linux-kernel - Re: [PATCH] comedi: Flush partial mappings in error case

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Message-ID: <4f531d06-9802-4086-8463-db4c9b6ba11c@mev.co.uk>
Date: Tue, 15 Oct 2024 11:19:45 +0100
From: Ian Abbott <abbotti@....co.uk>
To: Jann Horn <jannh@...gle.com>,
 H Hartley Sweeten <hsweeten@...ionengravers.com>,
 Frank Mori Hess <fmhess@...rs.sourceforge.net>
Cc: linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH] comedi: Flush partial mappings in error case

On 14/10/2024 21:50, Jann Horn wrote:
> If some remap_pfn_range() calls succeeded before one failed, we still have
> buffer pages mapped into the userspace page tables when we drop the buffer
> reference with comedi_buf_map_put(bm). The userspace mappings are only
> cleaned up later in the mmap error path.
> 
> Fix it by explicitly flushing all mappings in our VMA on the comedi_mmap()
> error path.
> 
> See commit 79a61cc3fc04 ("mm: avoid leaving partial pfn mappings around in
> error case").
> 
> Cc: stable@...r.kernel.org

Your patched version won't compile before 6.1 so you may want to 
indicate that in the Cc line.

> Fixes: ed9eccbe8970 ("Staging: add comedi core")
> Signed-off-by: Jann Horn <jannh@...gle.com>
> ---
> Note: compile-tested only; I don't actually have comedi hardware, and I
> don't know anything about comedi.
> ---
>   drivers/comedi/comedi_fops.c | 9 +++++++++
>   1 file changed, 9 insertions(+)
> 
> diff --git a/drivers/comedi/comedi_fops.c b/drivers/comedi/comedi_fops.c
> index 1b481731df96..0e573df8646f 100644
> --- a/drivers/comedi/comedi_fops.c
> +++ b/drivers/comedi/comedi_fops.c
> @@ -2414,6 +2414,15 @@ static int comedi_mmap(struct file *file, struct vm_area_struct *vma)
>   		vma->vm_private_data = bm;
>   
>   		vma->vm_ops->open(vma);
> +	} else {
> +		/*
> +		 * Leaving behind a partial mapping of a buffer we're about to
> +		 * drop is unsafe, see remap_pfn_range_notrack().
> +		 * We need to zap the range here ourselves instead of relying
> +		 * on the automatic zapping in remap_pfn_range() because we call
> +		 * remap_pfn_range() in a loop.
> +		 */
> +		zap_page_range_single(vma, vma->vm_start, size, NULL);
>   	}
>   
>   done:
> 
> ---
> base-commit: 6485cf5ea253d40d507cd71253c9568c5470cd27
> change-id: 20241014-comedi-tlb-400246505961

I guess this doesn't need to be done for the path that calls 
dma_mmap_coherent() since that does not do any range splitting. Would it 
be better to move it into the branch that calls remap_pfn_range() in a loop?

Note that I have no commit access to pulled-from repositories.  Greg-KH 
usually commits them on one of his repos, so could you Cc him too?  Thanks.

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