Re: [PATCH v4] rpmsg: char: Fix race between the release of rpmsg_ctrldev and cdev

[Mathieu Poirier]

On Wed, Dec 08, 2021 at 12:52:28PM -0800, Matthias Kaehlcke wrote:
> From: Sujit Kautkar <sujitka@xxxxxxxxxxxx>
> 
> From: Sujit Kautkar <sujitka@xxxxxxxxxxxx>
> 
> struct rpmsg_ctrldev contains a struct cdev. The current code frees
> the rpmsg_ctrldev struct in rpmsg_ctrldev_release_device(), but the
> cdev is a managed object, therefore its release is not predictable
> and the rpmsg_ctrldev could be freed before the cdev is entirely
> released, as in the backtrace below.
> 
> [   93.625603] ODEBUG: free active (active state 0) object type: timer_list hint: delayed_work_timer_fn+0x0/0x7c
> [   93.636115] WARNING: CPU: 0 PID: 12 at lib/debugobjects.c:488 debug_print_object+0x13c/0x1b0
> [   93.644799] Modules linked in: veth xt_cgroup xt_MASQUERADE rfcomm algif_hash algif_skcipher af_alg uinput ip6table_nat fuse uvcvideo videobuf2_vmalloc venus_enc venus_dec videobuf2_dma_contig hci_uart btandroid btqca snd_soc_rt5682_i2c bluetooth qcom_spmi_temp_alarm snd_soc_rt5682v
> [   93.715175] CPU: 0 PID: 12 Comm: kworker/0:1 Tainted: G    B             5.4.163-lockdep #26
> [   93.723855] Hardware name: Google Lazor (rev3 - 8) with LTE (DT)
> [   93.730055] Workqueue: events kobject_delayed_cleanup
> [   93.735271] pstate: 60c00009 (nZCv daif +PAN +UAO)
> [   93.740216] pc : debug_print_object+0x13c/0x1b0
> [   93.744890] lr : debug_print_object+0x13c/0x1b0
> [   93.749555] sp : ffffffacf5bc7940
> [   93.752978] x29: ffffffacf5bc7940 x28: dfffffd000000000
> [   93.758448] x27: ffffffacdb11a800 x26: dfffffd000000000
> [   93.763916] x25: ffffffd0734f856c x24: dfffffd000000000
> [   93.769389] x23: 0000000000000000 x22: ffffffd0733c35b0
> [   93.774860] x21: ffffffd0751994a0 x20: ffffffd075ec27c0
> [   93.780338] x19: ffffffd075199100 x18: 00000000000276e0
> [   93.785814] x17: 0000000000000000 x16: dfffffd000000000
> [   93.791291] x15: ffffffffffffffff x14: 6e6968207473696c
> [   93.796768] x13: 0000000000000000 x12: ffffffd075e2b000
> [   93.802244] x11: 0000000000000001 x10: 0000000000000000
> [   93.807723] x9 : d13400dff1921900 x8 : d13400dff1921900
> [   93.813200] x7 : 0000000000000000 x6 : 0000000000000000
> [   93.818676] x5 : 0000000000000080 x4 : 0000000000000000
> [   93.824152] x3 : ffffffd0732a0fa4 x2 : 0000000000000001
> [   93.829628] x1 : ffffffacf5bc7580 x0 : 0000000000000061
> [   93.835104] Call trace:
> [   93.837644]  debug_print_object+0x13c/0x1b0
> [   93.841963]  __debug_check_no_obj_freed+0x25c/0x3c0
> [   93.846987]  debug_check_no_obj_freed+0x18/0x20
> [   93.851669]  slab_free_freelist_hook+0xbc/0x1e4
> [   93.856346]  kfree+0xfc/0x2f4
> [   93.859416]  rpmsg_ctrldev_release_device+0x78/0xb8
> [   93.864445]  device_release+0x84/0x168
> [   93.868310]  kobject_cleanup+0x12c/0x298
> [   93.872356]  kobject_delayed_cleanup+0x10/0x18
> [   93.876948]  process_one_work+0x578/0x92c
> [   93.881086]  worker_thread+0x804/0xcf8
> [   93.884963]  kthread+0x2a8/0x314
> [   93.888303]  ret_from_fork+0x10/0x18
> 
> The cdev_device_add/del() API was created to address this issue
> (see commit 233ed09d7fda), use it instead of cdev add/del().
> 
> Signed-off-by: Sujit Kautkar <sujitka@xxxxxxxxxxxx>
> Signed-off-by: Matthias Kaehlcke <mka@xxxxxxxxxxxx>
> ---
> 
> Changes in v4:
> - call cdev_device_del() from rpmsg_chrdev_remove() instead of
>   rpmsg_ctrldev_release_device()
> - updated subject (was: "rpmsg: glink: Update cdev add/del API in
>   rpmsg_ctrldev_release_device()")
> - updated commit message
> - replaced backtrace in commit message with one that doesn't have
>   a dump_backtrace() call
> 
> Changes in v3:
> - Remove unecessary error check as per Matthias's comment
> 
> Changes in v2:
> - Fix typo in commit message
> 
>  drivers/rpmsg/rpmsg_char.c | 11 ++---------
>  1 file changed, 2 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/rpmsg/rpmsg_char.c b/drivers/rpmsg/rpmsg_char.c
> index b5907b80727c..b1b75ef04560 100644
> --- a/drivers/rpmsg/rpmsg_char.c
> +++ b/drivers/rpmsg/rpmsg_char.c
> @@ -459,7 +459,6 @@ static void rpmsg_ctrldev_release_device(struct device *dev)
>  
>  	ida_simple_remove(&rpmsg_ctrl_ida, dev->id);
>  	ida_simple_remove(&rpmsg_minor_ida, MINOR(dev->devt));
> -	cdev_del(&ctrldev->cdev);
>  	kfree(ctrldev);
>  }
>  
> @@ -494,19 +493,13 @@ static int rpmsg_chrdev_probe(struct rpmsg_device *rpdev)
>  	dev->id = ret;
>  	dev_set_name(&ctrldev->dev, "rpmsg_ctrl%d", ret);
>  
> -	ret = cdev_add(&ctrldev->cdev, dev->devt, 1);
> +	ret = cdev_device_add(&ctrldev->cdev, &ctrldev->dev);
>  	if (ret)
>  		goto free_ctrl_ida;
>  
>  	/* We can now rely on the release function for cleanup */
>  	dev->release = rpmsg_ctrldev_release_device;
>  
> -	ret = device_add(dev);
> -	if (ret) {
> -		dev_err(&rpdev->dev, "device_add failed: %d\n", ret);
> -		put_device(dev);
> -	}
> -
>  	dev_set_drvdata(&rpdev->dev, ctrldev);
>  
>  	return ret;
> @@ -532,7 +525,7 @@ static void rpmsg_chrdev_remove(struct rpmsg_device *rpdev)
>  	if (ret)
>  		dev_warn(&rpdev->dev, "failed to nuke endpoints: %d\n", ret);
>  
> -	device_del(&ctrldev->dev);
> +	cdev_device_del(&ctrldev->cdev, &ctrldev->dev);
>  	put_device(&ctrldev->dev);

Reviewed-by: Mathieu Poirier <mathieu.poirier@xxxxxxxxxx>

I'll let Bjorn pick this one to make sure it doesn't break anything for current
users of the driver.

Thanks,
Mathieu

>  }
>  
> -- 
> 2.34.1.400.ga245620fadb-goog
>