Subject: Re: [PATCH bpf 1/2] bpf: sockmap, af_unix sockets need to hold ref for pair sock

From: John Fastabend <[email-masked]>
Date: Tue, 24 Oct 2023 14:39:37 -0700

Jakub Sitnicki wrote:
> On Mon, Oct 16, 2023 at 12:08 PM -07, John Fastabend wrote:
> > AF_UNIX sockets are a paired socket. So sending on one of the pairs
> > will lookup the paired socket as part of the send operation. It is
> > possible however to put just one of the pairs in a BPF map. This
> > currently increments the refcnt on the sock in the sockmap to
> > ensure it is not free'd by the stack before sockmap cleans up its
> > state and stops any skbs being sent/recv'd to that socket.
> >
> > But we missed a case. If the peer socket is closed it will be
> > free'd by the stack. However, the paired socket can still be
> > referenced from BPF sockmap side because we hold a reference
> > there. Then if we are sending traffic through BPF sockmap to
> > that socket it will try to dereference the free'd pair in its
> > send logic creating a use after free.  And following splat,
> >
> >    [59.900375] BUG: KASAN: slab-use-after-free in sk_wake_async+0x31/0x1b0
> >    [59.901211] Read of size 8 at addr ffff88811acbf060 by task kworker/1:2/954
> >    [...]
> >    [59.905468] Call Trace:
> >    [59.905787]  <TASK>
> >    [59.906066]  dump_stack_lvl+0x130/0x1d0
> >    [59.908877]  print_report+0x16f/0x740
> >    [59.910629]  kasan_report+0x118/0x160
> >    [59.912576]  sk_wake_async+0x31/0x1b0
> >    [59.913554]  sock_def_readable+0x156/0x2a0
> >    [59.914060]  unix_stream_sendmsg+0x3f9/0x12a0
> >    [59.916398]  sock_sendmsg+0x20e/0x250
> >    [59.916854]  skb_send_sock+0x236/0xac0
> >    [59.920527]  sk_psock_backlog+0x287/0xaa0
> 
> Isn't the problem here that unix_stream_sendmsg doesn't grab a ref to
> peer sock? Unlike unix_dgram_sendmsg which uses the unix_peer_get
> helper.

It does by my read. In unix_stream_connect we have,

	sock_hold(sk);
	unix_peer(newsk)	= sk;
	newsk->sk_state		= TCP_ESTABLISHED;

where it assigns the peer sock. unix_dgram_connect() also calls
sock_hold() but through the path that does the socket lookup, such as
unix_find_other().

The problem I see is before the socket does the kfree on the
sock we need to be sure the backlog is canceled and the skb list
ingress_skb is purged. If we don't ensure this then the redirect
will 

My model is this,

         s1            c1
refcnt    1             1
connect   2             2
psock     3             3
send(s1) ...
close(s1) 2             1 <- close drops psock count also
close(c1) 0             0

The important bit here is the psock has a refcnt on the
underlying sock (psock->sk) and wont dec that until after
cancel_delayed_work_sync() completes. This ensures the
backlog wont try to sendmsg() on that sock after we free
it. We also check for SOCK_DEAD and abort to avoid sending
over a socket that has been marked DEAD.

So... After close(s1) the only thing keeping that sock
around is c1. Then we close(c1) that call path is

 unix_release
   close() 
   unix_release_sock()
     skpair = unix_peer(sk);
     ...
     sock_put(skpair);  <- trouble here

The release will call sock_put() on the pair socket and
dec it to 0 where it gets free'd through sk_free(). But
now the trouble is we haven't waited for cancel_delayed_work_sync()
on the c1 socket yet so backlog can still run. When it does
run it may try to send a pkg over socket s1. OK right up until
the sendmsg(s1, ...) does a peer lookup and derefs the peer
socket. The peer socket was free'd earlier so use after free. 

The question I had originally was this is odd, we are allowing
a sendmsg(s1) over a socket while its in unix_release(). We
used to take the sock lock from the backlog that was dropped
in the name of performance, but it creates these races.

Other fixes I considered. First adding sock lock back to
backlog. But that punishes the UDP and TCP cases that don't
have this problem. Set the SOCK_DEAD flag earlier or check
later but this just makes the race smaller doesn't really
eliminate it.

So this patch is what I came up with. 

> 
> >
> > To fix let BPF sockmap hold a refcnt on both the socket in the
> > sockmap and its paired socket.  It wasn't obvious how to contain
> > the fix to bpf_unix logic. The primarily problem with keeping this
> > logic in bpf_unix was: In the sock close() we could handle the
> > deref by having a close handler. But, when we are destroying the
> > psock through a map delete operation we wouldn't have gotten any
> > signal thorugh the proto struct other than it being replaced.
> > If we do the deref from the proto replace its too early because
> > we need to deref the skpair after the backlog worker has been
> > stopped.
> >
> > Given all this it seems best to just cache it at the end of the
> > psock and eat 8B for the af_unix and vsock users.
> >
> > Fixes: 94531cfcbe79 ("af_unix: Add unix_stream_proto for sockmap")
> > Signed-off-by: John Fastabend <[email-masked]>
> > ---
> 
> [...]