Re: [PATCH net v2] sock_map: avoid race between sock_map_close and sk_psock_put

[Jakub Sitnicki <jakub@cloudflare.com>]

On Fri, May 24, 2024 at 11:47 AM -03, 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.
>
> As suggested by Paolo Abeni, reorder the condition so the control flow is
> less convoluted.
>
> After that change, the reproducer does not trigger the WARN_ON_ONCE
> anymore.
>
> Suggested-by: Paolo Abeni <pabeni@redhat.com>
> 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>
> ---
>
> v2: change control flow as suggested by Paolo Abeni
>
> v1: https://lore.kernel.org/netdev/20240520214153.847619-1-cascardo@igalia.com/
>
> ---
>  net/core/sock_map.c | 16 ++++++++++------
>  1 file changed, 10 insertions(+), 6 deletions(-)
>
> diff --git a/net/core/sock_map.c b/net/core/sock_map.c
> index 9402889840bf..c3179567a99a 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);
> -	if (unlikely(!psock)) {
> -		rcu_read_unlock();
> -		release_sock(sk);
> -		saved_close = READ_ONCE(sk->sk_prot)->close;
> -	} else {
> +	psock = sk_psock(sk);
> +	if (likely(psock)) {
>  		saved_close = psock->saved_close;
>  		sock_map_remove_links(sk, psock);
> +		psock = sk_psock_get(sk);
> +		if (unlikely(!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 {
> +		saved_close = READ_ONCE(sk->sk_prot)->close;
> +no_psock:
> +		rcu_read_unlock();
> +		release_sock(sk);
>  	}
>  
>  	/* Make sure we do not recurse. This is a bug.

Thanks.

Acked-by: Jakub Sitnicki <jakub@cloudflare.com>