Re: [PATCH v4 08/23] vfio, mm: fix get_user_pages_remote() and FOLL_LONGTERM

On Tue, Nov 12, 2019 at 08:26:55PM -0800, John Hubbard wrote:

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.

Also, remove an unnessary pair of calls that were releasing and reacquiring the mmap_sem. There is no need to avoid holding mmap_sem just in order to call page_to_pfn().

Also, move the DAX check ("if a VMA is DAX, don't allow long term pinning") from the VFIO call site, all the way into the internals of get_user_pages_remote() and __gup_longterm_locked(). That is: get_user_pages_remote() calls __gup_longterm_locked(), which in turn calls check_dax_vmas(). It's lightly explained in the comments as well.

Thanks to Jason Gunthorpe for pointing out a clean way to fix this, and to Dan Williams for helping clarify the DAX refactoring.

Suggested-by: Jason Gunthorpe jgg@ziepe.ca Cc: Dan Williams dan.j.williams@intel.com Cc: Jerome Glisse jglisse@redhat.com Cc: Ira Weiny ira.weiny@intel.com

Reviewed-by: Ira Weiny ira.weiny@intel.com

Signed-off-by: John Hubbard jhubbard@nvidia.com

drivers/vfio/vfio_iommu_type1.c | 25 ++----------------------- mm/gup.c | 27 ++++++++++++++++++++++----- 2 files changed, 24 insertions(+), 28 deletions(-)

diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c index d864277ea16f..7301b710c9a4 100644 --- a/drivers/vfio/vfio_iommu_type1.c +++ b/drivers/vfio/vfio_iommu_type1.c @@ -340,7 +340,6 @@ static int vaddr_get_pfn(struct mm_struct *mm, unsigned long vaddr, { struct page *page[1]; struct vm_area_struct *vma;

• struct vm_area_struct *vmas[1]; unsigned int flags = 0; int ret;

@@ -348,33 +347,13 @@ 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]);
  

• }
  

• }
• up_read(&mm->mmap_sem);

• ret = get_user_pages_remote(NULL, mm, vaddr, 1, flags | FOLL_LONGTERM,

• 		    page, NULL, NULL);
  

  if (ret == 1) { *pfn = page_to_pfn(page[0]); return 0; }

• down_read(&mm->mmap_sem);
• vaddr = untagged_addr(vaddr);

vma = find_vma_intersection(mm, vaddr, vaddr + 1); diff --git a/mm/gup.c b/mm/gup.c index 933524de6249..83702b2e86c8 100644 --- a/mm/gup.c +++ b/mm/gup.c @@ -29,6 +29,13 @@ struct follow_page_context { unsigned int page_mask; }; +static __always_inline long __gup_longterm_locked(struct task_struct *tsk,

• 				  struct mm_struct *mm,
  

• 				  unsigned long start,
  

• 				  unsigned long nr_pages,
  

• 				  struct page **pages,
  

• 				  struct vm_area_struct **vmas,
  

• 				  unsigned int flags);
  

/*

• Return the compound head page with ref appropriately incremented,
• or NULL if that failed.

@@ -1167,13 +1174,23 @@ 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
  

• * Parts of FOLL_LONGTERM behavior are 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;
  

• /*
  

•  * This will check the vmas (even if our vmas arg is NULL)
  

•  * and return -ENOTSUPP if DAX isn't allowed in this case:
  

•  */
  

• return __gup_longterm_locked(tsk, mm, start, nr_pages, pages,
  

• 			     vmas, gup_flags | FOLL_TOUCH |
  

• 			     FOLL_REMOTE);
  

• }

return __get_user_pages_locked(tsk, mm, start, nr_pages, pages, vmas, locked, -- 2.24.0