The patch titled
Subject: mm/gup.c: fix invalid page pointer returned with FOLL_PIN gups
has been added to the -mm tree. Its filename is
mm-fix-invalid-page-pointer-returned-with-foll_pin-gups.patch
This patch should soon appear at
https://ozlabs.org/~akpm/mmots/broken-out/mm-fix-invalid-page-pointer-returned-with-foll_pin-gups.patch
and later at
https://ozlabs.org/~akpm/mmotm/broken-out/mm-fix-invalid-page-pointer-returned-with-foll_pin-gups.patch
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------------------------------------------------------
From: Peter Xu <peterx@xxxxxxxxxx>
Subject: mm/gup.c: fix invalid page pointer returned with FOLL_PIN gups
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.
Since at it, add another WARN_ON_ONCE() at the -EEXIST handling to make
sure we mustn't have **pages set when reaching there, because otherwise it
means the caller will try to read a garbage right after __get_user_pages()
returns.
Attaching the Fixes to the FOLL_PIN rework commit, as it happened later
than 1027e4436b6a.
Link: https://lkml.kernel.org/r/20220125033700.69705-1-peterx@xxxxxxxxxx
Fixes: 3faa52c03f44 ("mm/gup: track FOLL_PIN pages")
Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
Reported-by: Alex Williamson <alex.williamson@xxxxxxxxxx>
Debugged-by: Alex Williamson <alex.williamson@xxxxxxxxxx>
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>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/gup.c | 8 +++++++-
1 file changed, 7 insertions(+), 1 deletion(-)
--- a/mm/gup.c~mm-fix-invalid-page-pointer-returned-with-foll_pin-gups
+++ a/mm/gup.c
@@ -440,7 +440,7 @@ static int follow_pfn_pte(struct vm_area
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) {
@@ -1181,7 +1181,13 @@ retry:
/*
* Proper page table entry exists, but no corresponding
* struct page.
+ *
+ * Warn if we jumped over even with a valid **pages.
+ * It shouldn't trigger in practise, but when there's
+ * buggy returns on -EEXIST we'll warn before returning
+ * an invalid page pointer in the array.
*/
+ WARN_ON_ONCE(pages);
goto next_page;
} else if (IS_ERR(page)) {
ret = PTR_ERR(page);
_
Patches currently in -mm which might be from peterx@xxxxxxxxxx are
mm-fix-invalid-page-pointer-returned-with-foll_pin-gups.patch
