[1/4] mm: Fix invalid page pointer returned with FOLL_PIN gups

Commit Message

John Hubbard Jan. 31, 2022, 5:17 a.m. UTC

From: Peter Xu <[e-mail]>

Alex reported invalid page pointer returned with pin_user_pages_remote() from
vfio after upstream commit 4b6c33b32296 ("vfio/type1: Prepare for batched
pinning with struct vfio_batch").  This problem breaks NVIDIA vfio mdev.

It turns out that it's not the fault of the vfio commit; however after vfio
switches to a full page buffer to store the page pointers it starts to expose
the problem easier.

The problem is for VM_PFNMAP vmas we should normally fail with an -EFAULT then
vfio will carry on to handle the MMIO regions.  However when the bug triggered,
follow_page_mask() returned -EEXIST for such a page, which will jump over the
current page, leaving that entry in **pages untouched.  However the caller is
not aware of it, hence the caller will reference the page as usual even if the
pointer data can be anything.

We had that -EEXIST logic since commit 1027e4436b6a ("mm: make GUP handle pfn
mapping unless FOLL_GET is requested") which seems very reasonable.  It could
be that when we reworked GUP with FOLL_PIN we could have overlooked that
special path in commit 3faa52c03f44 ("mm/gup: track FOLL_PIN pages"), even if
that commit rightfully touched up follow_devmap_pud() on checking FOLL_PIN when
it needs to return an -EEXIST.

Attaching the Fixes to the FOLL_PIN rework commit, as it happened later than
1027e4436b6a.

Cc: Andrea Arcangeli <[e-mail]>
Cc: Andrew Morton <[e-mail]>
Cc: Jan Kara <[e-mail]>
Cc: Jérôme Glisse <[e-mail]>
Cc: Kirill A. Shutemov <[e-mail]>
Fixes: 3faa52c03f44 ("mm/gup: track FOLL_PIN pages")
Reviewed-by: John Hubbard <[e-mail]>
Reported-by: Alex Williamson <[e-mail]>
Debugged-by: Alex Williamson <[e-mail]>
Tested-by: Alex Williamson <[e-mail]>
Signed-off-by: Peter Xu <[e-mail]>
---
 mm/gup.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Comments

Christoph Hellwig Feb. 2, 2022, 2:04 p.m. UTC | #1

On Sun, Jan 30, 2022 at 09:17:49PM -0800, John Hubbard wrote:
> From: Peter Xu <[e-mail]>
> 
> Alex reported invalid page pointer returned with pin_user_pages_remote() from
> vfio after upstream commit 4b6c33b32296 ("vfio/type1: Prepare for batched
> pinning with struct vfio_batch").  This problem breaks NVIDIA vfio mdev.

I haven't actually seen any nvidia mdev module in the tree.

Patch

diff --git a/mm/gup.c b/mm/gup.c
index f0af462ac1e2..65575ae3602f 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -440,7 +440,7 @@  static int follow_pfn_pte(struct vm_area_struct *vma, unsigned long address,
 		pte_t *pte, unsigned int flags)
 {
 	/* No page to get reference */
-	if (flags & FOLL_GET)
+	if (flags & (FOLL_GET | FOLL_PIN))
 		return -EFAULT;
 
 	if (flags & FOLL_TOUCH) {