Re: [PATCH v2 21/26] userfaultfd: wp: add the writeprotect API to userfaultfd ioctl

[Peter Xu]

On Mon, Feb 25, 2019 at 11:03:51PM +0200, Mike Rapoport wrote:
> On Tue, Feb 12, 2019 at 10:56:27AM +0800, Peter Xu wrote:
> > From: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> > 
> > v1: From: Shaohua Li <shli@xxxxxx>
> > 
> > v2: cleanups, remove a branch.
> > 
> > [peterx writes up the commit message, as below...]
> > 
> > This patch introduces the new uffd-wp APIs for userspace.
> > 
> > Firstly, we'll allow to do UFFDIO_REGISTER with write protection
> > tracking using the new UFFDIO_REGISTER_MODE_WP flag.  Note that this
> > flag can co-exist with the existing UFFDIO_REGISTER_MODE_MISSING, in
> > which case the userspace program can not only resolve missing page
> > faults, and at the same time tracking page data changes along the way.
> > 
> > Secondly, we introduced the new UFFDIO_WRITEPROTECT API to do page
> > level write protection tracking.  Note that we will need to register
> > the memory region with UFFDIO_REGISTER_MODE_WP before that.
> > 
> > Signed-off-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> > [peterx: remove useless block, write commit message, check against
> >  VM_MAYWRITE rather than VM_WRITE when register]
> > Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
> > ---
> >  fs/userfaultfd.c                 | 82 +++++++++++++++++++++++++-------
> >  include/uapi/linux/userfaultfd.h | 11 +++++
> >  2 files changed, 77 insertions(+), 16 deletions(-)
> > 
> > diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> > index 3092885c9d2c..81962d62520c 100644
> > --- a/fs/userfaultfd.c
> > +++ b/fs/userfaultfd.c
> > @@ -304,8 +304,11 @@ static inline bool userfaultfd_must_wait(struct userfaultfd_ctx *ctx,
> >  	if (!pmd_present(_pmd))
> >  		goto out;
> > 
> > -	if (pmd_trans_huge(_pmd))
> > +	if (pmd_trans_huge(_pmd)) {
> > +		if (!pmd_write(_pmd) && (reason & VM_UFFD_WP))
> > +			ret = true;
> >  		goto out;
> > +	}
> > 
> >  	/*
> >  	 * the pmd is stable (as in !pmd_trans_unstable) so we can re-read it
> > @@ -318,6 +321,8 @@ static inline bool userfaultfd_must_wait(struct userfaultfd_ctx *ctx,
> >  	 */
> >  	if (pte_none(*pte))
> >  		ret = true;
> > +	if (!pte_write(*pte) && (reason & VM_UFFD_WP))
> > +		ret = true;
> >  	pte_unmap(pte);
> > 
> >  out:
> > @@ -1251,10 +1256,13 @@ static __always_inline int validate_range(struct mm_struct *mm,
> >  	return 0;
> >  }
> > 
> > -static inline bool vma_can_userfault(struct vm_area_struct *vma)
> > +static inline bool vma_can_userfault(struct vm_area_struct *vma,
> > +				     unsigned long vm_flags)
> >  {
> > -	return vma_is_anonymous(vma) || is_vm_hugetlb_page(vma) ||
> > -		vma_is_shmem(vma);
> > +	/* FIXME: add WP support to hugetlbfs and shmem */
> > +	return vma_is_anonymous(vma) ||
> > +		((is_vm_hugetlb_page(vma) || vma_is_shmem(vma)) &&
> > +		 !(vm_flags & VM_UFFD_WP));
> >  }
> > 
> >  static int userfaultfd_register(struct userfaultfd_ctx *ctx,
> > @@ -1286,15 +1294,8 @@ static int userfaultfd_register(struct userfaultfd_ctx *ctx,
> >  	vm_flags = 0;
> >  	if (uffdio_register.mode & UFFDIO_REGISTER_MODE_MISSING)
> >  		vm_flags |= VM_UFFD_MISSING;
> > -	if (uffdio_register.mode & UFFDIO_REGISTER_MODE_WP) {
> > +	if (uffdio_register.mode & UFFDIO_REGISTER_MODE_WP)
> >  		vm_flags |= VM_UFFD_WP;
> > -		/*
> > -		 * FIXME: remove the below error constraint by
