Re: [PATCH] hugetlbfs: check for pgoff value overflow

[Nic Losby <blurbdust@gmail.com>]

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Confirmed as patched! The kernel no longer crashes after the patch was
applied. Thank you for your time!

On Wed, Mar 7, 2018 at 5:59 PM, Mike Kravetz <mike.kravetz@oracle.com>
wrote:

> A vma with vm_pgoff large enough to overflow a loff_t type when
> converted to a byte offset can be passed via the remap_file_pages
> system call.  The hugetlbfs mmap routine uses the byte offset to
> calculate reservations and file size.
>
> A sequence such as:
>   mmap(0x20a00000, 0x600000, 0, 0x66033, -1, 0);
>   remap_file_pages(0x20a00000, 0x600000, 0, 0x20000000000000, 0);
> will result in the following when task exits/file closed,
>   kernel BUG at mm/hugetlb.c:749!
> Call Trace:
>   hugetlbfs_evict_inode+0x2f/0x40
>   evict+0xcb/0x190
>   __dentry_kill+0xcb/0x150
>   __fput+0x164/0x1e0
>   task_work_run+0x84/0xa0
>   exit_to_usermode_loop+0x7d/0x80
>   do_syscall_64+0x18b/0x190
>   entry_SYSCALL_64_after_hwframe+0x3d/0xa2
>
> The overflowed pgoff value causes hugetlbfs to try to set up a
> mapping with a negative range (end < start) that leaves invalid
> state which causes the BUG.
>
> Reported-by: Nic Losby <blurbdust@gmail.com>
> Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
> ---
>  fs/hugetlbfs/inode.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
> index 8fe1b0aa2896..cb288dec5564 100644
> --- a/fs/hugetlbfs/inode.c
> +++ b/fs/hugetlbfs/inode.c
> @@ -127,12 +127,13 @@ static int hugetlbfs_file_mmap(struct file *file,
> struct vm_area_struct *vma)
>         vma->vm_ops = &hugetlb_vm_ops;
>
>         /*
> -        * Offset passed to mmap (before page shift) could have been
> -        * negative when represented as a (l)off_t.
> +        * page based offset in vm_pgoff could be sufficiently large to
> +        * overflow a (l)off_t when converted to byte offset.
>          */
> -       if (((loff_t)vma->vm_pgoff << PAGE_SHIFT) < 0)
> +       if (vma->vm_pgoff && ((loff_t)vma->vm_pgoff << PAGE_SHIFT) <= 0)
>                 return -EINVAL;
>
> +       /* must be huge page aligned */
>         if (vma->vm_pgoff & (~huge_page_mask(h) >> PAGE_SHIFT))
>                 return -EINVAL;
>
> --
> 2.13.6
>
>

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