netdev - Re: [PATCH net] sctp: use call

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Message-ID: <20211222144050.6a13ac4a@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Wed, 22 Dec 2021 14:40:50 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Xin Long <lucien.xin@...il.com>
Cc:     network dev <netdev@...r.kernel.org>, linux-sctp@...r.kernel.org,
        davem@...emloft.net,
        Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
        Lee Jones <lee.jones@...aro.org>
Subject: Re: [PATCH net] sctp: use call_rcu to free endpoint

On Tue, 21 Dec 2021 14:51:52 -0500 Xin Long wrote:
> This patch is to delay the endpoint free by calling call_rcu() to fix
> another use-after-free issue in sctp_sock_dump():
> 
>   BUG: KASAN: use-after-free in __lock_acquire+0x36d9/0x4c20
>   Call Trace:
>     __lock_acquire+0x36d9/0x4c20 kernel/locking/lockdep.c:3218
>     lock_acquire+0x1ed/0x520 kernel/locking/lockdep.c:3844
>     __raw_spin_lock_bh include/linux/spinlock_api_smp.h:135 [inline]
>     _raw_spin_lock_bh+0x31/0x40 kernel/locking/spinlock.c:168
>     spin_lock_bh include/linux/spinlock.h:334 [inline]
>     __lock_sock+0x203/0x350 net/core/sock.c:2253
>     lock_sock_nested+0xfe/0x120 net/core/sock.c:2774
>     lock_sock include/net/sock.h:1492 [inline]
>     sctp_sock_dump+0x122/0xb20 net/sctp/diag.c:324
>     sctp_for_each_transport+0x2b5/0x370 net/sctp/socket.c:5091
>     sctp_diag_dump+0x3ac/0x660 net/sctp/diag.c:527
>     __inet_diag_dump+0xa8/0x140 net/ipv4/inet_diag.c:1049
>     inet_diag_dump+0x9b/0x110 net/ipv4/inet_diag.c:1065
>     netlink_dump+0x606/0x1080 net/netlink/af_netlink.c:2244
>     __netlink_dump_start+0x59a/0x7c0 net/netlink/af_netlink.c:2352
>     netlink_dump_start include/linux/netlink.h:216 [inline]
>     inet_diag_handler_cmd+0x2ce/0x3f0 net/ipv4/inet_diag.c:1170
>     __sock_diag_cmd net/core/sock_diag.c:232 [inline]
>     sock_diag_rcv_msg+0x31d/0x410 net/core/sock_diag.c:263
>     netlink_rcv_skb+0x172/0x440 net/netlink/af_netlink.c:2477
>     sock_diag_rcv+0x2a/0x40 net/core/sock_diag.c:274
> 
> This issue occurs when asoc is peeled off and the old sk is freed after
> getting sk by asoc->base.sk and before calling lock_sock(sk).
> 
> To prevent the ep/sk free, this patch is to call call_rcu to free the ep
> and hold it under rcu_read_lock to make sure that sk in sctp_sock_dump
> is still alive when calling lock_sock().

Could you clarify a little more where the RCU lock is held, it's not
obvious.

> Note that delaying endpint free won't delay the port release, as the port
> release happens in sctp_endpoint_destroy() before calling call_rcu().
> Also, freeing endpoint by call_rcu() makes it safe to access the sk by
> asoc->base.sk in sctp_assocs_seq_show() and sctp_rcv().
> 
> Thanks Jones to bring this issue up.
> 
> Reported-by: syzbot+9276d76e83e3bcde6c99@...kaller.appspotmail.com
> Reported-by: Lee Jones <lee.jones@...aro.org>
> Fixes: d25adbeb0cdb ("sctp: fix an use-after-free issue in sctp_sock_dump")
> Signed-off-by: Xin Long <lucien.xin@...il.com>

> diff --git a/net/sctp/endpointola.c b/net/sctp/endpointola.c
> index 48c9c2c7602f..81fb97d382d7 100644
> --- a/net/sctp/endpointola.c
> +++ b/net/sctp/endpointola.c
> @@ -184,6 +184,17 @@ void sctp_endpoint_free(struct sctp_endpoint *ep)
>  }
>  
>  /* Final destructor for endpoint.  */
> +static void sctp_endpoint_destroy_rcu(struct rcu_head *head)
> +{
> +	struct sctp_endpoint *ep = container_of(head, struct sctp_endpoint, rcu);
> +	struct sock *sk = ep->base.sk;
> +
> +	sctp_sk(sk)->ep = NULL;
> +	sock_put(sk);
> +
> +	SCTP_DBG_OBJCNT_DEC(ep);
> +}
> +
>  static void sctp_endpoint_destroy(struct sctp_endpoint *ep)
>  {
>  	struct sock *sk;
> @@ -213,18 +224,13 @@ static void sctp_endpoint_destroy(struct sctp_endpoint *ep)
>  	if (sctp_sk(sk)->bind_hash)
>  		sctp_put_port(sk);
>  
> -	sctp_sk(sk)->ep = NULL;
> -	/* Give up our hold on the sock */
> -	sock_put(sk);
> -
> -	kfree(ep);

where does this kfree() go after the change?

> -	SCTP_DBG_OBJCNT_DEC(ep);
> +	call_rcu(&ep->rcu, sctp_endpoint_destroy_rcu);
>  }