Re: [PATCH net] sctp: add chunks to sk_backlog when the newsk sk_socket is not set

[Jakub Kicinski <jakub.kicinski@netronome.com>]

On Tue,  8 Oct 2019 19:09:23 +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@redhat.com>
> Suggested-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
> Signed-off-by: Xin Long <lucien.xin@gmail.com>

Applied, queued for stable, thank you!