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

Mike Rapoport
2019-01-21T10:20:52Z

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.