linux-kernel - Re: [PATCH] media: videobuf2-core: fix use after free in vb2

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Date:   Sat, 17 Nov 2018 10:00:05 +0100
From:   Hans Verkuil <hverkuil@...all.nl>
To:     Sudip Mukherjee <sudipm.mukherjee@...il.com>,
        Pawel Osciak <pawel@...iak.com>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Kyungmin Park <kyungmin.park@...sung.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-media@...r.kernel.org,
        syzkaller-bugs@...glegroups.com
Subject: Re: [PATCH] media: videobuf2-core: fix use after free in vb2_mmap

Same patch was already posted and is waiting to be merged:

https://patchwork.linuxtv.org/patch/52944/

Regards,

	Hans

On 11/17/2018 12:42 AM, Sudip Mukherjee wrote:
> When we are using __find_plane_by_offset() to find the matching plane
> number and the buffer, the queue is not locked. So, after we have
> calculated the buffer number and assigned the pointer to vb, it can get
> freed. And if that happens then we get a use-after-free when we try to
> use this for the mmap and get the following calltrace:
> 
> [   30.623259] Call Trace:
> [   30.623531]  dump_stack+0x244/0x39d
> [   30.623914]  ? dump_stack_print_info.cold.1+0x20/0x20
> [   30.624439]  ? printk+0xa7/0xcf
> [   30.624777]  ? kmsg_dump_rewind_nolock+0xe4/0xe4
> [   30.625265]  print_address_description.cold.7+0x9/0x1ff
> [   30.625809]  kasan_report.cold.8+0x242/0x309
> [   30.626263]  ? vb2_mmap+0x712/0x790
> [   30.626637]  __asan_report_load8_noabort+0x14/0x20
> [   30.627201]  vb2_mmap+0x712/0x790
> [   30.627642]  ? vb2_poll+0x1d0/0x1d0
> [   30.628060]  vb2_fop_mmap+0x4b/0x70
> [   30.628458]  v4l2_mmap+0x153/0x200
> [   30.628929]  mmap_region+0xe85/0x1cd0
> 
> Lock the queue before we start finding the matching plane and buffer so
> that there is no chance of the memory being freed while we are about
> to use it.
> 
> Reported-by: syzbot+be93025dd45dccd8923c@...kaller.appspotmail.com
> Signed-off-by: Sudip Mukherjee <sudipm.mukherjee@...il.com>
> ---
>  drivers/media/common/videobuf2/videobuf2-core.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/media/common/videobuf2/videobuf2-core.c b/drivers/media/common/videobuf2/videobuf2-core.c
> index 975ff5669f72..a81320566e02 100644
> --- a/drivers/media/common/videobuf2/videobuf2-core.c
> +++ b/drivers/media/common/videobuf2/videobuf2-core.c
> @@ -2125,9 +2125,12 @@ int vb2_mmap(struct vb2_queue *q, struct vm_area_struct *vma)
>  	/*
>  	 * Find the plane corresponding to the offset passed by userspace.
>  	 */
> +	mutex_lock(&q->mmap_lock);
>  	ret = __find_plane_by_offset(q, off, &buffer, &plane);
> -	if (ret)
> +	if (ret) {
> +		mutex_unlock(&q->mmap_lock);
>  		return ret;
> +	}
>  
>  	vb = q->bufs[buffer];
>  
> @@ -2138,12 +2141,12 @@ int vb2_mmap(struct vb2_queue *q, struct vm_area_struct *vma)
>  	 */
>  	length = PAGE_ALIGN(vb->planes[plane].length);
>  	if (length < (vma->vm_end - vma->vm_start)) {
> +		mutex_unlock(&q->mmap_lock);
>  		dprintk(1,
>  			"MMAP invalid, as it would overflow buffer length\n");
>  		return -EINVAL;
>  	}
>  
> -	mutex_lock(&q->mmap_lock);
>  	ret = call_memop(vb, mmap, vb->planes[plane].mem_priv, vma);
>  	mutex_unlock(&q->mmap_lock);
>  	if (ret)
>