Subject: Re: [bpf PATCH] bpf: sockmap, remove bucket->lock from sock_{hash|map}_free

From: Jakub Sitnicki <[email-masked]>
Date: Wed, 03 Jun 2020 13:12:37 +0200

On Wed, Jun 03, 2020 at 08:13 AM CEST, Eric Dumazet wrote:
> On 3/10/20 9:41 AM, John Fastabend wrote:
>> The bucket->lock is not needed in the sock_hash_free and sock_map_free
>> calls, in fact it is causing a splat due to being inside rcu block.
>>
>>
>> | BUG: sleeping function called from invalid context at net/core/sock.c:2935
>> | in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 62, name: kworker/0:1
>> | 3 locks held by kworker/0:1/62:
>> |  #0: ffff88813b019748 ((wq_completion)events){+.+.}, at: process_one_work+0x1d7/0x5e0
>> |  #1: ffffc900000abe50 ((work_completion)(&map->work)){+.+.}, at: process_one_work+0x1d7/0x5e0
>> |  #2: ffff8881381f6df8 (&stab->lock){+...}, at: sock_map_free+0x26/0x180
>> | CPU: 0 PID: 62 Comm: kworker/0:1 Not tainted 5.5.0-04008-g7b083332376e #454
>> | Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS ?-20190727_073836-buildvm-ppc64le-16.ppc.fedoraproject.org-3.fc31 04/01/2014
>> | Workqueue: events bpf_map_free_deferred
>> | Call Trace:
>> |  dump_stack+0x71/0xa0
>> |  ___might_sleep.cold+0xa6/0xb6
>> |  lock_sock_nested+0x28/0x90
>> |  sock_map_free+0x5f/0x180
>> |  bpf_map_free_deferred+0x58/0x80
>> |  process_one_work+0x260/0x5e0
>> |  worker_thread+0x4d/0x3e0
>> |  kthread+0x108/0x140
>> |  ? process_one_work+0x5e0/0x5e0
>> |  ? kthread_park+0x90/0x90
>> |  ret_from_fork+0x3a/0x50
>>
>> The reason we have stab->lock and bucket->locks in sockmap code is to
>> handle checking EEXIST in update/delete cases. We need to be careful during
>> an update operation that we check for EEXIST and we need to ensure that the
>> psock object is not in some partial state of removal/insertion while we do
>> this. So both map_update_common and sock_map_delete need to guard from being
>> run together potentially deleting an entry we are checking, etc. But by the
>> time we get to the tear-down code in sock_{ma[|hash}_free we have already
>> disconnected the map and we just did synchronize_rcu() in the line above so
>> no updates/deletes should be in flight. Because of this we can drop the
>> bucket locks from the map free'ing code, noting no update/deletes can be
>> in-flight.
>>
>> Fixes: 604326b41a6f ("bpf, sockmap: convert to generic sk_msg interface")
>> Reported-by: Jakub Sitnicki <[email-masked]>
>> Suggested-by: Jakub Sitnicki <[email-masked]>
>> Signed-off-by: John Fastabend <[email-masked]>
>> ---
>>  net/core/sock_map.c |   12 ++++++++----
>>  1 file changed, 8 insertions(+), 4 deletions(-)
>>
>> diff --git a/net/core/sock_map.c b/net/core/sock_map.c
>> index 085cef5..b70c844 100644
>> --- a/net/core/sock_map.c
>> +++ b/net/core/sock_map.c
>> @@ -233,8 +233,11 @@ static void sock_map_free(struct bpf_map *map)
>>  	struct bpf_stab *stab = container_of(map, struct bpf_stab, map);
>>  	int i;
>>
>> +	/* After the sync no updates or deletes will be in-flight so it
>> +	 * is safe to walk map and remove entries without risking a race
>> +	 * in EEXIST update case.
>
>
> What prevents other cpus from deleting stuff in sock_hash_delete_elem() ?
>
> What state has been changed before the synchronize_rcu() call here,
> that other cpus check before attempting a delete ?
>
> Typically, synchronize_rcu() only makes sense if readers can not start a new cycle.
>
> A possible fix would be to check in sock_hash_delete_elem() (and possibly others methods)
> if map->refcnt is not zero.
>
> syzbot found : (no repro yet)
>
> general protection fault, probably for non-canonical address 0xfbd59c0000000024: 0000 [#1] PREEMPT SMP KASAN
> KASAN: maybe wild-memory-access in range [0xdead000000000120-0xdead000000000127]
> CPU: 2 PID: 14305 Comm: kworker/2:3 Not tainted 5.7.0-syzkaller #0
> Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.12.0-59-gc9ba5276e321-prebuilt.qemu.org 04/01/2014
> Workqueue: events bpf_map_free_deferred
> RIP: 0010:__write_once_size include/linux/compiler.h:279 [inline]
> RIP: 0010:__hlist_del include/linux/list.h:811 [inline]
> RIP: 0010:hlist_del_rcu include/linux/rculist.h:485 [inline]
> RIP: 0010:sock_hash_free+0x202/0x4a0 net/core/sock_map.c:1021
> Code: 0f 85 15 02 00 00 4c 8d 7b 28 4c 8b 63 20 4c 89 f8 48 c1 e8 03 80 3c 28 00 0f 85 47 02 00 00 4c 8b 6b 28 4c 89 e8 48 c1 e8 03 <80> 3c 28 00 0f 85 25 02 00 00 4d 85 e4 4d 89 65 00 74 20 e8 f6 82
> RSP: 0018:ffffc90000ba7c38 EFLAGS: 00010a06
> RAX: 1bd5a00000000024 RBX: ffff88801d866700 RCX: ffffffff8636ae84
> RDX: 0000000000000000 RSI: ffffffff8636afe9 RDI: ffff88801d866720
> RBP: dffffc0000000000 R08: ffff888022765080 R09: fffffbfff185f952
> R10: ffffffff8c2fca8f R11: fffffbfff185f951 R12: 0000000000000000
> R13: dead000000000122 R14: dead000000000122 R15: ffff88801d866728
> FS:  0000000000000000(0000) GS:ffff88802d000000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00007f27551a9db8 CR3: 0000000056530000 CR4: 0000000000340ee0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> Call Trace:
>  bpf_map_free_deferred+0xb2/0x100 kernel/bpf/syscall.c:471
>  process_one_work+0x965/0x16a0 kernel/workqueue.c:2268
>  worker_thread+0x96/0xe20 kernel/workqueue.c:2414
>  kthread+0x388/0x470 kernel/kthread.c:268
>  ret_from_fork+0x24/0x30 arch/x86/entry/entry_64.S:351
> Modules linked in:
> ---[ end trace da3ce2417ae8d343 ]---
> RIP: 0010:__write_once_size include/linux/compiler.h:279 [inline]
> RIP: 0010:__hlist_del include/linux/list.h:811 [inline]
> RIP: 0010:hlist_del_rcu include/linux/rculist.h:485 [inline]
> RIP: 0010:sock_hash_free+0x202/0x4a0 net/core/sock_map.c:1021
> Code: 0f 85 15 02 00 00 4c 8d 7b 28 4c 8b 63 20 4c 89 f8 48 c1 e8 03 80 3c 28 00 0f 85 47 02 00 00 4c 8b 6b 28 4c 89 e8 48 c1 e8 03 <80> 3c 28 00 0f 85 25 02 00 00 4d 85 e4 4d 89 65 00 74 20 e8 f6 82
> RSP: 0018:ffffc90000ba7c38 EFLAGS: 00010a06
> RAX: 1bd5a00000000024 RBX: ffff88801d866700 RCX: ffffffff8636ae84
> RDX: 0000000000000000 RSI: ffffffff8636afe9 RDI: ffff88801d866720
> RBP: dffffc0000000000 R08: ffff888022765080 R09: fffffbfff185f952
> R10: ffffffff8c2fca8f R11: fffffbfff185f951 R12: 0000000000000000
> R13: dead000000000122 R14: dead000000000122 R15: ffff88801d866728
> FS:  0000000000000000(0000) GS:ffff88802d000000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00007f1e5e3c6290 CR3: 000000001347f000 CR4: 0000000000340ee0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
>

