Re: [PATCH v2] mm: Fix invalid page pointer returned with FOLL_PIN gups

[John Hubbard <jhubbard@nvidia.com>]

On 1/27/22 22:19, 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.
> 
> Attaching the Fixes to the FOLL_PIN rework commit, as it happened later than
> 1027e4436b6a.
> 

Reviewed-by: John Hubbard <jhubbard@nvidia.com>


thanks,
-- 
John Hubbard
NVIDIA

> 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>
> Tested-by: Alex Williamson <alex.williamson@redhat.com>
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
> v2:
> - Drop the WARN_ON_ONCE() [Jason, John]
> - Add Alex's Tested-by too, as after dropping the WARN_ON_ONCE() then the patch
>    is exactly the one that Alex helped on bug verification, hence very safe to
>    grant the credit alongside.
> ---
>   mm/gup.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/gup.c b/mm/gup.c
> index f0af462ac1e2..65575ae3602f 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;
>   
>   	if (flags & FOLL_TOUCH) {