linux-kernel - Re: [PATCH] mm/memory.c: make remap_pfn

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Message-Id: <20200617154724.1b4d8a0353c7b8d0d7c79c0b@linux-foundation.org>
Date:   Wed, 17 Jun 2020 15:47:24 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Kaiyu Zhang <zhangalex@...gle.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/memory.c: make remap_pfn_range() reject unaligned
 addr

On Wed, 17 Jun 2020 15:34:14 -0700 Kaiyu Zhang <zhangalex@...gle.com> wrote:

> From: Alex Zhang <zhangalex@...gle.com>
> 
> This function implicitly assumes that the addr passed in is page aligned.
> A non page aligned addr could ultimately cause a kernel bug in
> remap_pte_range as the exit condition in the logic loop may never be
> satisfied.  This patch documents the need for the requirement, as
> well as explicitly adding a check for it.
> 
> ...
>
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2081,7 +2081,7 @@ static inline int remap_p4d_range(struct mm_struct *mm, pgd_t *pgd,
>  /**
>   * remap_pfn_range - remap kernel memory to userspace
>   * @vma: user vma to map to
> - * @addr: target user address to start at
> + * @addr: target page aligned user address to start at
>   * @pfn: page frame number of kernel physical memory address
>   * @size: size of mapping area
>   * @prot: page protection flags for this mapping
> @@ -2100,6 +2100,9 @@ int remap_pfn_range(struct vm_area_struct *vma, unsigned long addr,
>  	unsigned long remap_pfn = pfn;
>  	int err;
>  
> +	if (!PAGE_ALIGN(addr))
> +		return -EINVAL;
> +

That won't work.  PAGE_ALIGNED() will do so.

Also, as this is an error in the calling code it would be better to do

	if (WARN_ON_ONCE(!PAGE_ALIGNED(addr)))
		return -EINVAL;

so that the offending code can be fixed up.

Is there any code in the kernel tree which actually has this error?