Re: [PATCH 3/6] mremap: Don't allow MREMAP_DONTUNMAP on special_mappings and aio

[Brian Geffon]

I don't think this situation can ever happen MREMAP_DONTUNMAP is
already restricted to anonymous mappings (defined as not having
vm_ops) and vma_to_resize checks that the mapping is anonymous before
move_vma is called.



On Mon, Oct 12, 2020 at 6:34 PM Dmitry Safonov <dima@xxxxxxxxxx> wrote:
>
> As kernel expect to see only one of such mappings, any further
> operations on the VMA-copy may be unexpected by the kernel.
> Maybe it's being on the safe side, but there doesn't seem to be any
> expected use-case for this, so restrict it now.
>
> Fixes: commit e346b3813067 ("mm/mremap: add MREMAP_DONTUNMAP to mremap()")
> Signed-off-by: Dmitry Safonov <dima@xxxxxxxxxx>
> ---
>  arch/x86/kernel/cpu/resctrl/pseudo_lock.c | 2 +-
>  fs/aio.c                                  | 5 ++++-
>  include/linux/mm.h                        | 2 +-
>  mm/mmap.c                                 | 6 +++++-
>  mm/mremap.c                               | 2 +-
>  5 files changed, 12 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/kernel/cpu/resctrl/pseudo_lock.c b/arch/x86/kernel/cpu/resctrl/pseudo_lock.c
> index 0daf2f1cf7a8..e916646adc69 100644
> --- a/arch/x86/kernel/cpu/resctrl/pseudo_lock.c
> +++ b/arch/x86/kernel/cpu/resctrl/pseudo_lock.c
> @@ -1458,7 +1458,7 @@ static int pseudo_lock_dev_release(struct inode *inode, struct file *filp)
>         return 0;
>  }
>
> -static int pseudo_lock_dev_mremap(struct vm_area_struct *area)
> +static int pseudo_lock_dev_mremap(struct vm_area_struct *area, unsigned long flags)
>  {
>         /* Not supported */
>         return -EINVAL;
> diff --git a/fs/aio.c b/fs/aio.c
> index d5ec30385566..3be3c0f77548 100644
> --- a/fs/aio.c
> +++ b/fs/aio.c
> @@ -324,13 +324,16 @@ static void aio_free_ring(struct kioctx *ctx)
>         }
>  }
>
> -static int aio_ring_mremap(struct vm_area_struct *vma)
> +static int aio_ring_mremap(struct vm_area_struct *vma, unsigned long flags)
>  {
>         struct file *file = vma->vm_file;
>         struct mm_struct *mm = vma->vm_mm;
>         struct kioctx_table *table;
>         int i, res = -EINVAL;
>
> +       if (flags & MREMAP_DONTUNMAP)
> +               return -EINVAL;
> +
>         spin_lock(&mm->ioctx_lock);
>         rcu_read_lock();
>         table = rcu_dereference(mm->ioctx_table);
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 16b799a0522c..fd51a4a1f722 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -550,7 +550,7 @@ struct vm_operations_struct {
>         void (*open)(struct vm_area_struct * area);
>         void (*close)(struct vm_area_struct * area);
>         int (*split)(struct vm_area_struct * area, unsigned long addr);
> -       int (*mremap)(struct vm_area_struct * area);
> +       int (*mremap)(struct vm_area_struct *area, unsigned long flags);
>         vm_fault_t (*fault)(struct vm_fault *vmf);
>         vm_fault_t (*huge_fault)(struct vm_fault *vmf,
>                         enum page_entry_size pe_size);
> diff --git a/mm/mmap.c b/mm/mmap.c
> index bdd19f5b994e..50f853b0ec39 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -3372,10 +3372,14 @@ static const char *special_mapping_name(struct vm_area_struct *vma)
>         return ((struct vm_special_mapping *)vma->vm_private_data)->name;
>  }
>
> -static int special_mapping_mremap(struct vm_area_struct *new_vma)
> +static int special_mapping_mremap(struct vm_area_struct *new_vma,
> +                                 unsigned long flags)
>  {
>         struct vm_special_mapping *sm = new_vma->vm_private_data;
>
> +       if (flags & MREMAP_DONTUNMAP)
> +               return -EINVAL;
> +
>         if (WARN_ON_ONCE(current->mm != new_vma->vm_mm))
>                 return -EFAULT;
>
> diff --git a/mm/mremap.c b/mm/mremap.c
> index c248f9a52125..898e9818ba6d 100644
> --- a/mm/mremap.c
> +++ b/mm/mremap.c
> @@ -384,7 +384,7 @@ static unsigned long move_vma(struct vm_area_struct *vma,
>         if (moved_len < old_len) {
>                 err = -ENOMEM;
>         } else if (vma->vm_ops && vma->vm_ops->mremap) {
> -               err = vma->vm_ops->mremap(new_vma);
> +               err = vma->vm_ops->mremap(new_vma, flags);
>         }
>
>         if (unlikely(err)) {
> --
> 2.28.0
>