linux-kernel - Re: [PATCH v2] comedi: Flush partial mappings in error case

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Message-ID: <74ccca51-085a-4a31-a854-98937d18fb91@mev.co.uk>
Date: Wed, 16 Oct 2024 10:48:15 +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 v2] comedi: Flush partial mappings in error case

On 15/10/2024 19:26, 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.

It is possible to test it by loading the comedi_test driver module, 
which creates a comedi device in software.  But then you need to induce 
an error somehow.  I did it by hacking the comedi_mmap() function to 
break the loop that calls remap_pfn_range() early with an error.

> ---
> 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 | 10 ++++++++++
>   1 file changed, 10 insertions(+)
> 
> diff --git a/drivers/comedi/comedi_fops.c b/drivers/comedi/comedi_fops.c
> index 1b481731df96..68e5301e6281 100644
> --- a/drivers/comedi/comedi_fops.c
> +++ b/drivers/comedi/comedi_fops.c
> @@ -2407,6 +2407,16 @@ static int comedi_mmap(struct file *file, struct vm_area_struct *vma)
>   
>   			start += PAGE_SIZE;
>   		}
> +
> +		/*
> +		 * 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)
> +			zap_vma_ptes(vma, vma->vm_start, size);
>   	}
>   
>   	if (retval == 0) {
> 
> ---
> base-commit: 6485cf5ea253d40d507cd71253c9568c5470cd27
> change-id: 20241014-comedi-tlb-400246505961

I have tested it with the device created by the comedi_test.ko module 
using the comedi_test application from comedilib, by artificially 
breaking the partial mapping loop with an error half way to completion. 
I haven't noticed any problems from the zap_vma_ptes() call, so it is OK 
as far as I can tell.  (I don't really have any experience of calling 
functions like zap_vma_ptes().)

I note that there are a bunch of drivers in drivers/video/fbdev that 
also call remap_pfn_range() or io_remap_pfn_range() in a loop that 
probably require your attention!

Tested-by: Ian Abbott <abbotti@....co.uk>
Reviewed-by: Ian Abbott <abbotti@....co.uk>

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