Re: [PATCH 1/2] af_unix: Do not wait for garbage collector in sendmsg()

[Nam Cao <namcao@linutronix.de>]

sashiko-bot@kernel.org writes:
> Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
> - [High] Garbage collector is no longer scheduled when creating new inflight FDs, leading to permanent cycle accumulation and -ETOOMANYREFS failures in isolated environments.
> --
>
> commit 506189bd29f8ad56f5d9e51f3e0216203ebad5ec
> Author: Nam Cao <namcao@linutronix.de>
>
> af_unix: Do not wait for garbage collector in sendmsg()
>
> This patch removes the garbage collector scheduling and waiting from the
> sendmsg() path for AF_UNIX sockets. It aims to prevent priority inversion
> by relying on RLIMIT_NOFILE limits and asynchronous garbage collection.
>
>> diff --git a/net/unix/garbage.c b/net/unix/garbage.c
>> index 0783555e25266..f180c59b3da9d 100644
>> --- a/net/unix/garbage.c
>> +++ b/net/unix/garbage.c
>> @@ -300,8 +300,6 @@ int unix_prepare_fpl(struct scm_fp_list *fpl)
>>  	if (!fpl->edges)
>>  		goto err;
>>  
>> -	unix_schedule_gc(fpl->user);
>> -
>
> [Severity: High]
> Does this removal prevent the garbage collector from running when a process
> creates isolated socket cycles?
>
> If a process continuously creates socket cycles (e.g., passing FDs to each
> other) and closes its local FDs, the sockets reference each other in their
> receive queues, so their file reference counts will not reach zero.
>
> Because the refcounts do not drop to zero, unix_release_sock() is never
> triggered. By removing unix_schedule_gc() from unix_prepare_fpl() here,
> unix_release_sock() is left as the only trigger for garbage collection.
>
> This could allow the inflight count to accumulate indefinitely until it
> hits RLIMIT_NOFILE, causing subsequent sendmsg() calls to fail with
> -ETOOMANYREFS and permanently leaking the network namespace unless an
> unrelated AF_UNIX socket is closed system-wide.

Sashiko found a valid issue here. But this is a pre-existing issue. It
is not introduced in this patch.

unix_schedule_gc() does not actually schedule the garbage collector
unless user's inflight unix socket count exceeds UNIX_INFLIGHT_SANE_USER
(2024). So user can already accumulate up to 2023 inflight socket counts
without the garbage collector running until unix_release_sock() is called.

Nam