linux-kernel - Re: [PATCH v6 3/6] mm: introduce memfd_secret system call to create "secret" memory areas

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Date:   Tue, 29 Sep 2020 16:06:02 +0300
From:   Mike Rapoport <rppt@...nel.org>
To:     "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
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Subject: Re: [PATCH v6 3/6] mm: introduce memfd_secret system call to create
 "secret" memory areas

On Tue, Sep 29, 2020 at 04:58:44AM +0000, Edgecombe, Rick P wrote:
> On Thu, 2020-09-24 at 16:29 +0300, Mike Rapoport wrote:
> > Introduce "memfd_secret" system call with the ability to create
> > memory
> > areas visible only in the context of the owning process and not
> > mapped not
> > only to other processes but in the kernel page tables as well.
> > 
> > The user will create a file descriptor using the memfd_secret()
> > system call
> > where flags supplied as a parameter to this system call will define
> > the
> > desired protection mode for the memory associated with that file
> > descriptor.
> > 
> >  Currently there are two protection modes:
> > 
> > * exclusive - the memory area is unmapped from the kernel direct map
> > and it
> >               is present only in the page tables of the owning mm.
> 
> Seems like there were some concerns raised around direct map
> efficiency, but in case you are going to rework this...how does this
> memory work for the existing kernel functionality that does things like
> this?
> 
> get_user_pages(, &page);
> ptr = kmap(page);
> foo = *ptr;
> 
> Not sure if I'm missing something, but I think apps could cause the
> kernel to access a not-present page and oops.

The idea is that this memory should not be accessible by the kernel, so
the sequence you describe should indeed fail.

Probably oops would be to noisy and in this case the report needs to be
less verbose.

-- 
Sincerely yours,
Mike.