Re: [PATCH v2 14/15] mm/mmap: Use vms accounted pages in mmap_region()

[Kees Cook <kees@kernel.org>]

On Tue, Jun 25, 2024 at 03:11:44PM -0400, Liam R. Howlett wrote:
> From: "Liam R. Howlett" <Liam.Howlett@Oracle.com>
> 
> Change from nr_pages variable to vms.nr_accounted for the charged pages
> calculation.  This is necessary for a future patch.
> 
> This also avoids checking security_vm_enough_memory_mm() if the amount
> of memory won't change.

Is there a reason for making this change? (I.e. why not leave off the
"charged" test?)

Looking at the callbacks in the LSM, only capabilities and SELinux are
hooking this, and both are checking whether a process has elevated privs
and are ignoring the "pages" argument entirely, so I'm not sure it's
safe to change the logic for whether to make the call based on an unused
argument (i.e. the LSM may want to _always_ know about this). On the
other hand, it looks like it's purely an accounting issue, and if the
page count didn't change, there's no reason to bother calling into all
this to make no changes to the accounting.

I've added the LSM list to CC...

-Kees

> 
> Signed-off-by: Liam R. Howlett <Liam.Howlett@Oracle.com>
> Cc: Kees Cook <kees@kernel.org>
> ---
>  mm/mmap.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/mmap.c b/mm/mmap.c
> index f3edabf83975..adb0bb5ea344 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -2970,6 +2970,7 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
>  	} else {
>  		/* Minimal setup of vms */
>  		vms.nr_pages = 0;
> +		vms.nr_accounted = 0;
>  		next = vma_next(&vmi);
>  		prev = vma_prev(&vmi);
>  		if (prev)
> @@ -2981,9 +2982,10 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
>  	 */
>  	if (accountable_mapping(file, vm_flags)) {
>  		charged = pglen;
> -		charged -= nr_accounted;
> -		if (security_vm_enough_memory_mm(mm, charged))
> +		charged -= vms.nr_accounted;
> +		if (charged && security_vm_enough_memory_mm(mm, charged))
>  			goto abort_munmap;
> +
>  		vms.nr_accounted = 0;
>  		vm_flags |= VM_ACCOUNT;
>  	}
> -- 
> 2.43.0
> 

-- 
Kees Cook