linux-kernel - Re: [PATCH v2] i2c: dev: don't allow user-space to deadlock the kernel

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Message-ID: <CAMRc=Mdpv04oPe_r0S+fPcVvWc4Fkp6RiN89mdNTzDrjvfOG6Q@mail.gmail.com>
Date:   Wed, 18 Jan 2023 14:48:05 +0100
From:   Bartosz Golaszewski <brgl@...ev.pl>
To:     Wolfram Sang <wsa@...nel.org>
Cc:     linux-i2c@...r.kernel.org, linux-kernel@...r.kernel.org,
        Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
Subject: Re: [PATCH v2] i2c: dev: don't allow user-space to deadlock the kernel

On Wed, Jan 18, 2023 at 2:47 PM Bartosz Golaszewski <brgl@...ev.pl> 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.
>
> 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>
> ---

Sorry for the noise, it was supposed to be v3.

Bart