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

Andrew Morton
2018-07-10T20:49:03Z

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);
> ...
>