Re: [PATCH net] sock_map: avoid race between sock_map_close and sk_psock_put

[Jakub Sitnicki <jakub@cloudflare.com>]

On Thu, May 23, 2024 at 09:52 AM +02, Paolo Abeni wrote:
> On Wed, 2024-05-22 at 14:08 +0200, Paolo Abeni wrote:
>> On Mon, 2024-05-20 at 18:41 -0300, Thadeu Lima de Souza Cascardo wrote:
>> > sk_psock_get will return NULL if the refcount of psock has gone to 0, which
>> > will happen when the last call of sk_psock_put is done. However,
>> > sk_psock_drop may not have finished yet, so the close callback will still
>> > point to sock_map_close despite psock being NULL.
>> > 
>> > This can be reproduced with a thread deleting an element from the sock map,
>> > while the second one creates a socket, adds it to the map and closes it.
>> > 
>> > That will trigger the WARN_ON_ONCE:
>> > 
>> > ------------[ cut here ]------------
>> > WARNING: CPU: 1 PID: 7220 at net/core/sock_map.c:1701 sock_map_close+0x2a2/0x2d0 net/core/sock_map.c:1701
>> > Modules linked in:
>> > CPU: 1 PID: 7220 Comm: syz-executor380 Not tainted 6.9.0-syzkaller-07726-g3c999d1ae3c7 #0
>> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 04/02/2024
>> > RIP: 0010:sock_map_close+0x2a2/0x2d0 net/core/sock_map.c:1701
>> > Code: df e8 92 29 88 f8 48 8b 1b 48 89 d8 48 c1 e8 03 42 80 3c 20 00 74 08 48 89 df e8 79 29 88 f8 4c 8b 23 eb 89 e8 4f 15 23 f8 90 <0f> 0b 90 48 83 c4 08 5b 41 5c 41 5d 41 5e 41 5f 5d e9 13 26 3d 02
>> > RSP: 0018:ffffc9000441fda8 EFLAGS: 00010293
>> > RAX: ffffffff89731ae1 RBX: ffffffff94b87540 RCX: ffff888029470000
>> > RDX: 0000000000000000 RSI: ffffffff8bcab5c0 RDI: ffffffff8c1faba0
>> > RBP: 0000000000000000 R08: ffffffff92f9b61f R09: 1ffffffff25f36c3
>> > R10: dffffc0000000000 R11: fffffbfff25f36c4 R12: ffffffff89731840
>> > R13: ffff88804b587000 R14: ffff88804b587000 R15: ffffffff89731870
>> > FS:  000055555e080380(0000) GS:ffff8880b9500000(0000) knlGS:0000000000000000
>> > CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>> > CR2: 0000000000000000 CR3: 00000000207d4000 CR4: 0000000000350ef0
>> > Call Trace:
>> >  <TASK>
>> >  unix_release+0x87/0xc0 net/unix/af_unix.c:1048
>> >  __sock_release net/socket.c:659 [inline]
>> >  sock_close+0xbe/0x240 net/socket.c:1421
>> >  __fput+0x42b/0x8a0 fs/file_table.c:422
>> >  __do_sys_close fs/open.c:1556 [inline]
>> >  __se_sys_close fs/open.c:1541 [inline]
>> >  __x64_sys_close+0x7f/0x110 fs/open.c:1541
>> >  do_syscall_x64 arch/x86/entry/common.c:52 [inline]
>> >  do_syscall_64+0xf5/0x240 arch/x86/entry/common.c:83
>> >  entry_SYSCALL_64_after_hwframe+0x77/0x7f
>> > RIP: 0033:0x7fb37d618070
>> > Code: 00 00 48 c7 c2 b8 ff ff ff f7 d8 64 89 02 b8 ff ff ff ff eb d4 e8 10 2c 00 00 80 3d 31 f0 07 00 00 74 17 b8 03 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 48 c3 0f 1f 80 00 00 00 00 48 83 ec 18 89 7c
>> > RSP: 002b:00007ffcd4a525d8 EFLAGS: 00000202 ORIG_RAX: 0000000000000003
>> > RAX: ffffffffffffffda RBX: 0000000000000005 RCX: 00007fb37d618070