> > -		 * implementing the wprotect tracking mode.
> > -		 */
> > -		ret = -EINVAL;
> > -		goto out;
> > -	}
> > 
> >  	ret = validate_range(mm, uffdio_register.range.start,
> >  			     uffdio_register.range.len);
> > @@ -1342,7 +1343,7 @@ static int userfaultfd_register(struct userfaultfd_ctx *ctx,
> > 
> >  		/* check not compatible vmas */
> >  		ret = -EINVAL;
> > -		if (!vma_can_userfault(cur))
> > +		if (!vma_can_userfault(cur, vm_flags))
> >  			goto out_unlock;
> > 
> >  		/*
> > @@ -1370,6 +1371,8 @@ static int userfaultfd_register(struct userfaultfd_ctx *ctx,
> >  			if (end & (vma_hpagesize - 1))
> >  				goto out_unlock;
> >  		}
> > +		if ((vm_flags & VM_UFFD_WP) && !(cur->vm_flags & VM_MAYWRITE))
> > +			goto out_unlock;
> > 
> >  		/*
> >  		 * Check that this vma isn't already owned by a
> > @@ -1399,7 +1402,7 @@ static int userfaultfd_register(struct userfaultfd_ctx *ctx,
> >  	do {
> >  		cond_resched();
> > 
> > -		BUG_ON(!vma_can_userfault(vma));
> > +		BUG_ON(!vma_can_userfault(vma, vm_flags));
> >  		BUG_ON(vma->vm_userfaultfd_ctx.ctx &&
> >  		       vma->vm_userfaultfd_ctx.ctx != ctx);
> >  		WARN_ON(!(vma->vm_flags & VM_MAYWRITE));
> > @@ -1534,7 +1537,7 @@ static int userfaultfd_unregister(struct userfaultfd_ctx *ctx,
> >  		 * provides for more strict behavior to notice
> >  		 * unregistration errors.
> >  		 */
> > -		if (!vma_can_userfault(cur))
> > +		if (!vma_can_userfault(cur, cur->vm_flags))
> >  			goto out_unlock;
> > 
> >  		found = true;
> > @@ -1548,7 +1551,7 @@ static int userfaultfd_unregister(struct userfaultfd_ctx *ctx,
> >  	do {
> >  		cond_resched();
> > 
> > -		BUG_ON(!vma_can_userfault(vma));
> > +		BUG_ON(!vma_can_userfault(vma, vma->vm_flags));
> > 
> >  		/*
> >  		 * Nothing to do: this vma is already registered into this
> > @@ -1761,6 +1764,50 @@ static int userfaultfd_zeropage(struct userfaultfd_ctx *ctx,
> >  	return ret;
> >  }
> > 
> > +static int userfaultfd_writeprotect(struct userfaultfd_ctx *ctx,
> > +				    unsigned long arg)
> > +{
> > +	int ret;
> > +	struct uffdio_writeprotect uffdio_wp;
> > +	struct uffdio_writeprotect __user *user_uffdio_wp;
> > +	struct userfaultfd_wake_range range;
> > +
> > +	if (READ_ONCE(ctx->mmap_changing))
> > +		return -EAGAIN;
> > +
> > +	user_uffdio_wp = (struct uffdio_writeprotect __user *) arg;
> > +
> > +	if (copy_from_user(&uffdio_wp, user_uffdio_wp,
> > +			   sizeof(struct uffdio_writeprotect)))
> > +		return -EFAULT;
> > +
> > +	ret = validate_range(ctx->mm, uffdio_wp.range.start,
> > +			     uffdio_wp.range.len);
> > +	if (ret)
> > +		return ret;
> > +
> > +	if (uffdio_wp.mode & ~(UFFDIO_WRITEPROTECT_MODE_DONTWAKE |
> > +			       UFFDIO_WRITEPROTECT_MODE_WP))
> > +		return -EINVAL;
> > +	if ((uffdio_wp.mode & UFFDIO_WRITEPROTECT_MODE_WP) &&
> > +	     (uffdio_wp.mode & UFFDIO_WRITEPROTECT_MODE_DONTWAKE))
> > +		return -EINVAL;
> 
> Why _DONTWAKE cannot be used when setting write-protection?
> I can imagine a use-case when you'd want to freeze an application,
> write-protect several regions and then let the application continue.

This is the same question as the one in the other thread, which I've
had a longer reply there, hope it could be a bit clearer (sorry for
the confusion no matter what!).  I would be more than glad to know if
there could be any smarter way to define/renaming/... the flags.

Thanks!

-- 
Peter Xu