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.
--
Mike Kravetz
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