On Tue, Oct 08, 2019 at 07:09:23PM +0800, Xin Long wrote:
> This patch is to fix a NULL-ptr deref in selinux_socket_connect_helper:
>
> [...] kasan: GPF could be caused by NULL-ptr deref or user memory access
> [...] RIP: 0010:selinux_socket_connect_helper+0x94/0x460
> [...] Call Trace:
> [...] selinux_sctp_bind_connect+0x16a/0x1d0
> [...] security_sctp_bind_connect+0x58/0x90
> [...] sctp_process_asconf+0xa52/0xfd0 [sctp]
> [...] sctp_sf_do_asconf+0x785/0x980 [sctp]
> [...] sctp_do_sm+0x175/0x5a0 [sctp]
> [...] sctp_assoc_bh_rcv+0x285/0x5b0 [sctp]
> [...] sctp_backlog_rcv+0x482/0x910 [sctp]
> [...] __release_sock+0x11e/0x310
> [...] release_sock+0x4f/0x180
> [...] sctp_accept+0x3f9/0x5a0 [sctp]
> [...] inet_accept+0xe7/0x720
>
> It was caused by that the 'newsk' sk_socket was not set before going to
> security sctp hook when processing asconf chunk with SCTP_PARAM_ADD_IP
> or SCTP_PARAM_SET_PRIMARY:
>
> inet_accept()->
> sctp_accept():
> lock_sock():
> lock listening 'sk'
> do_softirq():
> sctp_rcv(): <-- [1]
> asconf chunk arrives and
> enqueued in 'sk' backlog
> sctp_sock_migrate():
> set asoc's sk to 'newsk'
> release_sock():
> sctp_backlog_rcv():
> lock 'newsk'
> sctp_process_asconf() <-- [2]
> unlock 'newsk'
> sock_graft():
> set sk_socket <-- [3]
>
> As it shows, at [1] the asconf chunk would be put into the listening 'sk'
> backlog, as accept() was holding its sock lock. Then at [2] asconf would
> get processed with 'newsk' as asoc's sk had been set to 'newsk'. However,
> 'newsk' sk_socket is not set until [3], while selinux_sctp_bind_connect()
> would deref it, then kernel crashed.
>
> Here to fix it by adding the chunk to sk_backlog until newsk sk_socket is
> set when .accept() is done.
>
> Note that sk->sk_socket can be NULL when the sock is closed, so SOCK_DEAD
> flag is also needed to check in sctp_newsk_ready().
>
> Thanks to Ondrej for reviewing the code.
>
> Fixes: d452930fd3b9 ("selinux: Add SCTP support")
> Reported-by: Ying Xu <yinxu@xxxxxxxxxx>
> Suggested-by: Marcelo Ricardo Leitner <marcelo.leitner@xxxxxxxxx>
> Signed-off-by: Xin Long <lucien.xin@xxxxxxxxx>
> ---
> include/net/sctp/sctp.h | 5 +++++
> net/sctp/input.c | 12 +++++++++---
> 2 files changed, 14 insertions(+), 3 deletions(-)
>
> diff --git a/include/net/sctp/sctp.h b/include/net/sctp/sctp.h
> index 5d60f13..3ab5c6b 100644
> --- a/include/net/sctp/sctp.h
> +++ b/include/net/sctp/sctp.h
> @@ -610,4 +610,9 @@ static inline __u32 sctp_min_frag_point(struct sctp_sock *sp, __u16 datasize)
> return sctp_mtu_payload(sp, SCTP_DEFAULT_MINSEGMENT, datasize);
> }
>
> +static inline bool sctp_newsk_ready(const struct sock *sk)
> +{
> + return sock_flag(sk, SOCK_DEAD) || sk->sk_socket;
> +}
> +
> #endif /* __net_sctp_h__ */
> diff --git a/net/sctp/input.c b/net/sctp/input.c
> index 5a070fb..f277137 100644
> --- a/net/sctp/input.c
> +++ b/net/sctp/input.c
> @@ -243,7 +243,7 @@ int sctp_rcv(struct sk_buff *skb)
> bh_lock_sock(sk);
> }
>
> - if (sock_owned_by_user(sk)) {
> + if (sock_owned_by_user(sk) || !sctp_newsk_ready(sk)) {
> if (sctp_add_backlog(sk, skb)) {
> bh_unlock_sock(sk);
> sctp_chunk_free(chunk);
> @@ -321,7 +321,7 @@ int sctp_backlog_rcv(struct sock *sk, struct sk_buff *skb)
> local_bh_disable();
> bh_lock_sock(sk);
>
> - if (sock_owned_by_user(sk)) {
> + if (sock_owned_by_user(sk) || !sctp_newsk_ready(sk)) {
> if (sk_add_backlog(sk, skb, sk->sk_rcvbuf))
> sctp_chunk_free(chunk);
> else
> @@ -336,7 +336,13 @@ int sctp_backlog_rcv(struct sock *sk, struct sk_buff *skb)
> if (backloged)
> return 0;
> } else {
> - sctp_inq_push(inqueue, chunk);
> + if (!sctp_newsk_ready(sk)) {
> + if (!sk_add_backlog(sk, skb, sk->sk_rcvbuf))
> + return 0;
> + sctp_chunk_free(chunk);
> + } else {
> + sctp_inq_push(inqueue, chunk);
> + }
> }
>
> done:
> --
> 2.1.0
>
>
Acked-by: Neil Horman <nhorman@xxxxxxxxxxxxx>
