[PATCH v3 08/23] vfio, mm: fix get_user_pages_remote() and FOLL_LONGTERM

[John Hubbard]

As it says in the updated comment in gup.c: current FOLL_LONGTERM
behavior is incompatible with FAULT_FLAG_ALLOW_RETRY because of the
FS DAX check requirement on vmas.

However, the corresponding restriction in get_user_pages_remote() was
slightly stricter than is actually required: it forbade all
FOLL_LONGTERM callers, but we can actually allow FOLL_LONGTERM callers
that do not set the "locked" arg.

Update the code and comments accordingly, and update the VFIO caller
to take advantage of this, fixing a bug as a result: the VFIO caller
is logically a FOLL_LONGTERM user.

Thanks to Jason Gunthorpe for pointing out a clean way to fix this.

Suggested-by: Jason Gunthorpe <jgg@xxxxxxxx>
Cc: Jerome Glisse <jglisse@xxxxxxxxxx>
Cc: Ira Weiny <ira.weiny@xxxxxxxxx>
Signed-off-by: John Hubbard <jhubbard@xxxxxxxxxx>
---
 drivers/vfio/vfio_iommu_type1.c | 30 +++++++++++++-----------------
 mm/gup.c                        | 13 ++++++++-----
 2 files changed, 21 insertions(+), 22 deletions(-)

diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
index d864277ea16f..017689b7c32b 100644
--- a/drivers/vfio/vfio_iommu_type1.c
+++ b/drivers/vfio/vfio_iommu_type1.c
@@ -348,24 +348,20 @@ static int vaddr_get_pfn(struct mm_struct *mm, unsigned long vaddr,
 		flags |= FOLL_WRITE;
 
 	down_read(&mm->mmap_sem);
-	if (mm == current->mm) {
-		ret = get_user_pages(vaddr, 1, flags | FOLL_LONGTERM, page,
-				     vmas);
-	} else {
-		ret = get_user_pages_remote(NULL, mm, vaddr, 1, flags, page,
-					    vmas, NULL);
-		/*
-		 * The lifetime of a vaddr_get_pfn() page pin is
-		 * userspace-controlled. In the fs-dax case this could
-		 * lead to indefinite stalls in filesystem operations.
-		 * Disallow attempts to pin fs-dax pages via this
-		 * interface.
-		 */
-		if (ret > 0 && vma_is_fsdax(vmas[0])) {
-			ret = -EOPNOTSUPP;
-			put_page(page[0]);
-		}
+	ret = get_user_pages_remote(NULL, mm, vaddr, 1, flags | FOLL_LONGTERM,
+				    page, vmas, NULL);
+	/*
+	 * The lifetime of a vaddr_get_pfn() page pin is
+	 * userspace-controlled. In the fs-dax case this could
+	 * lead to indefinite stalls in filesystem operations.
+	 * Disallow attempts to pin fs-dax pages via this
+	 * interface.
+	 */
+	if (ret > 0 && vma_is_fsdax(vmas[0])) {
+		ret = -EOPNOTSUPP;
+		put_page(page[0]);
 	}
+
 	up_read(&mm->mmap_sem);
 
 	if (ret == 1) {
diff --git a/mm/gup.c b/mm/gup.c
index 933524de6249..cfe6dc5fc343 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1167,13 +1167,16 @@ long get_user_pages_remote(struct task_struct *tsk, struct mm_struct *mm,
 		struct vm_area_struct **vmas, int *locked)
 {
 	/*
-	 * FIXME: Current FOLL_LONGTERM behavior is incompatible with
+	 * Current FOLL_LONGTERM behavior is incompatible with
 	 * FAULT_FLAG_ALLOW_RETRY because of the FS DAX check requirement on
-	 * vmas.  As there are no users of this flag in this call we simply
-	 * disallow this option for now.
+	 * vmas. However, this only comes up if locked is set, and there are
+	 * callers that do request FOLL_LONGTERM, but do not set locked. So,
+	 * allow what we can.
 	 */
-	if (WARN_ON_ONCE(gup_flags & FOLL_LONGTERM))
-		return -EINVAL;
+	if (gup_flags & FOLL_LONGTERM) {
+		if (WARN_ON_ONCE(locked))
+			return -EINVAL;
+	}
 
 	return __get_user_pages_locked(tsk, mm, start, nr_pages, pages, vmas,
 				       locked,
-- 
2.24.0