userfaultfd_remove() has to be execute before zapping the pagetables or
UFFDIO_COPY could keep filling pages after zap_page_range returned,
which would result in non zero data after a MADV_DONTNEED.
However userfaultfd_remove() may have to release the mmap_sem. This
was handled correctly in MADV_REMOVE, but MADV_DONTNEED accessed a
potentially stale vma (the very vma passed to zap_page_range(vma, ...)).
The fix consists in revalidating the vma in case userfaultfd_remove()
had to release the mmap_sem.
This also optimizes away an unnecessary down_read/up_read in the
MADV_REMOVE case if UFFD_EVENT_FORK had to be delivered.
It all remains zero runtime cost in case CONFIG_USERFAULTFD=n as
userfaultfd_remove() will be defined as "true" at build time.
Signed-off-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
---
fs/userfaultfd.c | 9 +++------
include/linux/userfaultfd_k.h | 7 +++----
mm/madvise.c | 44 ++++++++++++++++++++++++++++++++++++++++---
3 files changed, 47 insertions(+), 13 deletions(-)
diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 5087a69..2104811 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -694,8 +694,7 @@ void mremap_userfaultfd_complete(struct vm_userfaultfd_ctx *vm_ctx,
userfaultfd_event_wait_completion(ctx, &ewq);
}
-void userfaultfd_remove(struct vm_area_struct *vma,
- struct vm_area_struct **prev,
+bool userfaultfd_remove(struct vm_area_struct *vma,
unsigned long start, unsigned long end)
{
struct mm_struct *mm = vma->vm_mm;
@@ -704,13 +703,11 @@ void userfaultfd_remove(struct vm_area_struct *vma,
ctx = vma->vm_userfaultfd_ctx.ctx;
if (!ctx || !(ctx->features & UFFD_FEATURE_EVENT_REMOVE))
- return;
+ return true;
userfaultfd_ctx_get(ctx);
up_read(&mm->mmap_sem);
- *prev = NULL; /* We wait for ACK w/o the mmap semaphore */
-
msg_init(&ewq.msg);
ewq.msg.event = UFFD_EVENT_REMOVE;
@@ -719,7 +716,7 @@ void userfaultfd_remove(struct vm_area_struct *vma,
userfaultfd_event_wait_completion(ctx, &ewq);
- down_read(&mm->mmap_sem);
+ return false;
}
static bool has_unmap_ctx(struct userfaultfd_ctx *ctx, struct list_head *unmaps,
diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
index f2b79bf..48a3483 100644
--- a/include/linux/userfaultfd_k.h
+++ b/include/linux/userfaultfd_k.h
@@ -61,8 +61,7 @@ extern void mremap_userfaultfd_complete(struct vm_userfaultfd_ctx *,
unsigned long from, unsigned long to,
unsigned long len);
-extern void userfaultfd_remove(struct vm_area_struct *vma,
- struct vm_area_struct **prev,
+extern bool userfaultfd_remove(struct vm_area_struct *vma,
unsigned long start,
unsigned long end);
@@ -118,11 +117,11 @@ static inline void mremap_userfaultfd_complete(struct vm_userfaultfd_ctx *ctx,
{
}
-static inline void userfaultfd_remove(struct vm_area_struct *vma,
- struct vm_area_struct **prev,
+static inline bool userfaultfd_remove(struct vm_area_struct *vma,
unsigned long start,
unsigned long end)
{
+ return true;
}
static inline int userfaultfd_unmap_prep(struct vm_area_struct *vma,
diff --git a/mm/madvise.c b/mm/madvise.c
index dc5927c..7a2abf0 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -513,7 +513,43 @@ static long madvise_dontneed(struct vm_area_struct *vma,
if (!can_madv_dontneed_vma(vma))
return -EINVAL;
- userfaultfd_remove(vma, prev, start, end);
+ if (!userfaultfd_remove(vma, start, end)) {
+ *prev = NULL; /* mmap_sem has been dropped, prev is stale */
+
+ down_read(¤t->mm->mmap_sem);
+ vma = find_vma(current->mm, start);
+ if (!vma)
+ return -ENOMEM;
+ if (start < vma->vm_start) {
+ /*
+ * This "vma" under revalidation is the one
+ * with the lowest vma->vm_start where start
+ * is also < vma->vm_end. If start <
+ * vma->vm_start it means an hole materialized
+ * in the user address space within the
+ * virtual range passed to MADV_DONTNEED.
+ */
+ return -ENOMEM;
+ }
+ if (!can_madv_dontneed_vma(vma))
+ return -EINVAL;
+ if (end > vma->vm_end) {
+ /*
+ * Don't fail if end > vma->vm_end. If the old
+ * vma was splitted while the mmap_sem was
+ * released the effect of the concurrent
+ * operation may not cause MADV_DONTNEED to
+ * have an undefined result. There may be an
+ * adjacent next vma that we'll walk
+ * next. userfaultfd_remove() will generate an
+ * UFFD_EVENT_REMOVE repetition on the
+ * end-vma->vm_end range, but the manager can
+ * handle a repetition fine.
+ */
+ end = vma->vm_end;
+ }
+ VM_WARN_ON(start >= end);
+ }
zap_page_range(vma, start, end - start);
return 0;
}
@@ -554,8 +590,10 @@ static long madvise_remove(struct vm_area_struct *vma,
* mmap_sem.
*/
get_file(f);
- userfaultfd_remove(vma, prev, start, end);
- up_read(¤t->mm->mmap_sem);
+ if (userfaultfd_remove(vma, start, end)) {
+ /* mmap_sem was not released by userfaultfd_remove() */
+ up_read(¤t->mm->mmap_sem);
+ }
error = vfs_fallocate(f,
FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE,
offset, end - start);
--
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