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-return... and later at https://ozlabs.org/~akpm/mmotm/broken-out/mm-fix-invalid-page-pointer-return...

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------------------------------------------------------ From: Peter Xu peterx@redhat.com 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@redhat.com Fixes: 3faa52c03f44 ("mm/gup: track FOLL_PIN pages") Signed-off-by: Peter Xu peterx@redhat.com Reported-by: Alex Williamson alex.williamson@redhat.com Debugged-by: Alex Williamson alex.williamson@redhat.com Cc: Andrea Arcangeli aarcange@redhat.com Cc: Andrew Morton akpm@linux-foundation.org Cc: Jan Kara jack@suse.cz Cc: Jérôme Glisse jglisse@redhat.com Cc: John Hubbard jhubbard@nvidia.com Cc: Kirill A. Shutemov kirill.shutemov@linux.intel.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

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@redhat.com are

mm-fix-invalid-page-pointer-returned-with-foll_pin-gups.patch