Re: [RFC][PATCH 1/2] fs proc: make pagemap a privileged interface

From: Eric W. Biederman
Date: Fri Mar 13 2015 - 13:20:52 EST

Dave Hansen <dave@xxxxxxxx> writes:

> On 03/12/2015 03:35 PM, Andrew Morton wrote:
>> On Mon, 09 Mar 2015 13:43:21 -0700 Dave Hansen <dave@xxxxxxxx> wrote:
>>> From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
>>> Physical addresses are sensitive information. There are
>>> existing, known exploits that are made easier if physical
>>> information is available. Here is one example:
>> Do we really need to disable pagemap entirely? What happens if we just
>> obscure the addresses (ie: zero them)?
> I think we have 3 basic options:
> 1. Disable it entirely (-EPERM or whatever). Apps using it break
> quickly and fairly obviously (diagnosable with an strace)
> 2. Zero it, or return some nonsensical thing for the physical address
> portion, but maintain exporting the PTE flags. Apps only caring
> about PTE flags work, but anything trying to do lookups in
> /proc/kpageflags break. If we zero it, apps pay get confused
> thinking they have the _actual_ pfn=0.
> 3. Scramble it in some way obscuring the physical address. Unscramble
> it upon access to /proc/kpageflags.
> I think you're suggesting (2). Doesn't that risk silently breaking
> apps?

I think 3 where the scramble is something like AES crypto is likely to
scramble this well and still protect us from plain text attacks.

>>> pagemap is also the kind of feature that could be used to escalate
>>> privileged from root in to the kernel. It probably needs to be
>>> protected in the same way that /dev/mem or module loading is in
>>> cases where the kernel needs to be protected from root, thus the
>>> choice to use CAP_SYS_RAWIO.
>> Confused. If you have root, you can do mount -o notparanoid.
> Good point. I guess it doesn't protect us much here unless we also
> restrict the ability to remount.

And the ability to unmount...

A write-once sysctl or a boot time only parameter is much more likely to
be useful in the scenario where you are concerned about root.


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