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SubjectRe: [Patch V3] v4l2-core: fix use-after-free error
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On 4/15/19 5:36 PM, Sumit Gupta wrote:
> From: sumitg <sumitg@nvidia.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@xs4all.nl>
> Signed-off-by: Sumit Gupta <sumitg@nvidia.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;
> }
>
>

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