linux-kernel - Re: [PATCH v3] i2c: dev: don't allow user-space to deadlock the kernel

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Date:   Mon, 6 Feb 2023 16:38:56 +0100
From:   Bartosz Golaszewski <brgl@...ev.pl>
To:     Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
Cc:     Wolfram Sang <wsa@...nel.org>, linux-i2c@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
Subject: Re: [PATCH v3] i2c: dev: don't allow user-space to deadlock the kernel

On Mon, Feb 6, 2023 at 4:28 PM Uwe Kleine-König
<u.kleine-koenig@...gutronix.de> wrote:
>
> Hello,
>
> On Wed, Jan 18, 2023 at 02:49:40PM +0100, Bartosz Golaszewski wrote:
> > From: Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
> >
> > If we open an i2c character device and then unbind the underlying i2c
> > adapter (either by unbinding it manually via sysfs or - for a real-life
> > example - when unplugging a USB device with an i2c adaper), the kernel
> > thread calling i2c_del_adapter() will become blocked waiting for the
> > completion that only completes once all references to the character
> > device get dropped.
>
> Is this bad?
>
> > In order to fix that, we introduce a couple changes. They need to be
> > part of a single commit in order to preserve bisectability. First, drop
> > the dev_release completion. That removes the risk of a deadlock but
> > we now need to protect the character device structures against NULL
> > pointer dereferences. To that end introduce an rw semaphore. It will
> > protect the dummy i2c_client structure against dropping the adapter from
> > under it. It will be taken for reading by all file_operations callbacks
> > and for writing by the notifier's unbind handler. This way we don't
> > prohibit the syscalls that don't get in each other's way from running
> > concurrently but the adapter will not be unbound before all syscalls
> > return.
> >
> > Finally: upon being notified about an unbind event for the i2c adapter,
> > we take the lock for writing and set the adapter pointer in the character
> > device's structure to NULL. This "numbs down" the device - it still exists
> > but is no longer functional. Meanwhile every syscall callback checks that
> > pointer after taking the lock but before executing any code that requires
> > it. If it's NULL, we return an error to user-space.
> >
> > This way we can safely open an i2c device from user-space, unbind the
> > device without triggering a deadlock and any subsequent system-call for
> > the file descriptor associated with the removed adapter will gracefully
> > fail.
> >
> > Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
>
> @Bartosz, is that the patch you talked about on FOSDEM? I thought
> Wolfram had some concerns but I thought they were unaddressed still.
> What am I missing?

Hi Uwe, yes, this patch was dropped, I can see the rest of the
discussion on the list, have you got the rest of the email too?

Bart