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

[Naoya Horiguchi]

On Sat, Apr 15, 2017 at 03:58:59PM -0700, Mike Kravetz wrote:
> On 04/13/2017 08:32 PM, Naoya Horiguchi wrote:
> > 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,
> 
> I am pretty sure the checks are necessary.  You are correct in that
> vma->vm_{end|start|pgoff} are unsigned long.  However,  pgoff can be
> a REALLY big value that becomes negative when shifted.
> 
> Note that pgoff is simply the off_t offset value passed from the user cast
> to unsigned long and shifted right by PAGE_SHIFT.  There is nothing to
> prevent a user from passing a 'signed' negative value.  In the reproducer
> provided, the value passed from user space is 0x8000000000000000ULL.

OK, thank you for explanation. You're right.

- Naoya