>> > RDX: 0000000000000010 RSI: 00000000200001c0 RDI: 0000000000000004
>> > RBP: 0000000000000000 R08: 0000000100000000 R09: 0000000100000000
>> > R10: 0000000000000000 R11: 0000000000000202 R12: 0000000000000000
>> > R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
>> >  </TASK>
>> > 
>> > Use sk_psock, which will only check that the pointer is not been set to
>> > NULL yet, which should only happen after the callbacks are restored. If,
>> > then, a reference can still be gotten, we may call sk_psock_stop and cancel
>> > psock->work.
>> > 
>> > After that change, the reproducer does not trigger the WARN_ON_ONCE
>> > anymore.
>> > 
>> > Reported-by: syzbot+07a2e4a1a57118ef7355@syzkaller.appspotmail.com
>> > Closes: https://syzkaller.appspot.com/bug?extid=07a2e4a1a57118ef7355
>> > Fixes: aadb2bb83ff7 ("sock_map: Fix a potential use-after-free in sock_map_close()")
>> > Fixes: 5b4a79ba65a1 ("bpf, sockmap: Don't let sock_map_{close,destroy,unhash} call itself")
>> > Cc: stable@vger.kernel.org
>> > Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@igalia.com>
>> > ---
>> >  net/core/sock_map.c | 14 +++++++++-----
>> >  1 file changed, 9 insertions(+), 5 deletions(-)
>> > 
>> > diff --git a/net/core/sock_map.c b/net/core/sock_map.c
>> > index 9402889840bf..13267e667a4c 100644
>> > --- a/net/core/sock_map.c
>> > +++ b/net/core/sock_map.c
>> > @@ -1680,19 +1680,23 @@ void sock_map_close(struct sock *sk, long timeout)
>> >  
>> >  	lock_sock(sk);
>> >  	rcu_read_lock();
>> > -	psock = sk_psock_get(sk);
>> > +	psock = sk_psock(sk);
>> >  	if (unlikely(!psock)) {
>> > +		saved_close = READ_ONCE(sk->sk_prot)->close;
>> >  		rcu_read_unlock();
>> >  		release_sock(sk);
>> > -		saved_close = READ_ONCE(sk->sk_prot)->close;
>> >  	} else {
>> >  		saved_close = psock->saved_close;
>> >  		sock_map_remove_links(sk, psock);
>> > +		psock = sk_psock_get(sk);
>> >  		rcu_read_unlock();
>> > -		sk_psock_stop(psock);
>> > +		if (psock)
>> > +			sk_psock_stop(psock);
>> >  		release_sock(sk);
>> > -		cancel_delayed_work_sync(&psock->work);
>> > -		sk_psock_put(sk, psock);
>> > +		if (psock) {
>> > +			cancel_delayed_work_sync(&psock->work);
>> > +			sk_psock_put(sk, psock);
>> > +		}
>> >  	}
>> >  
>> >  	/* Make sure we do not recurse. This is a bug.
>> 
>> As a personal opinion I think the code will become simple reordering
>> the condition, something alike:
>> 
>> 	if (psock) {
>> 		saved_close = psock->saved_close;
>>  		sock_map_remove_links(sk, psock);
>> 		psock = sk_psock_get(sk);
>> 		if (!psock)
>> 			goto no_psock;
>>  		rcu_read_unlock();
>> 		sk_psock_stop(psock);
>>  		release_sock(sk);
>> 		cancel_delayed_work_sync(&psock->work);
>> 		sk_psock_put(sk, psock);
>> 	} else {
>> no_psock:
>> 		saved_close = READ_ONCE(sk->sk_prot)->close;
>
> FTR, the above is wrong, should be:
>
> 		saved_close = READ_ONCE(sk->sk_prot)->close;
> no_psock:
>
>>  		rcu_read_unlock();
>>  		release_sock(sk);
>> 	}
>
> /P

Paolo's version does read better to me as well.