linux-kernel - Re: [PATCH 1/6] mm: mmap: fix fput in error path

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Message-ID: <20201009151540.GX5177@ziepe.ca>
Date:   Fri, 9 Oct 2020 12:15:40 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Christian König <ckoenig.leichtzumerken@...il.com>
Cc:     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, akpm@...ux-foundation.org, daniel@...ll.ch,
        sumit.semwal@...aro.org, willy@...radead.org, jhubbard@...dia.com
Subject: Re: [PATCH 1/6] mm: mmap: fix fput in error path

On Fri, Oct 09, 2020 at 05:03:37PM +0200, Christian König wrote:
> 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.
> 
> Signed-off-by: Christian König <christian.koenig@....com>
>  drivers/dma-buf/dma-buf.c | 22 +++++-----------------
>  mm/mmap.c                 |  2 +-
>  2 files changed, 6 insertions(+), 18 deletions(-)
> 
> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> index a6ba4d598f0e..edd57402a48a 100644
> +++ 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 +1160,13 @@ 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;
> -	vma->vm_pgoff = pgoff;
> +	if (vma->vm_file)
> +		fput(vma->vm_file);

This if is redundant too

But otherwise

Reviewed-by: Jason Gunthorpe <jgg@...dia.com>

Thanks,
Jason