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

[Kees Cook]

On Mon, Mar 9, 2015 at 3:13 PM, Eric W. Biederman <ebiederm@xxxxxxxxxxxx> wrote:
> Dave Hansen <dave@xxxxxxxx> writes:
>
>> 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:
>>
>>       http://www.cs.columbia.edu/~vpk/papers/ret2dir.sec14.pdf
>>
>> If you know the physical address of something you also know at
>> which kernel virtual address you can find something (modulo
>> highmem).  It means that things that keep the kernel from
>> accessing user mappings (like SMAP/SMEP) can be worked around
>> because the _kernel_ mapping can get used instead.
>>
>> But, /proc/$pid/pagemap exposes the physical addresses of all
>> pages accessible to userspace.  This works against all of the
>> efforts to keep kernel addresses out of places where unprivileged
>> apps can find them.
>>
>> This patch introduces a "paranoid" option for /proc.  It can be
>> enabled like this:
>>
>>       mount -o remount,paranoid /proc
>>
>> Or when /proc is mounted initially.  When 'paranoid' mode is
>> active, opens to /proc/$pid/pagemap will return -EPERM for users
>> without CAP_SYS_RAWIO.  It can be disabled like this:
>>
>>       mount -o remount,notparanoid /proc
>>
>> The option is applied to the pid namespace, so an app that wanted
>> a separate policy from the rest of the system could get run in
>> its own pid namespace.
>>
>> I'm not really that stuck on the name.  I'm not opposed to making
>> it apply only to pagemap or to giving it a pagemap-specific
>> name.
>>
>> 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.
>
>
> There is already a way to make pagemap go away.  It is called
> CONFIG_PROC_PAGE_MONITOR.
>
> I suspect the right answer here is if you enable kernel address
> randomization you disable CONFIG_PROC_PAGE_MONTIOR.  Aka you make the
> two options conflict with each other.

It's not a good idea to make CONFIG options conflict with each other
like this as it puts distros is a tricky spot to decide which to use.
Allowing both and having a runtime flag of some kind tends to be the
better option (e.g. kASLR vs Hibernation).

> That is a lot less code and a lot less to maintain.
>
> On the other hand if this is truly a valuable interface that you can't
> part with we need an alternative to pagemaps that does the same job
> with out the exploit potential.  And I don't how to do that.
>
> Arguing in favor of just making the options conflict is the fact that
> kernel address randomization is pretty much snake oil.  At least on
> x86_64 the address pool is so small it can be trivially brute forced.  I
> think there are maybe 10 bits you can randomize within.
>
> As for a way to disable this I expect it would do better with something
> like a set once flag that prevents a process and all of it's children
> from accessing this file.
>
> *Blink* *Blink* Did you say you are worried about escalting privileges
> from root into the kernel space.  That is non-sense.  We give root the
> power to shot themselves in the foot and any proc option will be
> something that root will be able to get around.
>
> The pieces of the patch description don't add up.

No, that's an entirely valid use-case. You can trust the kernel but
not root. This is the point of the "trusted_kernel" patch series that
disables all sorts of dangerous interfaces that allow root to get at
physical memory.

This situation is more a memory leak than a direct compromise, so it
seems like providing at least some runtime control of it (separate
from potential future "trusted_kernel" stuff) makes sense.

-Kees

>
> Nacked-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
>
> Eric



-- 
Kees Cook
Chrome OS Security
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