linux-kernel - Re: [PATCH] virtio_blk: NULL out vqs to avoid double free on failed resume

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Message-ID: <20251021085030-mutt-send-email-mst@kernel.org>
Date: Tue, 21 Oct 2025 08:57:53 -0400
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Cong Zhang <cong.zhang@....qualcomm.com>
Cc: Jason Wang <jasowang@...hat.com>,
	Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
	Eugenio Pérez <eperezma@...hat.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Stefan Hajnoczi <stefanha@...hat.com>, Jens Axboe <axboe@...nel.dk>,
	linux-arm-msm@...r.kernel.org, virtualization@...ts.linux.dev,
	linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
	pavan.kondeti@....qualcomm.com
Subject: Re: [PATCH] virtio_blk: NULL out vqs to avoid double free on failed
 resume

On Tue, Oct 21, 2025 at 07:07:56PM +0800, Cong Zhang wrote:
> The vblk->vqs releases during freeze. If resume fails before vblk->vqs
> is allocated, later freeze/remove may attempt to free vqs again.
> Set vblk->vqs to NULL after freeing to avoid double free.
> 
> Signed-off-by: Cong Zhang <cong.zhang@....qualcomm.com>
> ---
> The patch fixes a double free issue that occurs in virtio_blk during
> freeze/resume.
> The issue is caused by:
> 1. During the first freeze, vblk->vqs is freed but pointer is not set to
>    NULL.
> 2. Virtio block device fails before vblk->vqs is allocated during resume.
> 3. During the next freeze, vblk->vqs gets freed again, causing the
>    double free crash.

this part I don't get. if restore fails, how can freeze be called
again?

> ---
>  drivers/block/virtio_blk.c | 13 ++++++++++++-
>  1 file changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
> index f061420dfb10c40b21765b173fab7046aa447506..746795066d7f56a01c9a9c0344d24f9fa06841eb 100644
> --- a/drivers/block/virtio_blk.c
> +++ b/drivers/block/virtio_blk.c
> @@ -1026,8 +1026,13 @@ static int init_vq(struct virtio_blk *vblk)
>  out:
>  	kfree(vqs);
>  	kfree(vqs_info);
> -	if (err)
> +	if (err) {
>  		kfree(vblk->vqs);
> +		/*
> +		 * Set to NULL to prevent freeing vqs again during freezing.
> +		 */
> +		vblk->vqs = NULL;
> +	}
>  	return err;
>  }
>  

> @@ -1598,6 +1603,12 @@ static int virtblk_freeze_priv(struct virtio_device *vdev)
>  
>  	vdev->config->del_vqs(vdev);
>  	kfree(vblk->vqs);
> +	/*
> +	 * Set to NULL to prevent freeing vqs again after a failed vqs
> +	 * allocation during resume. Note that kfree() already handles NULL
> +	 * pointers safely.
> +	 */
> +	vblk->vqs = NULL;
>  
>  	return 0;
>  }
> 
> ---
> base-commit: 8e2755d7779a95dd61d8997ebce33ff8b1efd3fb
> change-id: 20250926-virtio_double_free-7ab880d82a17
> 
> Best regards,
> -- 
> Cong Zhang <cong.zhang@....qualcomm.com>