Re: [PATCH resend RFC 1/9] s390/gmap: validate VMA in __gmap_zap()

[Claudio Imbrenda <imbrenda@linux.ibm.com>]

On Thu,  9 Sep 2021 18:22:40 +0200
David Hildenbrand <david@redhat.com> wrote:

> We should not walk/touch page tables outside of VMA boundaries when
> holding only the mmap sem in read mode. Evil user space can modify the
> VMA layout just before this function runs and e.g., trigger races with
> page table removal code since commit dd2283f2605e ("mm: mmap: zap pages
> with read mmap_sem in munmap"). The pure prescence in our guest_to_host
> radix tree does not imply that there is a VMA.
> 
> Further, we should not allocate page tables (via get_locked_pte()) outside
> of VMA boundaries: if evil user space decides to map hugetlbfs to these
> ranges, bad things will happen because we suddenly have PTE or PMD page
> tables where we shouldn't have them.
> 
> Similarly, we have to check if we suddenly find a hugetlbfs VMA, before
> calling get_locked_pte().
> 
> Note that gmap_discard() is different:
> zap_page_range()->unmap_single_vma() makes sure to stay within VMA
> boundaries.
> 
> Fixes: b31288fa83b2 ("s390/kvm: support collaborative memory management")
> Signed-off-by: David Hildenbrand <david@redhat.com>

Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>

> ---
>  arch/s390/mm/gmap.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/arch/s390/mm/gmap.c b/arch/s390/mm/gmap.c
> index 9bb2c7512cd5..b6b56cd4ca64 100644
> --- a/arch/s390/mm/gmap.c
> +++ b/arch/s390/mm/gmap.c
> @@ -673,6 +673,7 @@ EXPORT_SYMBOL_GPL(gmap_fault);
>   */
>  void __gmap_zap(struct gmap *gmap, unsigned long gaddr)
>  {
> +	struct vm_area_struct *vma;
>  	unsigned long vmaddr;
>  	spinlock_t *ptl;
>  	pte_t *ptep;
> @@ -682,6 +683,11 @@ void __gmap_zap(struct gmap *gmap, unsigned long gaddr)
>  						   gaddr >> PMD_SHIFT);
>  	if (vmaddr) {
>  		vmaddr |= gaddr & ~PMD_MASK;
> +
> +		vma = vma_lookup(gmap->mm, vmaddr);
> +		if (!vma || is_vm_hugetlb_page(vma))
> +			return;
> +
>  		/* Get pointer to the page table entry */
>  		ptep = get_locked_pte(gmap->mm, vmaddr, &ptl);
>  		if (likely(ptep))