Re: [PATCH v3 2/7] mm/munmap: Replace can_modify_mm with can_modify_vma

["Liam R. Howlett" <Liam.Howlett@oracle.com>]

* Pedro Falcato <pedro.falcato@gmail.com> [240816 20:18]:
> We were doing an extra mmap tree traversal just to check if the entire
> range is modifiable. This can be done when we iterate through the VMAs
> instead.
> 
> Signed-off-by: Pedro Falcato <pedro.falcato@gmail.com>

Although this has the side effect of potentially splitting the first vma
if it is not mseal()'ed, there really is no risk here since someone
making an invalid call that splits the vma for whatever reason can
modify the bad call to achieve the same split.  That is, this doesn't
help or hinder an attacker.

Reviewed-by: Liam R. Howlett <Liam.Howlett@Oracle.com>

> ---
>  mm/mmap.c | 11 +----------
>  mm/vma.c  | 19 ++++++++++++-------
>  2 files changed, 13 insertions(+), 17 deletions(-)
> 
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 3af256bacef3..30ae4cb5cec9 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -1740,16 +1740,7 @@ int do_vma_munmap(struct vma_iterator *vmi, struct vm_area_struct *vma,
>  		unsigned long start, unsigned long end, struct list_head *uf,
>  		bool unlock)
>  {
> -	struct mm_struct *mm = vma->vm_mm;
> -
> -	/*
> -	 * Check if memory is sealed, prevent unmapping a sealed VMA.
> -	 * can_modify_mm assumes we have acquired the lock on MM.
> -	 */
> -	if (unlikely(!can_modify_mm(mm, start, end)))
> -		return -EPERM;
> -
> -	return do_vmi_align_munmap(vmi, vma, mm, start, end, uf, unlock);
> +	return do_vmi_align_munmap(vmi, vma, vma->vm_mm, start, end, uf, unlock);
>  }
>  
>  /*
> diff --git a/mm/vma.c b/mm/vma.c
> index 84965f2cd580..5850f7c0949b 100644
> --- a/mm/vma.c
> +++ b/mm/vma.c
> @@ -712,6 +712,12 @@ do_vmi_align_munmap(struct vma_iterator *vmi, struct vm_area_struct *vma,
>  		if (end < vma->vm_end && mm->map_count >= sysctl_max_map_count)
>  			goto map_count_exceeded;
>  
> +		/* Don't bother splitting the VMA if we can't unmap it anyway */
> +		if (!can_modify_vma(vma)) {
> +			error = -EPERM;
> +			goto start_split_failed;
> +		}
> +
>  		error = __split_vma(vmi, vma, start, 1);
>  		if (error)
>  			goto start_split_failed;
> @@ -723,6 +729,11 @@ do_vmi_align_munmap(struct vma_iterator *vmi, struct vm_area_struct *vma,
>  	 */
>  	next = vma;
>  	do {
> +		if (!can_modify_vma(next)) {
> +			error = -EPERM;
> +			goto modify_vma_failed;
> +		}
> +
>  		/* Does it split the end? */
>  		if (next->vm_end > end) {
>  			error = __split_vma(vmi, next, end, 0);
> @@ -815,6 +826,7 @@ do_vmi_align_munmap(struct vma_iterator *vmi, struct vm_area_struct *vma,
>  	__mt_destroy(&mt_detach);
>  	return 0;
>  
> +modify_vma_failed:
>  clear_tree_failed:
>  userfaultfd_error:
>  munmap_gather_failed:
> @@ -860,13 +872,6 @@ int do_vmi_munmap(struct vma_iterator *vmi, struct mm_struct *mm,
>  	if (end == start)
>  		return -EINVAL;
>  
> -	/*
> -	 * Check if memory is sealed, prevent unmapping a sealed VMA.
> -	 * can_modify_mm assumes we have acquired the lock on MM.
> -	 */
> -	if (unlikely(!can_modify_mm(mm, start, end)))
> -		return -EPERM;
> -
>  	/* Find the first overlapping VMA */
>  	vma = vma_find(vmi, end);
>  	if (!vma) {
> 
> -- 
> 2.46.0
>