linux-kernel - Re: [Patch V3] v4l2-core: fix use-after-free error

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Date:   Wed, 24 Apr 2019 13:07:14 +0200
From:   Hans Verkuil <hverkuil-cisco@...all.nl>
To:     Sumit Gupta <sumitg@...dia.com>, mchehab@...nel.org,
        sakari.ailus@...ux.intel.com, paul.kocialkowski@...tlin.com,
        tfiga@...omium.org, keiichiw@...omium.org, bbasu@...dia.com
Cc:     linux-media@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [Patch V3] v4l2-core: fix use-after-free error

On 4/15/19 5:36 PM, Sumit Gupta wrote:
> From: sumitg <sumitg@...dia.com>
> 
> Fixing use-after-free within __v4l2_ctrl_handler_setup().
> Memory is being freed with kfree(new_ref) for duplicate
> control reference entry but ctrl->cluster pointer is still
> referring to freed duplicate entry resulting in error on
> access. Change done to update cluster pointer only when new
> control reference is added. Also, added check to add new
> ctrl to handler only if the cluster points to an entry.
> 
>  ==================================================================
>  BUG: KASAN: use-after-free in __v4l2_ctrl_handler_setup+0x388/0x428
>  Read of size 8 at addr ffffffc324e78618 by task systemd-udevd/312
> 
>  Allocated by task 312:
> 
>  Freed by task 312:
> 
>  The buggy address belongs to the object at ffffffc324e78600
>   which belongs to the cache kmalloc-64 of size 64
>  The buggy address is located 24 bytes inside of
>   64-byte region [ffffffc324e78600, ffffffc324e78640)
>  The buggy address belongs to the page:
>  page:ffffffbf0c939e00 count:1 mapcount:0 mapping:
> 					(null) index:0xffffffc324e78f80
>  flags: 0x4000000000000100(slab)
>  raw: 4000000000000100 0000000000000000 ffffffc324e78f80 000000018020001a
>  raw: 0000000000000000 0000000100000001 ffffffc37040fb80 0000000000000000
>  page dumped because: kasan: bad access detected
> 
>  Memory state around the buggy address:
>   ffffffc324e78500: fb fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
>   ffffffc324e78580: fb fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
>  >ffffffc324e78600: fb fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
>                              ^
>   ffffffc324e78680: 00 00 00 00 00 00 00 00 fc fc fc fc fc fc fc fc
>   ffffffc324e78700: 00 00 00 00 00 fc fc fc fc fc fc fc fc fc fc fc
>  ==================================================================
> 
> Suggested-by: Hans Verkuil <hverkuil-cisco@...all.nl>
> Signed-off-by: Sumit Gupta <sumitg@...dia.com>
> ---
> 
> v3:
> * update ctrl->cluster only when new control reference is added.
> * add new ctrl to handler only if the cluster points to an entry.
> 
> v2:
> * update ctrl->cluster only when new control reference is added.
> * check ctrl->ncontrols to avoid illegal access when cluster has zero controls.
> 
>  drivers/media/v4l2-core/v4l2-ctrls.c | 26 ++++++++++++++------------
>  1 file changed, 14 insertions(+), 12 deletions(-)
> 
> diff --git a/drivers/media/v4l2-core/v4l2-ctrls.c b/drivers/media/v4l2-core/v4l2-ctrls.c
> index 5e3806f..877c2ab 100644
> --- a/drivers/media/v4l2-core/v4l2-ctrls.c
> +++ b/drivers/media/v4l2-core/v4l2-ctrls.c
> @@ -2154,15 +2154,6 @@ static int handler_new_ref(struct v4l2_ctrl_handler *hdl,
>  	if (size_extra_req)
>  		new_ref->p_req.p = &new_ref[1];
>  
> -	if (ctrl->handler == hdl) {
> -		/* By default each control starts in a cluster of its own.
> -		   new_ref->ctrl is basically a cluster array with one
> -		   element, so that's perfect to use as the cluster pointer.
> -		   But only do this for the handler that owns the control. */
> -		ctrl->cluster = &new_ref->ctrl;
> -		ctrl->ncontrols = 1;
> -	}
> -
>  	INIT_LIST_HEAD(&new_ref->node);
>  
>  	mutex_lock(hdl->lock);
> @@ -2195,6 +2186,15 @@ static int handler_new_ref(struct v4l2_ctrl_handler *hdl,
>  	hdl->buckets[bucket] = new_ref;
>  	if (ctrl_ref)
>  		*ctrl_ref = new_ref;
> +	if (ctrl->handler == hdl) {
> +		/* By default each control starts in a cluster of its own.
> +		 * new_ref->ctrl is basically a cluster array with one
> +		 * element, so that's perfect to use as the cluster pointer.
> +		 * But only do this for the handler that owns the control.
> +		 */
> +		ctrl->cluster = &new_ref->ctrl;
> +		ctrl->ncontrols = 1;
> +	}

This is good.

>  
>  unlock:
>  	mutex_unlock(hdl->lock);
> @@ -2346,9 +2346,11 @@ static struct v4l2_ctrl *v4l2_ctrl_new(struct v4l2_ctrl_handler *hdl,
>  		kvfree(ctrl);
>  		return NULL;
>  	}
> -	mutex_lock(hdl->lock);
> -	list_add_tail(&ctrl->node, &hdl->ctrls);
> -	mutex_unlock(hdl->lock);
> +	if (ctrl->cluster) {
> +		mutex_lock(hdl->lock);
> +		list_add_tail(&ctrl->node, &hdl->ctrls);
> +		mutex_unlock(hdl->lock);
> +	}

But why change this? ctrl->cluster can never be NULL here.

If ctrl->cluster really is NULL for you, then something else
is wrong.

Regards,

	Hans

>  	return ctrl;
>  }
>  
>