Re: [lvc-project] [PATCH] [RFC] net: smc: fix fasync leak in smc_release()

On 2024/3/5 00:35, Dmitry Antipov wrote:

On 2/23/24 06:36, Wen Gu wrote:

quoted

One solution to this issue I can think of is to check whether
filp->private_data has been changed when the sock_fasync holds the sock lock,
but it inevitably changes the general code..

diff --git a/net/socket.c b/net/socket.c
index ed3df2f749bf..a28435195854 100644
--- a/net/socket.c
+++ b/net/socket.c

@@ -1443,6 +1443,11 @@ static int sock_fasync(int fd, struct file *filp, int on)

                 return -EINVAL;

         lock_sock(sk);
+       /* filp->private_data has changed */
+       if (on && unlikely(sock != filp->private_data)) {
+               release_sock(sk);
+               return -EAGAIN;
+       }
         fasync_helper(fd, filp, on, &wq->fasync_list);

         if (!wq->fasync_list)

Let's see if anyone else has a better idea.

IIUC this is not a solution just because it decreases the probability of the race
but doesn't eliminate it completely - an underlying socket switch (e.g. changing
'filp->private_data') may happen when 'fasync_helper()' is in progress.

Hi Dmitry,

IIUC, the fallback (or more precisely the private_data change) essentially
always happens when the lock_sock(smc->sk) is held, except in smc_listen_work()
or smc_listen_decline(), but at that moment, userspace program can not yet
acquire this new socket to add fasync entries to the fasync_list.

So IMHO, the above patch should work, since it checks the private_data under
the lock_sock(sk). But if I missed something, please correct me.

And I wonder if you can still see the leak with the patch above through
your reproducer or syzbot's reproducer? I once ran your reproducer for about
50 mins and didn't see the leak.

Thanks!

Dmitry