linux-kernel - Re: [PATCH] mm/userfaultfd: Support operation on multiple VMAs

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Date:   Mon, 13 Feb 2023 12:44:56 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Muhammad Usama Anjum <usama.anjum@...labora.com>,
        peterx@...hat.com, Andrew Morton <akpm@...ux-foundation.org>
Cc:     kernel@...labora.com, Paul Gofman <pgofman@...eweavers.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/userfaultfd: Support operation on multiple VMAs

On 13.02.23 11:43, Muhammad Usama Anjum wrote:
> mwriteprotect_range() errors out if [start, end) doesn't fall in one
> VMA. We are facing a use case where multiple VMAs are present in one
> range of interest. For example, the following pseudocode reproduces the
> error which we are trying to fix:
> 
> - Allocate memory of size 16 pages with PROT_NONE with mmap
> - Register userfaultfd
> - Change protection of the first half (1 to 8 pages) of memory to
>    PROT_READ | PROT_WRITE. This breaks the memory area in two VMAs.
> - Now UFFDIO_WRITEPROTECT_MODE_WP on the whole memory of 16 pages errors
>    out.
> 
> This is a simple use case where user may or may not know if the memory
> area has been divided into multiple VMAs.
> 
> Reported-by: Paul Gofman <pgofman@...eweavers.com>
> Signed-off-by: Muhammad Usama Anjum <usama.anjum@...labora.com>
> ---
>   mm/userfaultfd.c | 36 +++++++++++++++++++-----------------
>   1 file changed, 19 insertions(+), 17 deletions(-)
> 
> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> index 65ad172add27..46e0a014af68 100644
> --- a/mm/userfaultfd.c
> +++ b/mm/userfaultfd.c
> @@ -738,9 +738,11 @@ int mwriteprotect_range(struct mm_struct *dst_mm, unsigned long start,
>   			unsigned long len, bool enable_wp,
>   			atomic_t *mmap_changing)
>   {
> +	unsigned long end = start + len;
>   	struct vm_area_struct *dst_vma;
>   	unsigned long page_mask;
>   	int err;
> +	VMA_ITERATOR(vmi, dst_mm, start);
>   
>   	/*
>   	 * Sanitize the command parameters:
> @@ -762,26 +764,26 @@ int mwriteprotect_range(struct mm_struct *dst_mm, unsigned long start,
>   	if (mmap_changing && atomic_read(mmap_changing))
>   		goto out_unlock;
>   
> -	err = -ENOENT;
> -	dst_vma = find_dst_vma(dst_mm, start, len);
> -
> -	if (!dst_vma)
> -		goto out_unlock;
> -	if (!userfaultfd_wp(dst_vma))
> -		goto out_unlock;
> -	if (!vma_can_userfault(dst_vma, dst_vma->vm_flags))
> -		goto out_unlock;
> +	for_each_vma_range(vmi, dst_vma, end) {
> +		err = -ENOENT;
>   
> -	if (is_vm_hugetlb_page(dst_vma)) {
> -		err = -EINVAL;
> -		page_mask = vma_kernel_pagesize(dst_vma) - 1;
> -		if ((start & page_mask) || (len & page_mask))
> -			goto out_unlock;
> -	}
> +		if (!dst_vma->vm_userfaultfd_ctx.ctx)
> +			break;
> +		if (!userfaultfd_wp(dst_vma))
> +			break;
> +		if (!vma_can_userfault(dst_vma, dst_vma->vm_flags))
> +			break;
>   
> -	uffd_wp_range(dst_mm, dst_vma, start, len, enable_wp);
> +		if (is_vm_hugetlb_page(dst_vma)) {
> +			err = -EINVAL;
> +			page_mask = vma_kernel_pagesize(dst_vma) - 1;
> +			if ((start & page_mask) || (len & page_mask))
> +				break;
> +		}
>   
> -	err = 0;
> +		uffd_wp_range(dst_mm, dst_vma, start, len, enable_wp);

I suspect you should be adjusting the range to only cover that specific 
VMA here.

-- 
Thanks,

David / dhildenb