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SubjectRe: [PATCH RFC 07/24] userfaultfd: wp: add the writeprotect API to userfaultfd ioctl
On Mon, Jan 21, 2019 at 03:57:05PM +0800, Peter Xu wrote:
> From: Andrea Arcangeli <aarcange@redhat.com>
>
> v1: From: Shaohua Li <shli@fb.com>
>
> 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@redhat.com>
> [peterx: remove useless block, write commit message]
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
> fs/userfaultfd.c | 78 +++++++++++++++++++++++++-------
> include/uapi/linux/userfaultfd.h | 11 +++++
> 2 files changed, 73 insertions(+), 16 deletions(-)
>
> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> index bc9f6230a3f0..6ff8773d6797 100644
> --- a/fs/userfaultfd.c
> +++ b/fs/userfaultfd.c
> @@ -305,8 +305,11 @@ 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
> @@ -319,6 +322,8 @@ 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:
> @@ -1252,10 +1257,13 @@ 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,
> @@ -1287,15 +1295,8 @@ 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);
> @@ -1343,7 +1344,7 @@ 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;
>
> /*
> @@ -1371,6 +1372,8 @@ 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_WRITE))
> + goto out_unlock;

This is problematic for the non-cooperative use-case. Way may still want to
monitor a read-only area because it may eventually become writable, e.g. if
the monitored process runs mprotect().
Particularity, for using uffd-wp as a replacement for soft-dirty would
require it.

>
> /*
> * Check that this vma isn't already owned by a
> @@ -1400,7 +1403,7 @@ 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));
> @@ -1535,7 +1538,7 @@ 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;
> @@ -1549,7 +1552,7 @@ 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));
> WARN_ON(!(vma->vm_flags & VM_MAYWRITE));
>
> /*
> @@ -1760,6 +1763,46 @@ 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;
> +

In the non-cooperative mode the userfaultfd_writeprotect() may race with VM
layout changes, pretty much as uffdio_copy() [1]. My solution for uffdio_copy()
was to return -EAGAIN if such race is encountered. I think the same would
apply here.

> + 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;
> +
> + ret = mwriteprotect_range(ctx->mm, uffdio_wp.range.start,
> + uffdio_wp.range.len, uffdio_wp.mode &
> + UFFDIO_WRITEPROTECT_MODE_WP);
> + 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)
> {
> /*
> @@ -1837,6 +1880,9 @@ 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 48f1a7c2f1f0..11517f796275 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 {
> @@ -231,4 +234,12 @@ 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

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=df2cc96e77011cf7989208b206da9817e0321028

--
Sincerely yours,
Mike.

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