Re: [PATCH] hugetlbfs: fix offset overflow in huegtlbfs mmap

[Naoya Horiguchi]

On Tue, Apr 11, 2017 at 03:51:58PM -0700, Mike Kravetz wrote:
...
> diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
> index 7163fe0..dde8613 100644
> --- a/fs/hugetlbfs/inode.c
> +++ b/fs/hugetlbfs/inode.c
> @@ -136,17 +136,26 @@ static int hugetlbfs_file_mmap(struct file *file, struct vm_area_struct *vma)
>  	vma->vm_flags |= VM_HUGETLB | VM_DONTEXPAND;
>  	vma->vm_ops = &hugetlb_vm_ops;
>  
> +	/*
> +	 * Offset passed to mmap (before page shift) could have been
> +	 * negative when represented as a (l)off_t.
> +	 */
> +	if (((loff_t)vma->vm_pgoff << PAGE_SHIFT) < 0)
> +		return -EINVAL;
> +
>  	if (vma->vm_pgoff & (~huge_page_mask(h) >> PAGE_SHIFT))
>  		return -EINVAL;
>  
>  	vma_len = (loff_t)(vma->vm_end - vma->vm_start);
> +	len = vma_len + ((loff_t)vma->vm_pgoff << PAGE_SHIFT);
> +	/* check for overflow */
> +	if (len < vma_len)
> +		return -EINVAL;

Andrew sent this patch to Linus today, so I know it's a little too late, but
I think that getting len directly from vma like below might be a simpler fix.

  len = (loff_t)(vma->vm_end - vma->vm_start + (vma->vm_pgoff << PAGE_SHIFT)); 

This shouldn't overflow because vma->vm_{end|start|pgoff} are unsigned long,
but if worried you can add VM_BUG_ON_VMA(len < 0, vma).

Thanks,
Naoya Horiguchi

>  
>  	inode_lock(inode);
>  	file_accessed(file);
>  
>  	ret = -ENOMEM;
> -	len = vma_len + ((loff_t)vma->vm_pgoff << PAGE_SHIFT);
> -
>  	if (hugetlb_reserve_pages(inode,
>  				vma->vm_pgoff >> huge_page_order(h),
>  				len >> huge_page_shift(h), vma,
> @@ -155,7 +164,7 @@ static int hugetlbfs_file_mmap(struct file *file, struct vm_area_struct *vma)
>  
>  	ret = 0;
>  	if (vma->vm_flags & VM_WRITE && inode->i_size < len)
> -		inode->i_size = len;
> +		i_size_write(inode, len);
>  out:
>  	inode_unlock(inode);
>  
> -- 
> 2.7.4
> 
>