Re: [PATCH net] sk_msg: Keep reference on socket file while psock lives

[John Fastabend <john.fastabend@gmail.com>]

Jakub Sitnicki wrote:
> Hi Daniel,
> 
> On Tue, Feb 19, 2019 at 05:00 PM CET, Daniel Borkmann wrote:
> > On 02/11/2019 10:09 AM, Jakub Sitnicki wrote:
> >> Backlog work for psock (sk_psock_backlog) might sleep while waiting for
> >> memory to free up when sending packets. While sleeping, socket can
> >> disappear from under our feet together with its wait queue because the
> >> userspace has closed it.
> >>
> >> This breaks an assumption in sk_stream_wait_memory, which expects the
> >> wait queue to be still there when it wakes up resulting in a
> >> use-after-free:
> >>
> >> ==================================================================
> >> BUG: KASAN: use-after-free in remove_wait_queue+0x31/0x70
> >> Write of size 8 at addr ffff888069a0c4e8 by task kworker/0:2/110
> >>
> >> CPU: 0 PID: 110 Comm: kworker/0:2 Not tainted 5.0.0-rc2-00335-g28f9d1a3d4fe-dirty #14
> >> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-2.fc27 04/01/2014
> >> Workqueue: events sk_psock_backlog
> >> Call Trace:
> >>  print_address_description+0x6e/0x2b0
> >>  ? remove_wait_queue+0x31/0x70
> >>  kasan_report+0xfd/0x177
> >>  ? remove_wait_queue+0x31/0x70
> >>  ? remove_wait_queue+0x31/0x70
> >>  remove_wait_queue+0x31/0x70
> >>  sk_stream_wait_memory+0x4dd/0x5f0
> >>  ? sk_stream_wait_close+0x1b0/0x1b0
> >>  ? wait_woken+0xc0/0xc0
> >>  ? tcp_current_mss+0xc5/0x110
> >>  tcp_sendmsg_locked+0x634/0x15d0

[...]

> >>
> >> Avoid it by keeping a reference to the socket file until the psock gets
> >> destroyed.
> >>
> >> While at it, rearrange the order of reference grabbing and
> >> initialization to match the destructor in reverse.
> >>
> >> Reported-by: Marek Majkowski <marek@cloudflare.com>
> >> Link: https://lore.kernel.org/netdev/CAJPywTLwgXNEZ2dZVoa=udiZmtrWJ0q5SuBW64aYs0Y1khXX3A@mail.gmail.com
> >> Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
> >> ---
> >>  net/core/skmsg.c | 8 +++++++-
> >>  1 file changed, 7 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/net/core/skmsg.c b/net/core/skmsg.c
> >> index 8c826603bf36..a38442b8580b 100644
> >> --- a/net/core/skmsg.c
> >> +++ b/net/core/skmsg.c
> >> @@ -493,8 +493,13 @@ struct sk_psock *sk_psock_init(struct sock *sk, int node)
> >>  	sk_psock_set_state(psock, SK_PSOCK_TX_ENABLED);
> >>  	refcount_set(&psock->refcnt, 1);
> >>
> >> -	rcu_assign_sk_user_data(sk, psock);
> >> +	/* Hold on to socket wait queue. Backlog work waits on it for
> >> +	 * memory when sending. We must cancel work before socket wait
> >> +	 * queue can go away.
> >> +	 */
> >> +	get_file(sk->sk_socket->file);
> >>  	sock_hold(sk);
> >> +	rcu_assign_sk_user_data(sk, psock);
> >>
> >>  	return psock;
> >>  }
> >> @@ -558,6 +563,7 @@ static void sk_psock_destroy_deferred(struct work_struct *gc)
> >>  	if (psock->sk_redir)
> >>  		sock_put(psock->sk_redir);
> >>  	sock_put(psock->sk);
> >> +	fput(psock->sk->sk_socket->file);
> >
> > Thanks for the report (and sorry for the late reply). I think holding ref on
> > the struct file just so we keep it alive till deferred destruction might be
> > papering over the actual underlying bug. Nothing obvious pops out from staring
> > at the code right now; as a reproducer to run, did you use the prog in the link
> > above and hit it after your strparser fix?
> 
> I get you, I actually sat on this fix for a moment because I had a
> similar concern, that holding a file ref is a heavy-handed fix and I'm
> not seeing the real problem.
> 
> For me it came down to this:

> 1. tcp_sendmsg_locked that we call from psock backlog work can end up
>    waiting for memory. We somehow need to ensure that the socket wait
>    queue does not disappear until tcp_sendmsg_locked returns.
> 
> 2. KCM, which I assume must have the same problem, holds a reference on
>    the socket file.
> 
> I'm curious if there is another angle to it.
> 
> To answer your actual questions - your guesses are correct on both
> accounts.
> 
> For the reproducer, I've used the TCP echo program from Marek [1]. On
> the client side, I had something like:
> 
>   while :; do
>     nc 10.0.0.1 12345 > /dev/null < /dev/zero &
>     pid=$!
>     sleep 0.1
>     kill $pid
>   done
> 
> I can dig out the test scripts or help testing any patches.
> 
> I was testing with the strparser fix applied, 1d79895aef18 ("sk_msg:
> Always cancel strp work before freeing the psock"), which unfortunately
> was not enough.
> 
> The explanation there was that the socket descriptor can get closed, and
> in consequence the socket file can get destroyed, before the deferred
> destructor for psock runs. So psock backlog work can be still very much
> alive and running while the socket file is gone.
> 
> Thanks for looking into it,
> -Jakub
> 
> [1] https://gist.github.com/majek/a09bcbeb8ab548cde6c18c930895c3f2

In the sendpage case we set the MSG_DONTWAIT flag, I think we should
set it in the sendmsg case as well. This would result in
tcp_sendmsg_locked() setting timeo via sock_sndtimeo to zero and should
avoid any waiting. And then we avoid above use after free bug.

Then handle the error in sk_psock_backlog() correctly it returns an
EAGAIN (I think?) in this case so we will deduct any sent bytes and
increment the offset as needed and try again here. Except we will
have purged the ingress_skb list so we should abort.

And the work queue should be flushed before destroying psock so we
can be assured that the psock reference is not lost.

Here is what I'm thinking still untested. Because skmsg is the only
user of skb_send_sock_locked() this should be OK to do. If you have
time running your tests again would be great. I can also try to
repro.

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index e89be6282693..eadfd16be7db 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -2337,6 +2337,7 @@ int skb_send_sock_locked(struct sock *sk, struct sk_buff *skb, int offset,
                kv.iov_base = skb->data + offset;
                kv.iov_len = slen;
                memset(&msg, 0, sizeof(msg));
+               msg->flags = MSG_DONTWAIT;
 
                ret = kernel_sendmsg_locked(sk, &msg, &kv, 1, slen);
                if (ret <= 0)