[022/227] mm: fix invalid page pointer returned with FOLL_PIN gups

Commit Message

Andrew Morton March 22, 2022, 9:39 p.m. UTC

From: Peter Xu <[e-mail]>
Subject: mm: fix invalid page pointer returned with FOLL_PIN gups

Patch series "mm/gup: some cleanups", v5.


This patch (of 5):

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").

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.

[[e-mail]: added some tags, removed a reference to an out of tree module.]
Link: https://lkml.kernel.org/r/[e-mail]
Link: https://lkml.kernel.org/r/[e-mail]
Link: https://lkml.kernel.org/r/[e-mail]
Fixes: 3faa52c03f44 ("mm/gup: track FOLL_PIN pages")
Signed-off-by: Peter Xu <[e-mail]>
Signed-off-by: John Hubbard <[e-mail]>
Reviewed-by: Claudio Imbrenda <[e-mail]>
Reported-by: Alex Williamson <[e-mail]>
Debugged-by: Alex Williamson <[e-mail]>
Tested-by: Alex Williamson <[e-mail]>
Reviewed-by: Christoph Hellwig <[e-mail]>
Reviewed-by: Jan Kara <[e-mail]>
Cc: Andrea Arcangeli <[e-mail]>
Cc: Kirill A. Shutemov <[e-mail]>
Cc: Jason Gunthorpe <[e-mail]>
Cc: David Hildenbrand <[e-mail]>
Cc: Lukas Bulwahn <[e-mail]>
Cc: Matthew Wilcox (Oracle) <[e-mail]>
Cc: Jason Gunthorpe <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
---

 mm/gup.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Patch

--- a/mm/gup.c~mm-fix-invalid-page-pointer-returned-with-foll_pin-gups
+++ a/mm/gup.c
@@ -465,7 +465,7 @@  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) {