linux-kernel - Re: [PATCH 1/2] mm: mmap: fix fput in error path v2

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Message-ID: <be6d248d-01d4-f2a3-5e33-7f311daef0b7@gmail.com>
Date:   Wed, 4 Nov 2020 09:03:15 +0100
From:   Christian König <ckoenig.leichtzumerken@...il.com>
To:     akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linaro-mm-sig@...ts.linaro.org,
        dri-devel@...ts.freedesktop.org, linux-media@...r.kernel.org,
        chris@...is-wilson.co.uk, airlied@...hat.com, daniel@...ll.ch,
        sumit.semwal@...aro.org, willy@...radead.org, jhubbard@...dia.com,
        jgg@...pe.ca, linmiaohe@...wei.com
Subject: Re: [PATCH 1/2] mm: mmap: fix fput in error path v2

If nobody comes up with an objections I'm going to merge that through 
drm-misc-next.

Thanks,
Christian.

Am 12.10.20 um 10:52 schrieb Christian König:
> Patch "495c10cc1c0c CHROMIUM: dma-buf: restore args..."
> adds a workaround for a bug in mmap_region.
>
> As the comment states ->mmap() callback can change
> vma->vm_file and so we might call fput() on the wrong file.
>
> Revert the workaround and proper fix this in mmap_region.
>
> v2: drop the extra if in dma_buf_mmap as well
>
> Signed-off-by: Christian König <christian.koenig@....com>
> Reviewed-by: Jason Gunthorpe <jgg@...dia.com>
> ---
>   drivers/dma-buf/dma-buf.c | 20 +++-----------------
>   mm/mmap.c                 |  2 +-
>   2 files changed, 4 insertions(+), 18 deletions(-)
>
> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> index a6ba4d598f0e..08630d057cf2 100644
> --- a/drivers/dma-buf/dma-buf.c
> +++ b/drivers/dma-buf/dma-buf.c
> @@ -1143,9 +1143,6 @@ EXPORT_SYMBOL_GPL(dma_buf_end_cpu_access);
>   int dma_buf_mmap(struct dma_buf *dmabuf, struct vm_area_struct *vma,
>   		 unsigned long pgoff)
>   {
> -	struct file *oldfile;
> -	int ret;
> -
>   	if (WARN_ON(!dmabuf || !vma))
>   		return -EINVAL;
>   
> @@ -1163,22 +1160,11 @@ int dma_buf_mmap(struct dma_buf *dmabuf, struct vm_area_struct *vma,
>   		return -EINVAL;
>   
>   	/* readjust the vma */
> -	get_file(dmabuf->file);
> -	oldfile = vma->vm_file;
> -	vma->vm_file = dmabuf->file;
> +	fput(vma->vm_file);
> +	vma->vm_file = get_file(dmabuf->file);
>   	vma->vm_pgoff = pgoff;
>   
> -	ret = dmabuf->ops->mmap(dmabuf, vma);
> -	if (ret) {
> -		/* restore old parameters on failure */
> -		vma->vm_file = oldfile;
> -		fput(dmabuf->file);
> -	} else {
> -		if (oldfile)
> -			fput(oldfile);
> -	}
> -	return ret;
> -
> +	return dmabuf->ops->mmap(dmabuf, vma);
>   }
>   EXPORT_SYMBOL_GPL(dma_buf_mmap);
>   
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 40248d84ad5f..3a2670d73355 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -1852,8 +1852,8 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
>   	return addr;
>   
>   unmap_and_free_vma:
> +	fput(vma->vm_file);
>   	vma->vm_file = NULL;
> -	fput(file);
>   
>   	/* Undo any partial mapping done by a device driver. */
>   	unmap_region(mm, vma, prev, vma->vm_start, vma->vm_end);