My initial reasoning behind the change was that sock_hash_delete_elem()
callers hold a ref to sockhash [0]. Either because there is an open FD
for the map, or the map is in use by loaded BPF program. The same
applies to updates.

If that holds, map->refcnt is > 0, and we should not see the map being
freed at the same time as sock_hash_delete_elem() happens.

But then there is also sock_hash_delete_from_link() that deletes from
sockhash when a sock/psock unlinks itself from the map. This operation
happens without holding a ref to the map, so that sockets won't keep the
map "alive". There is no corresponding *_update_from_link() for updates
without holding a ref.

Sadly, I can't spot anything preventing list mutation, hlist_del_rcu(),
from happening both in sock_hash_delete_elem() and sock_hash_free()
concurrently, now that the bucket spin-lock doesn't protect it any
more. That is what I understand syzbot is reporting.

synchronize_rcu() before we walk the htable doesn't rule it out, because
as you point out, new readers can start a new cycle, and we don't change
any state that would signal that the map is going away.

I'm not sure that the check for map->refcnt when sock is unlinking
itself from the map will do it. I worry we will then have issues when
sockhash is unlinking itself from socks (so the other way around) in
sock_hash_free(). We could no longer assume that the sock & psock
exists.

What comes to mind is to reintroduce the spin-lock protected critical
section in sock_hash_free(), but delay the processing of sockets to be
unlinked from sockhash. We could grab a ref to sk_psock while holding a
spin-lock and unlink it while no longer in atomic critical section.

Either way, Eric, thank you for the report and the pointers.

John, WDYT?

[0] https://lore.kernel.org/netdev/[email-masked]/

>> +	 */
>>  	synchronize_rcu();
>> -	raw_spin_lock_bh(&stab->lock);
>>  	for (i = 0; i < stab->map.max_entries; i++) {
>>  		struct sock **psk = &stab->sks[i];
>>  		struct sock *sk;
>> @@ -248,7 +251,6 @@ static void sock_map_free(struct bpf_map *map)
>>  			release_sock(sk);
>>  		}
>>  	}
>> -	raw_spin_unlock_bh(&stab->lock);
>>
>>  	/* wait for psock readers accessing its map link */
>>  	synchronize_rcu();
>> @@ -863,10 +865,13 @@ static void sock_hash_free(struct bpf_map *map)
>>  	struct hlist_node *node;
>>  	int i;
>>
>> +	/* After the sync no updates or deletes will be in-flight so it
>> +	 * is safe to walk map and remove entries without risking a race
>> +	 * in EEXIST update case.
>> +	 */
>>  	synchronize_rcu();
>>  	for (i = 0; i < htab->buckets_num; i++) {
>>  		bucket = sock_hash_select_bucket(htab, i);
>> -		raw_spin_lock_bh(&bucket->lock);
>>  		hlist_for_each_entry_safe(elem, node, &bucket->head, node) {
>>  			hlist_del_rcu(&elem->node);
>>  			lock_sock(elem->sk);
>> @@ -875,7 +880,6 @@ static void sock_hash_free(struct bpf_map *map)
>>  			rcu_read_unlock();
>>  			release_sock(elem->sk);
>>  		}
>> -		raw_spin_unlock_bh(&bucket->lock);
>>  	}
>>
>>  	/* wait for psock readers accessing its map link */
>>