Messages in this thread | | | Date | Wed, 26 Jan 2022 16:15:02 -0800 | Subject | Re: [PATCH] mm: Fix invalid page pointer returned with FOLL_PIN gups | From | John Hubbard <> |
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On 1/24/22 19:37, Peter Xu wrote: > 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. > > 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> > Fixes: 3faa52c03f44 ("mm/gup: track FOLL_PIN pages") > Reported-by: Alex Williamson <alex.williamson@redhat.com> > Debugged-by: Alex Williamson <alex.williamson@redhat.com> > Signed-off-by: Peter Xu <peterx@redhat.com> > --- > mm/gup.c | 8 +++++++- > 1 file changed, 7 insertions(+), 1 deletion(-) > > diff --git a/mm/gup.c b/mm/gup.c > index f0af462ac1e2..8ebc04058e97 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;
Yes. This clearly fixes the problem that the patch describes, and also clearly matches up with the Fixes tag. So that's correct.
> > if (flags & FOLL_TOUCH) { > @@ -1181,7 +1181,13 @@ static long __get_user_pages(struct mm_struct *mm, > /* > * 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);
Here, however, I think we need to consider this a little more carefully, and attempt to actually fix up this case. It is never going to be OK here, to return a **pages array that has these little landmines of potentially uninitialized pointers. And so continuing on *at all* seems very wrong.
Can we bail out at this point, without breaking the world? I think we can...
Also: this part, even if it remains as is, should be a separate fix and a separate patch, IMHO.
thanks, -- John Hubbard NVIDIA
> goto next_page; > } else if (IS_ERR(page)) { > ret = PTR_ERR(page);
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