Re: [PATCH 2/2] mm/frame-vec: use FOLL_LONGTERM

[Jan Kara]

On Mon 05-10-20 14:38:54, Jason Gunthorpe wrote:
> When get_vaddr_frames() does its hacky follow_pfn() loop it should never
> be allowed to extract a struct page from a normal VMA. This could allow a
> serious use-after-free problem on any kernel memory.
> 
> Restrict this to only work on VMA's with one of VM_IO | VM_PFNMAP
> set. This limits the use-after-free problem to only IO memory, which while
> still serious, is an improvement.
> 
> Cc: stable@xxxxxxxxxxxxxxx
> Fixes: 8025e5ddf9c1 ("[media] mm: Provide new get_vaddr_frames() helper")
> Signed-off-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
> ---
>  mm/frame_vector.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/mm/frame_vector.c b/mm/frame_vector.c
> index 10f82d5643b6de..26cb20544b6c37 100644
> --- a/mm/frame_vector.c
> +++ b/mm/frame_vector.c
> @@ -99,6 +99,10 @@ int get_vaddr_frames(unsigned long start, unsigned int nr_frames,
>  		if (ret >= nr_frames || start < vma->vm_end)
>  			break;
>  		vma = find_vma_intersection(mm, start, start + 1);
> +		if (!(vma->vm_flags & (VM_IO | VM_PFNMAP))) {
> +			ret = -EINVAL;
> +			goto out;
> +		}
>  	} while (vma && vma->vm_flags & (VM_IO | VM_PFNMAP));

Hum, I fail to see how this helps. If vma has no VM_IO or VM_PFNMAP flag,
we'd exit the loop (to out: label) anyway due to the loop termination
condition and why not return the frames we already have? Furthermore
find_vma_intersection() can return NULL which would oops in your check
then. What am I missing?

								Honza

>  out:
>  	if (locked)
> -- 
> 2.28.0
> 
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR