Re: [PATCH RFC 06/24] userfaultfd: wp: support write protection for userfault vma range

[Mike Rapoport <rppt@linux.ibm.com>]

On Mon, Jan 21, 2019 at 03:57:04PM +0800, Peter Xu wrote:
> From: Shaohua Li <shli@fb.com>
> 
> Add API to enable/disable writeprotect a vma range. Unlike mprotect,
> this doesn't split/merge vmas.
> 
> Cc: Andrea Arcangeli <aarcange@redhat.com>
> Cc: Pavel Emelyanov <xemul@parallels.com>
> Cc: Rik van Riel <riel@redhat.com>
> Cc: Kirill A. Shutemov <kirill@shutemov.name>
> Cc: Mel Gorman <mgorman@suse.de>
> Cc: Hugh Dickins <hughd@google.com>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Signed-off-by: Shaohua Li <shli@fb.com>
> Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
>  include/linux/userfaultfd_k.h |  2 ++
>  mm/userfaultfd.c              | 52 +++++++++++++++++++++++++++++++++++
>  2 files changed, 54 insertions(+)
> 
> diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
> index 38f748e7186e..e82f3156f4e9 100644
> --- a/include/linux/userfaultfd_k.h
> +++ b/include/linux/userfaultfd_k.h
> @@ -37,6 +37,8 @@ extern ssize_t mfill_zeropage(struct mm_struct *dst_mm,
>  			      unsigned long dst_start,
>  			      unsigned long len,
>  			      bool *mmap_changing);
> +extern int mwriteprotect_range(struct mm_struct *dst_mm,
> +		unsigned long start, unsigned long len, bool enable_wp);
> 
>  /* mm helpers */
>  static inline bool is_mergeable_vm_userfaultfd_ctx(struct vm_area_struct *vma,
> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> index 458acda96f20..c38903f501c7 100644
> --- a/mm/userfaultfd.c
> +++ b/mm/userfaultfd.c
> @@ -615,3 +615,55 @@ ssize_t mfill_zeropage(struct mm_struct *dst_mm, unsigned long start,
>  {
>  	return __mcopy_atomic(dst_mm, start, 0, len, true, mmap_changing);
>  }
> +
> +int mwriteprotect_range(struct mm_struct *dst_mm, unsigned long start,
> +	unsigned long len, bool enable_wp)
> +{
> +	struct vm_area_struct *dst_vma;
> +	pgprot_t newprot;
> +	int err;
> +
> +	/*
> +	 * Sanitize the command parameters:
> +	 */
> +	BUG_ON(start & ~PAGE_MASK);
> +	BUG_ON(len & ~PAGE_MASK);
> +
> +	/* Does the address range wrap, or is the span zero-sized? */
> +	BUG_ON(start + len <= start);
> +
> +	down_read(&dst_mm->mmap_sem);
> +
> +	/*
> +	 * Make sure the vma is not shared, that the dst range is
> +	 * both valid and fully within a single existing vma.
> +	 */
> +	err = -EINVAL;

In non-cooperative mode, there can be a race between VM layout changes and
mcopy_atomic [1]. I believe the same races are possible here, so can we
please make err = -ENOENT for consistency with mcopy?

> +	dst_vma = find_vma(dst_mm, start);
> +	if (!dst_vma || (dst_vma->vm_flags & VM_SHARED))
> +		goto out_unlock;
> +	if (start < dst_vma->vm_start ||
> +	    start + len > dst_vma->vm_end)
> +		goto out_unlock;
> +
> +	if (!dst_vma->vm_userfaultfd_ctx.ctx)
> +		goto out_unlock;
> +	if (!userfaultfd_wp(dst_vma))
> +		goto out_unlock;
> +
> +	if (!vma_is_anonymous(dst_vma))
> +		goto out_unlock;

The sanity checks here seem to repeat those in mcopy_atomic(). I'd suggest
splitting them out to a helper function.

> +	if (enable_wp)
> +		newprot = vm_get_page_prot(dst_vma->vm_flags & ~(VM_WRITE));
> +	else
> +		newprot = vm_get_page_prot(dst_vma->vm_flags);
> +
> +	change_protection(dst_vma, start, start + len, newprot,
> +				!enable_wp, 0);
> +
> +	err = 0;
> +out_unlock:
> +	up_read(&dst_mm->mmap_sem);
> +	return err;
> +}
> -- 
> 2.17.1

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=27d02568f529e908399514dfbee8ee43bdfd5299

-- 
Sincerely yours,
Mike.