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Date:   Fri, 12 Apr 2019 13:47:40 +0000
From:   Sumit Gupta <sumitg@...dia.com>
To:     Hans Verkuil <hverkuil-cisco@...all.nl>,
        "mchehab@...nel.org" <mchehab@...nel.org>,
        "sakari.ailus@...ux.intel.com" <sakari.ailus@...ux.intel.com>,
        "paul.kocialkowski@...tlin.com" <paul.kocialkowski@...tlin.com>,
        "tfiga@...omium.org" <tfiga@...omium.org>,
        "keiichiw@...omium.org" <keiichiw@...omium.org>,
        Bibek Basu <bbasu@...dia.com>
CC:     "linux-media@...r.kernel.org" <linux-media@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [Patch V2] v4l2-core: fix use-after-free error


On 4/11/19 6:01 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 cluster is still referring to the duplicate entry. 
> Change done to update ctrl->cluster only when new control reference is 
> added. Also, checking if
> ctrl->ncontrols is non-zero before accessing controls from
> cluster pointer to avoid illegal access when cluster has zero 
> controls.
> 
>  ==================================================================
>  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>
> ---
> 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 | 53 
> +++++++++++++++++++-----------------
>  1 file changed, 28 insertions(+), 25 deletions(-)
> 
> diff --git a/drivers/media/v4l2-core/v4l2-ctrls.c 
> b/drivers/media/v4l2-core/v4l2-ctrls.c
> index 5e3806f..3d0a952 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;
> +	}
>  
>  unlock:
>  	mutex_unlock(hdl->lock);

This looks good.

> @@ -2725,25 +2725,28 @@ int __v4l2_ctrl_handler_setup(struct v4l2_ctrl_handler *hdl)
>  		ctrl->done = false;
>  
>  	list_for_each_entry(ctrl, &hdl->ctrls, node) {
> -		struct v4l2_ctrl *master = ctrl->cluster[0];
> -		int i;
> -
> -		/* Skip if this control was already handled by a cluster. */
> -		/* Skip button controls and read-only controls. */
> -		if (ctrl->done || ctrl->type == V4L2_CTRL_TYPE_BUTTON ||
> -		    (ctrl->flags & V4L2_CTRL_FLAG_READ_ONLY))
> -			continue;
> +		if (ctrl->ncontrols) {
> +			struct v4l2_ctrl *master = ctrl->cluster[0];
> +			int i;
> +
> +			/* Skip if this control was already handled by a */
> +			/* cluster. Skip button controls and read-only */
> +			/* controls. */
> +			if (ctrl->done || ctrl->type == V4L2_CTRL_TYPE_BUTTON ||
> +				(ctrl->flags & V4L2_CTRL_FLAG_READ_ONLY))
> +				continue;
>  
> -		for (i = 0; i < master->ncontrols; i++) {
> -			if (master->cluster[i]) {
> -				cur_to_new(master->cluster[i]);
> -				master->cluster[i]->is_new = 1;
> -				master->cluster[i]->done = true;
> +			for (i = 0; i < master->ncontrols; i++) {
> +				if (master->cluster[i]) {
> +					cur_to_new(master->cluster[i]);
> +					master->cluster[i]->is_new = 1;
> +					master->cluster[i]->done = true;
> +				}
>  			}
> +			ret = call_op(master, s_ctrl);
> +			if (ret)
> +				break;
>  		}
> -		ret = call_op(master, s_ctrl);
> -		if (ret)
> -			break;
>  	}
>  
>  	return ret;
> 

These changes shouldn't be needed: when you get here all controls are guaranteed to have a non-0 ncontrols value.

If it is possible to encounter a control where ncontrols == 0, then that's a bug elsewhere. But as far as I can tell that isn't the case.

Regards,

	Hans

Yes, I am getting a control where ncontrols is zero as printed below.
It can be another bug as you said. I think, adding check here will be
helpful to not crash in such cases. Please share if you think otherwise,
then will remove the check.

  int __v4l2_ctrl_handler_setup(struct v4l2_ctrl_handler *hdl)
                  ctrl->done = false;
 
          list_for_each_entry(ctrl, &hdl->ctrls, node) {
  +               if (ctrl->ncontrols == 0)
  +                       pr_crit("<func:%s><ctrl->name:%s><ctrl->ncontrols:%d>\n",
  +                                       __func__, ctrl->name, ctrl->ncontrols);

Log:
  [    8.354464] <func:__v4l2_ctrl_handler_setup><ctrl->name:Group Hold><ctrl->ncontrols:0>


Regards,
Sumit Gupta

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