Re: [RFC PATCH] mm: extend memfd with ability to create "secret" memory areas

[Mike Rapoport <rppt@kernel.org>]

On Wed, Feb 12, 2020 at 02:10:29PM -0700, Jonathan Corbet wrote:
> On Thu, 30 Jan 2020 18:23:41 +0200
> Mike Rapoport <rppt@kernel.org> wrote:
> 
> > Hi,
> > 
> > This is essentially a resend of my attempt to implement "secret" mappings
> > using a file descriptor [1]. 
> 
> So one little thing I was curious about as I read through the patch...
> 
> > +static int secretmem_check_limits(struct vm_fault *vmf)
> > +{
> > +	struct secretmem_state *state = vmf->vma->vm_file->private_data;
> > +	struct inode *inode = file_inode(vmf->vma->vm_file);
> > +	unsigned long limit;
> > +
> > +	if (((loff_t)vmf->pgoff << PAGE_SHIFT) >= i_size_read(inode))
> > +		return -EINVAL;
> > +
> > +	limit = rlimit(RLIMIT_MEMLOCK) >> PAGE_SHIFT;
> > +	if (state->nr_pages + 1 >= limit)
> > +		return -EPERM;
> > +
> > +	return 0;
> > +}
> 
> If I'm not mistaken, this means each memfd can be RLIMIT_MEMLOCK in length,
> with no global limit on the number of locked pages.  What's keeping me from
> creating 1000 of these things and locking down lots of RAM?

Indeed, it's possible to lock down RLIMIT_MEMLOCK * RLIMIT_NOFILE of RAM
with this implementation, thanks for catching this.

I'll surely update the resource limiting once we've settle on the API
selection :)
 
> Thanks,
> 
> jon
> 

-- 
Sincerely yours,
Mike.