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From: "Edgecombe, Rick P" <rick.p.edgecombe@intel.com>
To: "rppt@kernel.org" <rppt@kernel.org>
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Subject: Re: [PATCH v6 3/6] mm: introduce memfd_secret system call to create "secret" memory areas
Date: Tue, 29 Sep 2020 20:06:03 +0000
Message-ID: <839fbb26254dc9932dcff3c48a3a4ab038c016ea.camel@intel.com> (raw)
In-Reply-To: <20200929130602.GF2142832@kernel.org>

On Tue, 2020-09-29 at 16:06 +0300, Mike Rapoport wrote:
> 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.

I was more concerned that it could cause kernel instabilities.

I see, so it should not be accessed even at the userspace address? I
wonder if it should be prevented somehow then. At least
get_user_pages() should be prevented I think. Blocking copy_*_user()
access might not be simple.

I'm also not so sure that a user would never have any possible reason
to copy data from this memory into the kernel, even if it's just
convenience. In which case a user setup could break if a specific
kernel implementation switched to get_user_pages()/kmap() from using
copy_*_user(). So seems maybe a bit thorny without fully blocking
access from the kernel, or deprecating that pattern.

You should probably call out these "no passing data to/from the kernel"
expectations, unless I missed them somewhere.
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Thread overview: 57+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-09-24 13:28 [PATCH v6 0/6] " Mike Rapoport
2020-09-24 13:28 ` [PATCH v6 1/6] mm: add definition of PMD_PAGE_ORDER Mike Rapoport
2020-09-24 13:29 ` [PATCH v6 2/6] mmap: make mlock_future_check() global Mike Rapoport
2020-09-24 13:29 ` [PATCH v6 3/6] mm: introduce memfd_secret system call to create "secret" memory areas Mike Rapoport
2020-09-29  4:58   ` Edgecombe, Rick P
2020-09-29 13:06     ` Mike Rapoport
2020-09-29 20:06       ` Edgecombe, Rick P [this message]
2020-09-30 10:35         ` Mike Rapoport
2020-09-30 20:11           ` Edgecombe, Rick P
2020-10-11  9:42             ` Mike Rapoport
2020-09-24 13:29 ` [PATCH v6 4/6] arch, mm: wire up memfd_secret system call were relevant Mike Rapoport
2020-09-24 13:29 ` [PATCH v6 5/6] mm: secretmem: use PMD-size pages to amortize direct map fragmentation Mike Rapoport
2020-09-25  7:41   ` Peter Zijlstra
2020-09-25  9:00     ` David Hildenbrand
2020-09-25  9:50       ` Peter Zijlstra
2020-09-25 10:31         ` Mark Rutland
2020-09-25 14:57           ` Tycho Andersen
2020-09-29 14:04           ` Mike Rapoport
2020-09-29 13:07         ` Mike Rapoport
2020-09-29 13:06       ` Mike Rapoport
2020-09-29 13:05     ` Mike Rapoport
2020-09-29 14:12       ` Peter Zijlstra
2020-09-29 14:31         ` Dave Hansen
2020-09-29 14:58         ` Mike Rapoport
2020-09-29 15:15           ` Peter Zijlstra
2020-09-30 10:27             ` Mike Rapoport
2020-09-30 14:39               ` James Bottomley
2020-09-30 14:45                 ` David Hildenbrand
2020-09-30 15:17                   ` James Bottomley
2020-09-30 15:25                     ` David Hildenbrand
2020-09-30 15:09               ` Matthew Wilcox
2020-10-01  8:14                 ` Mike Rapoport
2020-09-29 15:03         ` James Bottomley
2020-09-30 10:20         ` Mike Rapoport
2020-09-30 10:43           ` Peter Zijlstra
2020-09-24 13:29 ` [PATCH v6 6/6] secretmem: test: add basic selftest for memfd_secret(2) Mike Rapoport
2020-09-24 13:35 ` [PATCH] man2: new page describing memfd_secret() system call Mike Rapoport
2020-09-24 14:55   ` Alejandro Colomar
2020-10-03  9:32     ` Alejandro Colomar
2020-10-05  7:32       ` Mike Rapoport
2020-11-16 21:01         ` [PATCH v2] memfd_secret.2: New " Alejandro Colomar
2020-11-17  6:26           ` Mike Rapoport
2020-09-25  2:34 ` [PATCH v6 0/6] mm: introduce memfd_secret system call to create "secret" memory areas Andrew Morton
2020-09-25  6:42   ` Mike Rapoport
2020-11-01 11:09 ` Hagen Paul Pfeifer
2020-11-02 15:40   ` Mike Rapoport
2020-11-03 13:52     ` Hagen Paul Pfeifer
2020-11-03 16:30       ` Mike Rapoport
2020-11-04 11:39         ` Hagen Paul Pfeifer
2020-11-04 17:02           ` Mike Rapoport
2020-11-09 10:41             ` Hagen Paul Pfeifer
2020-11-02  9:11 ` David Hildenbrand
2020-11-02  9:31   ` David Hildenbrand
2020-11-02 17:43   ` Mike Rapoport
2020-11-02 17:51     ` David Hildenbrand
2020-11-03  9:52       ` Mike Rapoport
2020-11-03 10:11         ` David Hildenbrand

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