linux-kernel - Re: [PATCH] [media] v4l2-core: fix use-after-free error

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Date:   Wed, 10 Apr 2019 16:52:41 +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
Cc:     linux-media@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] [media] v4l2-core: fix use-after-free error

On 4/10/19 4:28 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 point cluster to
> original reference instead of duplicate which is freed.
> 
>  ==================================================================
>  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
>  ==================================================================
> 
> Signed-off-by: sumitg <sumitg@...dia.com>
> ---
>  drivers/media/v4l2-core/v4l2-ctrls.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/media/v4l2-core/v4l2-ctrls.c b/drivers/media/v4l2-core/v4l2-ctrls.c
> index 5e3806f..e971bab 100644
> --- a/drivers/media/v4l2-core/v4l2-ctrls.c
> +++ b/drivers/media/v4l2-core/v4l2-ctrls.c
> @@ -2182,6 +2182,7 @@ static int handler_new_ref(struct v4l2_ctrl_handler *hdl,
>  			continue;
>  		/* Don't add duplicates */
>  		if (ref->ctrl->id == id) {
> +			ctrl->cluster = &ref->ctrl;
>  			kfree(new_ref);
>  			goto unlock;
>  		}
> 

I don't like this solution. Instead move this code:

        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;
        }

to after the insert_in_hash label.

That way the ctrl->cluster pointer is only updated if everything is OK.

Nice catch, BTW!

Regards,

	Hans