linux-kernel - Re: [PATCH] tpm: add support for partial reads

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Date:   Thu, 19 Jul 2018 12:52:59 -0700
From:   James Bottomley <James.Bottomley@...senPartnership.com>
To:     Tadeusz Struk <tadeusz.struk@...el.com>,
        jarkko.sakkinen@...ux.intel.com
Cc:     jgg@...pe.ca, linux-integrity@...r.kernel.org,
        linux-security-module@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tpm: add support for partial reads

On Thu, 2018-07-19 at 12:05 -0700, Tadeusz Struk wrote:
> On 07/19/2018 11:47 AM, James Bottomley wrote:
> > On Thu, 2018-07-19 at 10:54 -0700, Tadeusz Struk wrote:
> > > On 07/19/2018 10:19 AM, James Bottomley wrote:
> > > > That's just an implementation, though, what's the use case?
> > > 
> > > Hi James,
> > > The use case is described in the TCTI spec [1] in the
> > > "3.2.5.2 receive" section.
> > 
> > Well, yes, but I think we've all agreed that the /dev/tpm and
> > /dev/tpmrmX aren't TCTI interfaces, although you can layer TCTI on
> > top of them, so why not simply do fragmentation on top if you need
> > it?
> > 
> > The reason for not doing it in the interface is that it alters the
> > ABI.  Before this patch we had a hard packet boundary: one packet
> > per read, one per write and a -EFAULT if you fail to provide a
> > correctly sized buffer.  Now if you provide a buffer too small but
> > don't know about the fragmentation you're going to misprocess a
> > packet (because you think you got a whole reply but you didn't) and
> > then you get a -EBUSY on your next command which you don't know how
> > to handle.  The only way out is to update the applications to
> > handle the new behaviour, which is a no-no in Linux ABI terms.
> 
> Don't all the existing applications that read a response in one go
> do a 4K read now? So nothing will change for them. They will work
> exactly the same with this change as they do without it.
> This doesn't break the ABI.

The ABI break is the error case as I outlined above.  We can't assume
everyone uses the current interface without getting an error and one
error and your hosed is a nasty failure case to change the interface
to.  Plus, if you assume everyone is passing 4k buffers, why would you
even need the fragmentation case?

> > It might be possible to layer the behaviour you want compatibly
> > into the current device structure (say an ioctl to switch to the
> > fragment behaviour) but I've got to ask why we'd go to the
> > complexity without a use case?
> 
> New IOCTL would add extra complexity, which isn't necessary.

So what's wrong with fragmenting in the layer above the device driver
(in userspace) and not actually changing the kernel?

James