Re: [PATCH 1/2] mm: Fix vma_is_anonymous() false-positives

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 10 Jul 2018 16:48:20 +0300 "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> wrote:

> vma_is_anonymous() relies on ->vm_ops being NULL to detect anonymous
> VMA. This is unreliable as ->mmap may not set ->vm_ops.
> 
> False-positive vma_is_anonymous() may lead to crashes:
> 
> ...
> 
> This can be fixed by assigning anonymous VMAs own vm_ops and not relying
> on it being NULL.
> 
> If ->mmap() failed to set ->vm_ops, mmap_region() will set it to
> dummy_vm_ops. This way we will have non-NULL ->vm_ops for all VMAs.

Is there a smaller, simpler fix which we can use for backporting
purposes and save the larger rework for development kernels?

>
> ...
>
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -71,6 +71,9 @@ int mmap_rnd_compat_bits __read_mostly = CONFIG_ARCH_MMAP_RND_COMPAT_BITS;
>  static bool ignore_rlimit_data;
>  core_param(ignore_rlimit_data, ignore_rlimit_data, bool, 0644);
>  
> +const struct vm_operations_struct anon_vm_ops = {};
> +const struct vm_operations_struct dummy_vm_ops = {};

Some nice comments here would be useful.  Especially for dummy_vm_ops. 
Why does it exist, what is its role, etc.

>  static void unmap_region(struct mm_struct *mm,
>  		struct vm_area_struct *vma, struct vm_area_struct *prev,
>  		unsigned long start, unsigned long end);
> @@ -561,6 +564,8 @@ static unsigned long count_vma_pages_range(struct mm_struct *mm,
>  void __vma_link_rb(struct mm_struct *mm, struct vm_area_struct *vma,
>  		struct rb_node **rb_link, struct rb_node *rb_parent)
>  {
> +	WARN_ONCE(!vma->vm_ops, "missing vma->vm_ops");
> +
>  	/* Update tracking information for the gap following the new vma. */
>  	if (vma->vm_next)
>  		vma_gap_update(vma->vm_next);
> @@ -1774,12 +1779,19 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
>  		 */
>  		WARN_ON_ONCE(addr != vma->vm_start);
>  
> +		/* All mappings must have ->vm_ops set */
> +		if (!vma->vm_ops)
> +			vma->vm_ops = &dummy_vm_ops;

Can this happen?  Can we make it a rule that file_operations.mmap(vma)
must initialize vma->vm_ops?  Should we have a WARN here to detect when
the fs implementation failed to do that?

>  		addr = vma->vm_start;
>  		vm_flags = vma->vm_flags;
>  	} else if (vm_flags & VM_SHARED) {
>  		error = shmem_zero_setup(vma);
>  		if (error)
>  			goto free_vma;
> +	} else {
> +		/* vma_is_anonymous() relies on this. */
> +		vma->vm_ops = &anon_vm_ops;
>  	}
>  
>  	vma_link(mm, vma, prev, rb_link, rb_parent);
> ...
>