linux-kernel - Re: [PATCH v3] comedi: Flush partial mappings in error case

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Message-ID: <10f30e5b-dbf0-4f7d-9688-5ae256e2c252@mev.co.uk>
Date: Fri, 18 Oct 2024 12:35:27 +0100
From: Ian Abbott <abbotti@....co.uk>
To: Jann Horn <jannh@...gle.com>,
 H Hartley Sweeten <hsweeten@...ionengravers.com>,
 Frank Mori Hess <fmh6jj@...il.com>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH v3] comedi: Flush partial mappings in error case

On 17/10/2024 20:07, 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 error path.
> 
> See commit 79a61cc3fc04 ("mm: avoid leaving partial pfn mappings around in
> error case").
> 
> Cc: stable@...r.kernel.org
> 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.
> ---
> Changes in v3:
> - gate zapping ptes on CONFIG_MMU (Intel kernel test robot)
> - Link to v2: https://lore.kernel.org/r/20241015-comedi-tlb-v2-1-cafb0e27dd9a@google.com
> 
> Changes in v2:
> - only do the zapping in the pfnmap path (Ian Abbott)
> - use zap_vma_ptes() instead of zap_page_range_single() (Ian Abbott)
> - Link to v1: https://lore.kernel.org/r/20241014-comedi-tlb-v1-1-4b699144b438@google.com
> ---
>   drivers/comedi/comedi_fops.c | 12 ++++++++++++
>   1 file changed, 12 insertions(+)
> 
> diff --git a/drivers/comedi/comedi_fops.c b/drivers/comedi/comedi_fops.c
> index 1b481731df96..b9df9b19d4bd 100644
> --- a/drivers/comedi/comedi_fops.c
> +++ b/drivers/comedi/comedi_fops.c
> @@ -2407,6 +2407,18 @@ static int comedi_mmap(struct file *file, struct vm_area_struct *vma)
>   
>   			start += PAGE_SIZE;
>   		}
> +
> +#ifdef CONFIG_MMU
> +		/*
> +		 * 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.
> +		 */
> +		if (retval)

Perhaps that condition should be changed to `retval && i` since there 
will be no partial mappings left behind if the first call to 
`remap_pfn_range` failed.

> +			zap_vma_ptes(vma, vma->vm_start, size);
> +#endif
>   	}
>   
>   	if (retval == 0) {
> 
> ---
> base-commit: 6485cf5ea253d40d507cd71253c9568c5470cd27
> change-id: 20241014-comedi-tlb-400246505961


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