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

[Liam R. Howlett]

* Kees Cook <kees at kernel.org> [240626 12:32]:
> On Tue, Jun 25, 2024 at 03:11:44PM -0400, Liam R. Howlett wrote:
> > From: "Liam R. Howlett" <Liam.Howlett at 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?)

Before, the munmap() completed prior to mmap()'ing the MAP_FIXED vma.
If we don't remove the nr_accounted from the charged, we risk hitting
the maximum limit.

> 
> 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 didn't see any reason not to avoid the call, but your statement is
valid.  I didn't see anything looking at the callbacks that would have
issue with skipping it - but I'd like to hear what LSM has to say.

I don't have any objections to removing the extra check, if anyone
thinks it could be an issue.

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

Thank you, and thanks for looking at this.

> 
> -Kees
> 
> > 
> > Signed-off-by: Liam R. Howlett <Liam.Howlett at Oracle.com>
> > Cc: Kees Cook <kees at 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