Re: [PATCH v2 4/6] mm/mremap: Replace can_modify_mm with can_modify_vma

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

* Pedro Falcato <pedro.falcato@gmail.com> [240807 17:13]:
> Delegate all can_modify checks to the proper places. Unmap checks are
> done in do_unmap (et al).
> 
> This patch allows for mremap partial failure in certain cases (for
> instance, when destination VMAs aren't sealed, but the source VMA is).
> It shouldn't be too troublesome, as you'd need to go out of your way to
> do illegal operations on a VMA.

As mseal() is supposed to be a security thing, is the illegal operation
not a concern?

> 
> Signed-off-by: Pedro Falcato <pedro.falcato@gmail.com>
> ---
> v2:
> 	- Removed a superfluous check in mremap (Jeff Xu)
> 
>  mm/mremap.c | 30 ++++--------------------------
>  1 file changed, 4 insertions(+), 26 deletions(-)
> 
> diff --git a/mm/mremap.c b/mm/mremap.c
> index e7ae140fc64..35afb3e38a8 100644
> --- a/mm/mremap.c
> +++ b/mm/mremap.c
> @@ -821,6 +821,10 @@ static struct vm_area_struct *vma_to_resize(unsigned long addr,
>  	if (!vma)
>  		return ERR_PTR(-EFAULT);
>  
> +	/* Don't allow vma expansion when it has already been sealed */
> +	if (!can_modify_vma(vma))
> +		return ERR_PTR(-EPERM);
> +
>  	/*
>  	 * !old_len is a special case where an attempt is made to 'duplicate'
>  	 * a mapping.  This makes no sense for private mappings as it will
> @@ -902,19 +906,6 @@ static unsigned long mremap_to(unsigned long addr, unsigned long old_len,
>  	if ((mm->map_count + 2) >= sysctl_max_map_count - 3)
>  		return -ENOMEM;
>  
> -	/*
> -	 * In mremap_to().
> -	 * Move a VMA to another location, check if src addr is sealed.
> -	 *
> -	 * Place can_modify_mm here because mremap_to()
> -	 * does its own checking for address range, and we only
> -	 * check the sealing after passing those checks.
> -	 *
> -	 * can_modify_mm assumes we have acquired the lock on MM.
> -	 */
> -	if (unlikely(!can_modify_mm(mm, addr, addr + old_len)))
> -		return -EPERM;
> -
>  	if (flags & MREMAP_FIXED) {
>  		/*
>  		 * In mremap_to().
> @@ -1079,19 +1070,6 @@ SYSCALL_DEFINE5(mremap, unsigned long, addr, unsigned long, old_len,
>  		goto out;
>  	}
>  
> -	/*
> -	 * Below is shrink/expand case (not mremap_to())
> -	 * Check if src address is sealed, if so, reject.
> -	 * In other words, prevent shrinking or expanding a sealed VMA.
> -	 *
> -	 * Place can_modify_mm here so we can keep the logic related to
> -	 * shrink/expand together.
> -	 */
> -	if (unlikely(!can_modify_mm(mm, addr, addr + old_len))) {
> -		ret = -EPERM;
> -		goto out;
> -	}
> -
>  	/*
>  	 * Always allow a shrinking remap: that just unmaps
>  	 * the unnecessary pages..
> -- 
> 2.46.0
>