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 <aarcange@xxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Jan Kara <jack@xxxxxxx>
Cc: Jérôme Glisse <jglisse@xxxxxxxxxx>
Cc: John Hubbard <jhubbard@xxxxxxxxxx>
Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Fixes: 3faa52c03f44 ("mm/gup: track FOLL_PIN pages")
Reported-by: Alex Williamson <alex.williamson@xxxxxxxxxx>
Debugged-by: Alex Williamson <alex.williamson@xxxxxxxxxx>
Tested-by: Alex Williamson <alex.williamson@xxxxxxxxxx>
Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
---
v2:
- Drop the WARN_ON_ONCE() [Jason, John]
- Add Alex's Tested-by too, as after dropping the WARN_ON_ONCE() then the patch
is exactly the one that Alex helped on bug verification, hence very safe to
grant the credit alongside.
---
mm/gup.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
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) {
--
2.32.0
