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

Wed Mar 6 21:07:53 MSK 2024

On 3/6/24 17:45, Wen Gu wrote:
> 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.

Well, the whole picture is somewhat more complicated. Consider the
following diagram (an underlying kernel socket is in [], e.g. [smc->sk]):
Thread 0 Thread 1
ioctl(sock, FIOASYNC, [1])
...
sock = filp->private_data;
lock_sock(sock [smc->sk]);
sock_fasync(sock, ..., 1) ; new fasync_struct linked to smc->sk
release_sock(sock [smc->sk]);
 ...
 lock_sock([smc->sk]);
 ...
 smc_switch_to_fallback()
 ...
 smc->clcsock->file->private_data = smc->clcsock;
 ...
 release_sock([smc->sk]);
ioctl(sock, FIOASYNC, [0])
...
sock = filp->private_data;
lock_sock(sock [smc->clcsock]);
sock_fasync(sock, ..., 0) ; nothing to unlink from smc->clcsock
 ; since fasync entry was linked to smc->sk
release_sock(sock [smc->clcsock]);
 ...
 close(sock [smc->clcsock]);
 __fput(...);
 file->f_op->fasync(sock, [0]) ; always failed -
 ; should use
 ; smc->sk instead
 file->f_op->release()
 ...
 smc_restore_fallback_changes()
 ...
 file->private_data = smc->sk.sk_socket;
That is, smc_restore_fallback_changes() restores filp->private_data to
smc->sk. If __fput() would have called file->f_op->release() _before_
file->f_op->fasync(), the fix would be as simple as adding
smc->sk.sk_socket->wq.fasync_list = smc->clcsock->wq.fasync_list;
to smc_restore_fallback_changes(). But since file->f_op->fasync() is called
before file->f_op->release(), the former always makes an attempt to unlink fasync
entry from smc->clcsock instead of smc->sk, thus introducing the memory leak.
And an idea with shared wait queue was intended in attempt to eliminate
this chicken-egg lookalike problem completely.
Dmitry