Subject: Re: [PATCH][unix] missing barriers in some of unix_sock ->addr and ->path accesses

From: Al Viro <[email-masked]>
Date: Mon, 18 Feb 2019 21:32:46 +0000

On Mon, Feb 18, 2019 at 09:14:33PM +0000, Sasha Levin wrote:
> Hi,
> 
> [This is an automated email]
> 
> This commit has been processed because it contains a -stable tag.

Ugh...  Should've removed Cc; stable from netdev posting; my apologies.

> How should we proceed with this patch?

Wait for it to get into davem's tree, for starters?

Sorry about that, again...

FWIW, further adventures in net/unix land:

unix_dgram_poll() contains
                /* connection hasn't started yet? */
                if (sk->sk_state == TCP_SYN_SENT)
                        return mask;
and nothing in there sets TCP_SYN_SENT state (not that it would've made
any sense of AF_UNIX).

unix_poll() contains
        /* Connection-based need to check for termination and startup */
        if ((sk->sk_type == SOCK_STREAM || sk->sk_type == SOCK_SEQPACKET) &&
            sk->sk_state == TCP_CLOSE)
while it can only be called as ->poll of unix_stream_ops, which means
that sk->sk_type can't be anything other that SOCK_STREAM in there.

static void scan_children(struct sock *x, void (*func)(struct unix_sock *),
                          struct sk_buff_head *hitlist)
{
        if (x->sk_state != TCP_LISTEN) {
                scan_inflight(x, func, hitlist);
        } else {
...
has no exclusion or barriers to deal with the store of TCP_LISTEN into
->sk_state inside unix_listen().  That one's potentially nasty - we won't
find SCM_RIGHTS already queued to embrios in x's queue until we notice
that x->sk_state == TCP_LISTEN, which can happen between two calls of
scan_children() in the same unix_gc() run.  The race is narrow, but not
impossible, AFAICS.  Reasonably easy to fix - lift locking the queue
out of scan_inflight(), grab the queue lock before checking if it's
a listener and have unix_listen() either grab the queue lock around the
assignment to ->sk_state, or pump it up and down before dropping
unix_state_lock() (at which point connect() might be able to find it, etc.)

Al, still digging through net/unix...