netdev - Re: [RESEND 2/2] sctp: hold cached endpoints to prevent possible UAF

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Date:   Wed, 15 Dec 2021 17:48:18 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Lee Jones <lee.jones@...aro.org>
Cc:     linux-kernel@...r.kernel.org, Vlad Yasevich <vyasevich@...il.com>,
        Neil Horman <nhorman@...driver.com>,
        Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        lksctp developers <linux-sctp@...r.kernel.org>,
        "H.P. Yarroll" <piggy@....org>,
        Karl Knutson <karl@...ena.chicago.il.us>,
        Jon Grimm <jgrimm@...ibm.com>,
        Xingang Guo <xingang.guo@...el.com>,
        Hui Huang <hui.huang@...ia.com>,
        Sridhar Samudrala <sri@...ibm.com>,
        Daisy Chang <daisyc@...ibm.com>,
        Ryan Layer <rmlayer@...ibm.com>,
        Kevin Gao <kevin.gao@...el.com>, netdev@...r.kernel.org,
        Xin Long <lucien.xin@...il.com>
Subject: Re: [RESEND 2/2] sctp: hold cached endpoints to prevent possible
 UAF

On Tue, 14 Dec 2021 21:57:32 +0000 Lee Jones wrote:
> The cause of the resultant dump_stack() reported below is a
> dereference of a freed pointer to 'struct sctp_endpoint' in
> sctp_sock_dump().
> 
> This race condition occurs when a transport is cached into its
> associated hash table followed by an endpoint/sock migration to a new
> association in sctp_assoc_migrate() prior to their subsequent use in
> sctp_diag_dump() which uses sctp_for_each_transport() to walk the hash
> table calling into sctp_sock_dump() where the dereference occurs.
> 
>   BUG: KASAN: use-after-free in sctp_sock_dump+0xa8/0x438 [sctp_diag]
>   Call trace:
>    dump_backtrace+0x0/0x2dc
>    show_stack+0x20/0x2c
>    dump_stack+0x120/0x144
>    print_address_description+0x80/0x2f4
>    __kasan_report+0x174/0x194
>    kasan_report+0x10/0x18
>    __asan_load8+0x84/0x8c
>    sctp_sock_dump+0xa8/0x438 [sctp_diag]
>    sctp_for_each_transport+0x1e0/0x26c [sctp]
>    sctp_diag_dump+0x180/0x1f0 [sctp_diag]
>    inet_diag_dump+0x12c/0x168
>    netlink_dump+0x24c/0x5b8
>    __netlink_dump_start+0x274/0x2a8
>    inet_diag_handler_cmd+0x224/0x274
>    sock_diag_rcv_msg+0x21c/0x230
>    netlink_rcv_skb+0xe0/0x1bc
>    sock_diag_rcv+0x34/0x48
>    netlink_unicast+0x3b4/0x430
>    netlink_sendmsg+0x4f0/0x574
>    sock_write_iter+0x18c/0x1f0
>    do_iter_readv_writev+0x230/0x2a8
>    do_iter_write+0xc8/0x2b4
>    vfs_writev+0xf8/0x184
>    do_writev+0xb0/0x1a8
>    __arm64_sys_writev+0x4c/0x5c
>    el0_svc_common+0x118/0x250
>    el0_svc_handler+0x3c/0x9c
>    el0_svc+0x8/0xc
> 
> To prevent this from happening we need to take a references to the
> to-be-used/dereferenced 'struct sock' and 'struct sctp_endpoint's
> until such a time when we know it can be safely released.
> 
> When KASAN is not enabled, a similar, but slightly different NULL
> pointer derefernce crash occurs later along the thread of execution in
> inet_sctp_diag_fill() this time.

Are you able to identify where the bug was introduced? Fixes tag would
be good to have here. 

You should squash the two patches together.

> diff --git a/net/sctp/diag.c b/net/sctp/diag.c
> index 760b367644c12..2029b240b6f24 100644
> --- a/net/sctp/diag.c
> +++ b/net/sctp/diag.c
> @@ -301,6 +301,8 @@ static int sctp_sock_dump(struct sctp_transport *tsp, void *p)
>  	struct sctp_association *assoc;
>  	int err = 0;
>  
> +	sctp_endpoint_hold(ep);
> +	sock_hold(sk);
>  	lock_sock(sk);
>  	list_for_each_entry(assoc, &ep->asocs, asocs) {
>  		if (cb->args[4] < cb->args[1])
> @@ -341,6 +343,8 @@ static int sctp_sock_dump(struct sctp_transport *tsp, void *p)
>  	cb->args[4] = 0;
>  release:
>  	release_sock(sk);
> +	sock_put(sk);
> +	sctp_endpoint_put(ep);
>  	return err;
>  }
>