Re: [PATCH v2] misc: bcm_vk: Fix potential deadlock on &vk->ctx_lock

[Greg KH <gregkh@linuxfoundation.org>]

On Wed, Jun 28, 2023 at 11:29:58AM +0000, Chengfeng Ye wrote:
> As &vk->ctx_lock is acquired by timer bcm_vk_hb_poll() under softirq
> context, other process context code should disable irq or bottom-half
> before acquire the same lock, otherwise deadlock could happen if the
> timer preempt the execution while the lock is held in process context
> on the same CPU.
> 
> Possible deadlock scenario
> bcm_vk_open()
>     -> bcm_vk_get_ctx()
>     -> spin_lock(&vk->ctx_lock)
> 	<timer iterrupt>
> 	-> bcm_vk_hb_poll()
> 	-> bcm_vk_blk_drv_access()
> 	-> spin_lock_irqsave(&vk->ctx_lock, flags) (deadlock here)
> 
> This flaw was found using an experimental static analysis tool we are
> developing for irq-related deadlock, which reported the following
> warning when analyzing the linux kernel 6.4-rc7 release.
> 
> [Deadlock]: &vk->ctx_lock
>   [Interrupt]: bcm_vk_hb_poll
>     -->/root/linux/drivers/misc/bcm-vk/bcm_vk_msg.c:176
>     -->/root/linux/drivers/misc/bcm-vk/bcm_vk_dev.c:512
>   [Locking Unit]: bcm_vk_ioctl
>     -->/root/linux/drivers/misc/bcm-vk/bcm_vk_dev.c:1181
>     -->/root/linux/drivers/misc/bcm-vk/bcm_vk_dev.c:512
> 
> [Deadlock]: &vk->ctx_lock
>   [Interrupt]: bcm_vk_hb_poll
>     -->/root/linux/drivers/misc/bcm-vk/bcm_vk_msg.c:176
>     -->/root/linux/drivers/misc/bcm-vk/bcm_vk_dev.c:512
>   [Locking Unit]: bcm_vk_ioctl
>     -->/root/linux/drivers/misc/bcm-vk/bcm_vk_dev.c:1169
> 
> [Deadlock]: &vk->ctx_lock
>   [Interrupt]: bcm_vk_hb_poll
>     -->/root/linux/drivers/misc/bcm-vk/bcm_vk_msg.c:176
>     -->/root/linux/drivers/misc/bcm-vk/bcm_vk_dev.c:512
>   [Locking Unit]: bcm_vk_open
>     -->/root/linux/drivers/misc/bcm-vk/bcm_vk_msg.c:216
> 
> [Deadlock]: &vk->ctx_lock
>   [Interrupt]: bcm_vk_hb_poll
>     -->/root/linux/drivers/misc/bcm-vk/bcm_vk_msg.c:176
>     -->/root/linux/drivers/misc/bcm-vk/bcm_vk_dev.c:512
>   [Locking Unit]: bcm_vk_release
>     -->/root/linux/drivers/misc/bcm-vk/bcm_vk_msg.c:306
> 
> The tentative patch fix the potential deadlock by spin_lock_irqsave().
> 
> Signed-off-by: Chengfeng Ye <dg573847474@gmail.com>
> ---

You do not mention how you tested to verify that your change is correct
:(