Re: [PATCH] rpmsg: char: take ept_lock in the SET_INCOMING_FLOWCONTROL ioctl

[Mathieu Poirier <mathieu.poirier@linaro.org>]

Hi,

On Mon, Oct 26, 2026 at 08:03:06PM +1000, Yu Zhang wrote:
> rpmsg_eptdev_ioctl() handles RPMSG_SET_INCOMING_FLOWCONTROL by calling
> rpmsg_set_flow_control(eptdev->ept, ...) without holding eptdev->ept_lock
> and without checking eptdev->ept for NULL. rpmsg_set_flow_control()
> dereferences ept->ops, so this is the only eptdev->ept user that does not
> honour the ept_lock discipline.
> 
> eptdev->ept is torn down under ept_lock: rpmsg_chrdev_eptdev_destroy()
> (reached via RPMSG_DESTROY_EPT_IOCTL on a second fd, or when the rpmsg
> device is removed) and rpmsg_eptdev_release() both do
> 
> 	mutex_lock(&eptdev->ept_lock);
> 	rpmsg_destroy_ept(eptdev->ept);
> 	eptdev->ept = NULL;
> 	mutex_unlock(&eptdev->ept_lock);
> 
> and rpmsg_destroy_ept() drops the endpoint's last reference and frees it.
> A SET_INCOMING_FLOWCONTROL ioctl running concurrently can therefore read
> eptdev->ept just as it is freed and dereference ept->ops -> use-after-free
> (or a NULL dereference once ept has been cleared).
> 
> The other eptdev->ept users get this right: rpmsg_eptdev_write_iter() and
> rpmsg_eptdev_poll() take ept_lock and re-check eptdev->ept before use. Do
> the same in the ioctl. Lock only this case: RPMSG_DESTROY_EPT_IOCTL calls
> rpmsg_chrdev_eptdev_destroy(), which takes ept_lock itself, so the switch
> as a whole must not hold it.

Please get rid of the above paragraph.

> 
> Fixes: 5550201c0fe2 ("rpmsg: char: Add RPMSG GET/SET FLOWCONTROL IOCTL support")
> Cc: stable@vger.kernel.org
> Signed-off-by: Yu Zhang <yuz08559@gmail.com>
> ---
>  drivers/rpmsg/rpmsg_char.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/rpmsg/rpmsg_char.c b/drivers/rpmsg/rpmsg_char.c
> index 550fda2..49c0784 100644
> --- a/drivers/rpmsg/rpmsg_char.c
> +++ b/drivers/rpmsg/rpmsg_char.c
> @@ -343,7 +343,15 @@ static long rpmsg_eptdev_ioctl(struct file *fp, unsigned int cmd,
>  			break;
>  		}
>  		set = !!arg;
> -		ret = rpmsg_set_flow_control(eptdev->ept, set, eptdev->chinfo.dst);
> +		ret = mutex_lock_interruptible(&eptdev->ept_lock);
> +		if (ret)
> +			break;
> +		if (eptdev->ept)
> +			ret = rpmsg_set_flow_control(eptdev->ept, set,
> +						     eptdev->chinfo.dst);
> +		else
> +			ret = -EPIPE;
> +		mutex_unlock(&eptdev->ept_lock);


                ret = mutex_lock_interruptible(&eptdev->ept_lock);
		if (ret)
 			break;

                if (!eptdev->ept) {
                        ret = -EPIEP;
                        break;
                }
		ret = rpmsg_set_flow_control(eptdev->ept, set,
                                             eptdev->chinfo.dst);
		mutex_unlock(&eptdev->ept_lock);


Thanks,
Mathieu

>  		break;
>  	case RPMSG_DESTROY_EPT_IOCTL:
>  		/* Don't allow to destroy a default endpoint. */
> -- 
> 2.43.0
>