Re: [PATCH] mm: Fix invalid page pointer returned with FOLL_PIN gups

[John Hubbard <jhubbard@nvidia.com>]

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);