAM
within 3 minutes of running the test program.

[  273.864203] ksoftirqd/0: page allocation failure: order:0, mode:0x40a20(GFP_ATOMIC|__GFP_COMP), nodemask=(null),cpuset=/,mems_allowed=0
[  273.876426] CPU: 0 PID: 12 Comm: ksoftirqd/0 Not tainted 5.18.7-rc1-00013-g52b92343db13 #74
[  273.884775] Hardware name: CZ.NIC Turris Mox Board (DT)
[  273.889994] Call trace:
[  273.892437]  dump_backtrace.part.0+0xc8/0xd4
[  273.896721]  show_stack+0x18/0x70
[  273.900039]  dump_stack_lvl+0x68/0x84
[  273.903703]  dump_stack+0x18/0x34
[  273.907017]  warn_alloc+0x114/0x1a0
[  273.910508]  __alloc_pages+0xbb0/0xbe0
[  273.914257]  cache_grow_begin+0x60/0x300
[  273.918183]  fallback_alloc+0x184/0x220
[  273.922017]  ____cache_alloc_node+0x174/0x190
[  273.926373]  kmem_cache_alloc+0x1a4/0x220
[  273.930381]  fdb_create+0x40/0x430
[  273.933784]  br_fdb_update+0x198/0x210
[  273.937532]  br_handle_frame_finish+0x244/0x530
[  273.942063]  br_handle_frame+0x1c0/0x270
[  273.945986]  __netif_receive_skb_core.constprop.0+0x29c/0xd30
[  273.951734]  __netif_receive_skb_list_core+0xe8/0x210
[  273.956784]  netif_receive_skb_list_internal+0x180/0x29c
[  273.962091]  napi_gro_receive+0x174/0x190
[  273.966099]  mvneta_rx_swbm+0x6b8/0xb40
[  273.969935]  mvneta_poll+0x684/0x900
[  273.973506]  __napi_poll+0x38/0x18c
[  273.976988]  net_rx_action+0xe8/0x280
[  273.980643]  __do_softirq+0x124/0x2a0
[  273.984299]  run_ksoftirqd+0x4c/0x60
[  273.987871]  smpboot_thread_fn+0x23c/0x270
[  273.991963]  kthread+0x10c/0x110
[  273.995188]  ret_from_fork+0x10/0x20

(followed by lots upon lots of vomiting, followed by ...)

[  311.138590] Out of memory and no killable processes...
[  311.143774] Kernel panic - not syncing: System is deadlocked on memory
[  311.150295] CPU: 0 PID: 6 Comm: kworker/0:0 Not tainted 5.18.7-rc1-00013-g52b92343db13 #74
[  311.158550] Hardware name: CZ.NIC Turris Mox Board (DT)
[  311.163766] Workqueue: events rht_deferred_worker
[  311.168477] Call trace:
[  311.170916]  dump_backtrace.part.0+0xc8/0xd4
[  311.175188]  show_stack+0x18/0x70
[  311.178501]  dump_stack_lvl+0x68/0x84
[  311.182159]  dump_stack+0x18/0x34
[  311.185466]  panic+0x168/0x328
[  311.188515]  out_of_memory+0x568/0x584
[  311.192261]  __alloc_pages+0xb04/0xbe0
[  311.196006]  __alloc_pages_bulk+0x15c/0x604
[  311.200185]  alloc_pages_bulk_array_mempolicy+0xbc/0x24c
[  311.205491]  __vmalloc_node_range+0x238/0x550
[  311.209843]  __vmalloc_node_range+0x1c0/0x550
[  311.214195]  kvmalloc_node+0xe0/0x124
[  311.217856]  bucket_table_alloc.isra.0+0x40/0x150
[  311.222554]  rhashtable_rehash_alloc.isra.0+0x20/0x8c
[  311.227599]  rht_deferred_worker+0x7c/0x540
[  311.231775]  process_one_work+0x1d0/0x320
[  311.235779]  worker_thread+0x70/0x440
[  311.239435]  kthread+0x10c/0x110
[  311.242661]  ret_from_fork+0x10/0x20
[  311.246238] SMP: stopping secondary CPUs
[  311.250161] Kernel Offset: disabled
[  311.253642] CPU features: 0x000,00020009,00001086
[  311.258338] Memory Limit: none
[  311.261390] ---[ end Kernel panic - not syncing: System is deadlocked on memory ]---

That can't be quite alright? Shouldn't we have some sort of protection
in the bridge itself too, not just tell hardware driver writers to deal
with it? Or is it somewhere, but it needs to be enabled/configured?

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