linux-kernel - Re: [PATCH] net: Do not break out of sk_stream_wait_memory() with TIF_NOTIFY

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Date:   Fri, 27 Oct 2023 14:04:32 +0200
From:   Sascha Hauer <s.hauer@...gutronix.de>
To:     Paolo Abeni <pabeni@...hat.com>
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, Jens Axboe <axboe@...nel.dk>,
        kernel@...gutronix.de
Subject: Re: [PATCH] net: Do not break out of sk_stream_wait_memory() with
 TIF_NOTIFY_SIGNAL

On Thu, Oct 26, 2023 at 10:49:18AM +0200, Paolo Abeni wrote:
> On Thu, 2023-10-26 at 09:03 +0200, Sascha Hauer wrote:
> > On Tue, Oct 24, 2023 at 03:56:17PM +0200, Paolo Abeni wrote:
> > > On Mon, 2023-10-23 at 14:13 +0200, Sascha Hauer wrote:
> > > > It can happen that a socket sends the remaining data at close() time.
> > > > With io_uring and KTLS it can happen that sk_stream_wait_memory() bails
> > > > out with -512 (-ERESTARTSYS) because TIF_NOTIFY_SIGNAL is set for the
> > > > current task. This flag has been set in io_req_normal_work_add() by
> > > > calling task_work_add().
> > > > 
> > > > It seems signal_pending() is too broad, so this patch replaces it with
> > > > task_sigpending(), thus ignoring the TIF_NOTIFY_SIGNAL flag.
> > > 
> > > This looks dangerous, at best. Other possible legit users setting
> > > TIF_NOTIFY_SIGNAL will be broken.
> > > 
> > > Can't you instead clear TIF_NOTIFY_SIGNAL in io_run_task_work() ?
> > 
> > I don't have an idea how io_run_task_work() comes into play here, but it
> > seems it already clears TIF_NOTIFY_SIGNAL:
> > 
> > static inline int io_run_task_work(void)
> > {
> >         /*
> >          * Always check-and-clear the task_work notification signal. With how
> >          * signaling works for task_work, we can find it set with nothing to
> >          * run. We need to clear it for that case, like get_signal() does.
> >          */
> >         if (test_thread_flag(TIF_NOTIFY_SIGNAL))
> >                 clear_notify_signal();
> > 	...
> > }
> 
> I see, io_run_task_work() is too late, sk_stream_wait_memory() is
> already woken up.
> 
> I still think this patch is unsafe. What about explicitly handling the
> restart in tls_sw_release_resources_tx() ? The main point is that such
> function is called by inet_release() and the latter can't be re-
> started.

I don't think there's anything I can do in tls_sw_release_resources_tx().
When entering this function TIF_NOTIFY_SIGNAL is not (yet) set. It gets
set at some point while tls_sw_release_resources_tx() is running. I find
it set when tls_tx_records() returns with -ERESTARTSYS. I tried clearing
TIF_NOTIFY_SIGNAL then and called tls_tx_records() again, but that doesn't
work.

Sascha

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