Re: [PATCH v2 1/2] mm: prevent droppable mappings from being locked

["David Hildenbrand (Arm)" <david@kernel.org>]

On 3/10/26 16:58, Anthony Yznaga wrote:
> Droppable mappings must not be lockable. There is a check for VMAs with
> VM_DROPPABLE set in mlock_fixup() along with checks for other types of
> unlockable VMAs which ensures this when calling mlock()/mlock2().
> 
> For mlockall(MCL_FUTURE), the check for unlockable VMAs is different.
> In apply_mlockall_flags(), if the flags parameter has MCL_FUTURE set, the
> current task's mm's default VMA flag field mm->def_flags has VM_LOCKED
> applied to it. VM_LOCKONFAULT is also applied if MCL_ONFAULT is also set.
> When these flags are set as default in this manner they are cleared in
> __mmap_complete() for new mappings that do not support mlock. A check for
> VM_DROPPABLE in __mmap_complete() is missing resulting in droppable
> mappings created with VM_LOCKED set. To fix this and reduce that chance of
> similar bugs in the future, introduce and use vma_supports_mlock().
> 
> Fixes: 9651fcedf7b9 ("mm: add MAP_DROPPABLE for designating always lazily freeable mappings")

Should we Cc: stable? I think we should, to fix mlockall(MCL_FUTURE)
behavior.

> Suggested-by: David Hildenbrand <david@kernel.org>
> Signed-off-by: Anthony Yznaga <anthony.yznaga@oracle.com>
> ---
> v2:
>  - Implement vma_supports_mlock() instead of vma flags mask (DavidH)
>  - Add selftests (Lorenzo)
> 
>  include/linux/hugetlb_inline.h    |  2 +-
>  mm/internal.h                     | 10 ++++++++++
>  mm/mlock.c                        | 10 ++++++----
>  mm/vma.c                          |  4 +---
>  tools/testing/vma/include/stubs.h |  5 +++++
>  5 files changed, 23 insertions(+), 8 deletions(-)
> 
> diff --git a/include/linux/hugetlb_inline.h b/include/linux/hugetlb_inline.h
> index 593f5d4e108b..755281fab23d 100644
> --- a/include/linux/hugetlb_inline.h
> +++ b/include/linux/hugetlb_inline.h
> @@ -30,7 +30,7 @@ static inline bool is_vma_hugetlb_flags(const vma_flags_t *flags)
>  
>  #endif
>  
> -static inline bool is_vm_hugetlb_page(struct vm_area_struct *vma)
> +static inline bool is_vm_hugetlb_page(const struct vm_area_struct *vma)
>  {
>  	return is_vm_hugetlb_flags(vma->vm_flags);
>  }
> diff --git a/mm/internal.h b/mm/internal.h
> index cb0af847d7d9..8c67637abcdd 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -1218,6 +1218,16 @@ static inline struct file *maybe_unlock_mmap_for_io(struct vm_fault *vmf,
>  	}
>  	return fpin;
>  }
> +
> +static inline bool vma_supports_mlock(const struct vm_area_struct *vma)
> +{
> +	if (vma->vm_flags & (VM_SPECIAL | VM_DROPPABLE))
> +		return false;
> +	if (vma_is_dax(vma) || is_vm_hugetlb_page(vma))
> +		return false;
> +	return vma != get_gate_vma(current->mm);

As discussed, it would be great to find out whether checking
get_gate_vma() makes any sense here. Likely not. :)

Acked-by: David Hildenbrand (Arm) <david@kernel.org>

-- 
Cheers,

David