linux-kernel - Re: [GIT PULL] keys: Miscellaneous fixes/changes

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Date:   Tue, 21 Mar 2023 13:16:08 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        David Howells <dhowells@...hat.com>
Cc:     Jarkko Sakkinen <jarkko@...nel.org>,
        Bharath SM <bharathsm@...rosoft.com>,
        Shyam Prasad N <nspmangalore@...il.com>,
        Steve French <smfrench@...il.com>,
        Robbie Harwood <rharwood@...hat.com>,
        Eric Biederman <ebiederm@...ssion.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        keyrings@...r.kernel.org, linux-cifs@...r.kernel.org,
        linux-crypto@...r.kernel.org, kexec@...ts.infradead.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [GIT PULL] keys: Miscellaneous fixes/changes

On 3/21/23 12:48?PM, Linus Torvalds wrote:
> On Tue, Mar 21, 2023 at 9:43?AM David Howells <dhowells@...hat.com> wrote:
>>
>>  (1) Fix request_key() so that it doesn't cache a looked up key on the
>>      current thread if that thread is a kernel thread.  The cache is
>>      cleared during notify_resume - but that doesn't happen in kernel
>>      threads.  This is causing cifs DNS keys to be un-invalidateable.
> 
> I've pulled this, but I'd like people to look a bit more at this.
> 
> The issue with TIF_NOTIFY_RESUME is that it is only done on return to
> user space.
> 
> And these days, PF_KTHREAD isn't the only case that never returns to
> user space. PF_IO_WORKER has the exact same behaviour.
> 
> Now, to counteract this, as of this merge window (and marked for
> stable) IO threads do a fake "return to user mode" handling in
> io_run_task_work(), and so I think we're all good, but I'd like people
> to at least think about this.

I haven't seen the patch yet as it hasn't been pushed, but can imagine
what it looks like. It may make sense to add some debug check for
PF_KTHREAD having TIF_NOTIFY_RESUME set, or task_work pending for that
matter, as that is generally not workable without doing something to
handle it explicitly.

For PF_IO_WORKER, with the commit you mentioned, those threads should
deal with TIF_NOTIFY_RESUME just fine. Until something else gets added
that is also run from exit_to_user_mode...

-- 
Jens Axboe