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

[Mike Rapoport]

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.

> +
> +	ret = mwriteprotect_range(ctx->mm, uffdio_wp.range.start,
> +				  uffdio_wp.range.len, uffdio_wp.mode &
> +				  UFFDIO_WRITEPROTECT_MODE_WP,
> +				  &ctx->mmap_changing);
> +	if (ret)
> +		return ret;
> +
> +	if (!(uffdio_wp.mode & UFFDIO_WRITEPROTECT_MODE_DONTWAKE)) {
> +		range.start = uffdio_wp.range.start;
> +		range.len = uffdio_wp.range.len;
> +		wake_userfault(ctx, &range);
> +	}
> +	return ret;
> +}
> +
>  static inline unsigned int uffd_ctx_features(__u64 user_features)
>  {
>  	/*
> @@ -1838,6 +1885,9 @@ static long userfaultfd_ioctl(struct file *file, unsigned cmd,
>  	case UFFDIO_ZEROPAGE:
>  		ret = userfaultfd_zeropage(ctx, arg);
>  		break;
> +	case UFFDIO_WRITEPROTECT:
> +		ret = userfaultfd_writeprotect(ctx, arg);
> +		break;
>  	}
>  	return ret;
>  }
> diff --git a/include/uapi/linux/userfaultfd.h b/include/uapi/linux/userfaultfd.h
> index 297cb044c03f..1b977a7a4435 100644
> --- a/include/uapi/linux/userfaultfd.h
> +++ b/include/uapi/linux/userfaultfd.h
> @@ -52,6 +52,7 @@
>  #define _UFFDIO_WAKE			(0x02)
>  #define _UFFDIO_COPY			(0x03)
>  #define _UFFDIO_ZEROPAGE		(0x04)
> +#define _UFFDIO_WRITEPROTECT		(0x06)
>  #define _UFFDIO_API			(0x3F)
> 
>  /* userfaultfd ioctl ids */
> @@ -68,6 +69,8 @@
>  				      struct uffdio_copy)
>  #define UFFDIO_ZEROPAGE		_IOWR(UFFDIO, _UFFDIO_ZEROPAGE,	\
>  				      struct uffdio_zeropage)
> +#define UFFDIO_WRITEPROTECT	_IOWR(UFFDIO, _UFFDIO_WRITEPROTECT, \
> +				      struct uffdio_writeprotect)
> 
>  /* read() structure */
>  struct uffd_msg {
> @@ -232,4 +235,12 @@ struct uffdio_zeropage {
>  	__s64 zeropage;
>  };
> 
> +struct uffdio_writeprotect {
> +	struct uffdio_range range;
> +	/* !WP means undo writeprotect. DONTWAKE is valid only with !WP */
> +#define UFFDIO_WRITEPROTECT_MODE_WP		((__u64)1<<0)
> +#define UFFDIO_WRITEPROTECT_MODE_DONTWAKE	((__u64)1<<1)
> +	__u64 mode;
> +};
> +
>  #endif /* _LINUX_USERFAULTFD_H */
> -- 
> 2.17.1
> 

-- 
Sincerely yours,
Mike.