Re: [tpmdd-devel] [RFC] tpm2-space: add handling for global session exhaustion

[James Bottomley]

On Mon, 2017-01-30 at 23:58 +0200, Jarkko Sakkinen wrote:
> On Mon, Jan 30, 2017 at 08:04:55AM -0800, James Bottomley wrote:
> > On Sun, 2017-01-29 at 19:52 -0500, Ken Goldman wrote:
> > > On 1/27/2017 5:04 PM, James Bottomley wrote:
> > > 
> > > > > Beware the nasty corner case:
> > > > > 
> > > > > - Application asks for a session and gets 02000000
> > > > > 
> > > > > - Time elapses and 02000000 gets forcibly flushed
> > > > > 
> > > > > - Later, app comes back, asks for a second session and again
> > > > > gets
> > > > > 02000000.
> > > > > 
> > > > > - App gets very confused.
> > > > > 
> > > > > May it be better to close the connection completely, which
> > > > > the
> > > > > application can detect, than flush a session and give this
> > > > > corner
> > > > > case?
> > > > 
> > > > if I look at the code I've written, I don't know what the
> > > > session
> > > > number is, I just save sessionHandle in a variable for later
> > > > use 
> > > > (lets say to v1).  If I got the same session number returned at
> > > > a 
> > > > later time and placed it in v2, all I'd notice is that an 
> > > > authorization using v1 would fail.  I'm not averse to killing
> > > > the 
> > > > entire connection but, assuming you have fallback, it might be 
> > > > kinder simply to ensure that the operations with the reclaimed 
> > > > session fail (which is what the code currently does).
> > > 
> > > My worry is that this session failure cannot be detected by the 
> > > application.  An HMAC failure could cause the app to tell a user
> > > that
> > > they entered the wrong password.  Misleading.  On the TPM, it
> > > could 
> > > trigger the dictionary attack lockout.  For a PIN index, it could
> > > consume a failure count.  Killing a policy session that has e.g.,
> > > a 
> > > policy signed term could cause the application to go back to some
> > > external entity for another authorization signature.
> > > 
> > > Let's go up to the stack.  What's the attack?
> > > 
> > > If we're worried about many simultaneous applications (wouldn't
> > > that 
> > > be wonderful), why not just let startauthsession fail?  The 
> > > application can just retry periodically.
> > 
> > How in that scenario do we ensure that a session becomes available?
> >  Once that's established, there's no real difference between
> > retrying
> > the startauthsession in the kernel when we know the session is
> > available and forcing userspace to do the retry except that the
> > former
> > has a far greater chance of success (and it's only about 6 lines of
> > code).
> > 
> > >   Just allocate them in triples so there's no deadlock.
> > 
> > Is this the application or the kernel?  If it's the kernel, that
> > adds a
> > lot of complexity.
> > 
> > > If we're worried about a DoS attack, killing a session just helps
> > > the
> > > attacker.  The attacker can create a few connections and spin on 
> > > startauthsession, locking everyone out anyway.
> > 
> > There are two considerations here: firstly we'd need to introduce a
> > mechanism to "kill" the connection.  Probably we'd simply error
> > every
> > command on the space until it was closed.  The second is which
> > scenario
> > is more reasonable: Say the application simply forgot to flush the
> > session and will never use it again.  Simply reclaiming the session
> > would produce no effect at all on the application in this scenario.
> >  However, I have no data to say what's likely.
> > 
> > > ~~
> > > 
> > > Also, let's remember that this is a rare application.  Sessions
> > > are 
> > > only needed for remote access (requiring encryption, HMAC or
> > > salt), 
> > > or policy sessions.
> > 
> > This depends what your threat model is.  For ssh keys, you worry
> > that
> > someone might be watching, so you use HMAC authority even for a
> > local
> > TPM.  In the cloud, you don't quite know where the TPM is, so again
> > you'd use HMAC sessions ... however, in both use cases the sessions
> > should be very short lived.
> > 
> > > ~~
> > > 
> > > Should the code also reserve a session for the kernel?  Mark it
> > > not 
> > > kill'able?
> > 
> > At the moment, the kernel doesn't use sessions, so let's worry
> > about
> > that problem at the point it arises (if it ever arises).
> > 
> > James
> 
> It does. My trusted keys implementation actually uses sessions.

But as I read the code, I can't find where the kernel creates a
session.  It looks like the session and hmac are passed in as option
arguments, aren't they?

> I'm kind dilating to an opinion that we would leave this commit out 
> from the first kernel release that will contain the resource manager 
> with similar rationale as Jason gave me for whitelisting: get the 
> basic stuff in and once it is used with some workloads whitelisting 
> and exhaustion will take eventually the right form.
> 
> How would you feel about this?

As long as we get patch 1/2 then applications using sessions will
actually work with spaces, so taking more time with 2/2 is fine by me.

James