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SubjectRe: [PATCH 1/1] kthread: introduce kthread_get_run() to fix __nbd_ioctl()
On Sun, Oct 25, 2015 at 03:27:13PM +0100, Oleg Nesterov wrote:
> It is not safe to use the task_struct returned by kthread_run(threadfn)
> if threadfn() can exit before the "owner" does kthread_stop(), nothing
> protects this task_struct.
>
> So __nbd_ioctl() looks buggy; a killed nbd_thread_send() can exit, free
> its task_struct, and then kthread_stop() can use the freed/reused memory.
>
> Add the new trivial helper, kthread_get_run(). Hopefully it will have more
> users, this patch changes __nbd_ioctl() as an example.

Thanks.

Acked-by: Markus Pargmann <mpa@pengutronix.de>

However I am not sure this is important for 4.3 final. This bug is
present since at least 2008 (didn't look further).

Best Regards,

Markus

>
> Signed-off-by: Oleg Nesterov <oleg@redhat.com>
> ---
> drivers/block/nbd.c | 5 +++--
> include/linux/kthread.h | 12 ++++++++++++
> 2 files changed, 15 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
> index 93b3f99..b85e7a0 100644
> --- a/drivers/block/nbd.c
> +++ b/drivers/block/nbd.c
> @@ -754,8 +754,8 @@ static int __nbd_ioctl(struct block_device *bdev, struct nbd_device *nbd,
> else
> blk_queue_flush(nbd->disk->queue, 0);
>
> - thread = kthread_run(nbd_thread_send, nbd, "%s",
> - nbd_name(nbd));
> + thread = kthread_get_run(nbd_thread_send, nbd, "%s",
> + nbd_name(nbd));
> if (IS_ERR(thread)) {
> mutex_lock(&nbd->tx_lock);
> return PTR_ERR(thread);
> @@ -765,6 +765,7 @@ static int __nbd_ioctl(struct block_device *bdev, struct nbd_device *nbd,
> error = nbd_thread_recv(nbd);
> nbd_dev_dbg_close(nbd);
> kthread_stop(thread);
> + put_task_struct(thread);
>
> mutex_lock(&nbd->tx_lock);
>
> diff --git a/include/linux/kthread.h b/include/linux/kthread.h
> index 13d5520..b0465cc 100644
> --- a/include/linux/kthread.h
> +++ b/include/linux/kthread.h
> @@ -37,6 +37,18 @@ struct task_struct *kthread_create_on_cpu(int (*threadfn)(void *data),
> __k; \
> })
>
> +/* Same as kthread_run() but also pin the task_struct */
> +#define kthread_get_run(threadfn, data, namefmt, ...) \
> +({ \
> + struct task_struct *__k \
> + = kthread_create(threadfn, data, namefmt, ## __VA_ARGS__); \
> + if (!IS_ERR(__k)) { \
> + get_task_struct(__k); \
> + wake_up_process(__k); \
> + } \
> + __k; \
> +})
> +
> void kthread_bind(struct task_struct *k, unsigned int cpu);
> void kthread_bind_mask(struct task_struct *k, const struct cpumask *mask);
> int kthread_stop(struct task_struct *k);
> --
> 1.5.5.1
>
>
>

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