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

John Hubbard
2019-11-12T00:09:46Z

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@ziepe.ca>
Cc: Jerome Glisse <jglisse@redhat.com>
Cc: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
---
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