[patch 045/166] userfaultfd: wp: add the writeprotect API to userfaultfd ioctl

Andrew Morton
2020-04-07T03:06:16Z

From: Andrea Arcangeli <aarcange@redhat.com>
Subject: userfaultfd: wp: add the writeprotect API to userfaultfd ioctl

Introduce 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.

[peterx@redhat.com: write up the commit message]
[peterx@redhat.com: remove useless block, write commit message, check against
VM_MAYWRITE rather than VM_WRITE when register]
Link: http://lkml.kernel.org/r/20200220163112.11409-14-peterx@redhat.com
Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Jerome Glisse <jglisse@redhat.com>
Cc: Bobby Powers <bobbypowers@gmail.com>
Cc: Brian Geffon <bgeffon@google.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Denis Plotnikov <dplotnikov@virtuozzo.com>
Cc: "Dr . David Alan Gilbert" <dgilbert@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: "Kirill A . Shutemov" <kirill@shutemov.name>
Cc: Martin Cracauer <cracauer@cons.org>
Cc: Marty McFadden <mcfadden8@llnl.gov>
Cc: Maya Gokhale <gokhale2@llnl.gov>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: Pavel Emelyanov <xemul@openvz.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: Shaohua Li <shli@fb.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

fs/userfaultfd.c | 82 +++++++++++++++++++++++------
include/uapi/linux/userfaultfd.h | 23 ++++++++
2 files changed, 89 insertions(+), 16 deletions(-)

--- a/fs/userfaultfd.c~userfaultfd-wp-add-the-writeprotect-api-to-userfaultfd-ioctl
+++ a/fs/userfaultfd.c
@@ -314,8 +314,11 @@ static inline bool userfaultfd_must_wait
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
@@ -328,6 +331,8 @@ static inline bool userfaultfd_must_wait
*/
if (pte_none(*pte))
ret = true;
+ if (!pte_write(*pte) && (reason & VM_UFFD_WP))
+ ret = true;
pte_unmap(pte);

out:
@@ -1287,10 +1292,13 @@ static __always_inline int validate_rang
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,
@@ -1322,15 +1330,8 @@ static int userfaultfd_register(struct u
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);
@@ -1380,7 +1381,7 @@ static int userfaultfd_register(struct u

/* check not compatible vmas */
ret = -EINVAL;
- if (!vma_can_userfault(cur))
+ if (!vma_can_userfault(cur, vm_flags))
goto out_unlock;

/*
@@ -1408,6 +1409,8 @@ static int userfaultfd_register(struct u
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
@@ -1437,7 +1440,7 @@ static int userfaultfd_register(struct u
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));
@@ -1575,7 +1578,7 @@ static int userfaultfd_unregister(struct
* 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;
@@ -1589,7 +1592,7 @@ static int userfaultfd_unregister(struct
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
@@ -1802,6 +1805,50 @@ out:
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;
+
+ 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)
{
/*
@@ -1883,6 +1930,9 @@ static long userfaultfd_ioctl(struct fil
case UFFDIO_ZEROPAGE:
ret = userfaultfd_zeropage(ctx, arg);
break;
+ case UFFDIO_WRITEPROTECT:
+ ret = userfaultfd_writeprotect(ctx, arg);
+ break;
}
return ret;
}
--- a/include/uapi/linux/userfaultfd.h~userfaultfd-wp-add-the-writeprotect-api-to-userfaultfd-ioctl
+++ a/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 +235,24 @@ struct uffdio_zeropage {
__s64 zeropage;
};

+struct uffdio_writeprotect {
+ struct uffdio_range range;
+/*
+ * UFFDIO_WRITEPROTECT_MODE_WP: set the flag to write protect a range,
+ * unset the flag to undo protection of a range which was previously
+ * write protected.
+ *
+ * UFFDIO_WRITEPROTECT_MODE_DONTWAKE: set the flag to avoid waking up
+ * any wait thread after the operation succeeds.
+ *
+ * NOTE: Write protecting a region (WP=1) is unrelated to page faults,
+ * therefore DONTWAKE flag is meaningless with WP=1. Removing write
+ * protection (WP=0) in response to a page fault wakes the faulting
+ * task unless DONTWAKE is set.
+ */
+#define UFFDIO_WRITEPROTECT_MODE_WP ((__u64)1<<0)
+#define UFFDIO_WRITEPROTECT_MODE_DONTWAKE ((__u64)1<<1)
+ __u64 mode;
+};
+
#endif /* _LINUX_USERFAULTFD_H */
_