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

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Message-ID: <20180723214846.GG532@ziepe.ca>
Date:   Mon, 23 Jul 2018 15:48:46 -0600
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Tadeusz Struk <tadeusz.struk@...el.com>
Cc:     jarkko.sakkinen@...ux.intel.com, 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, Jul 19, 2018 at 08:52:32AM -0700, Tadeusz Struk wrote:
> Currently to read a response from the TPM device an application needs
> provide "big enough" buffer for the whole response and read it in one go.
> The application doesn't know how big the response it beforehand so it
> always needs to maintain a 4K buffer and read the max (4K).
> In case if the user of the TSS library doesn't provide big enough buffer
> the TCTI spec says that the library should set the required size and return
> TSS2_TCTI_RC_INSUFFICIENT_BUFFER error code so that the application could
> allocate a bigger buffer and call receive again.
> To make it possible in the TSS library this requires being able to do
> partial reads from the driver.
> The library would read the header first to get the actual size of the
> response from the header and then read the rest of the response.
> This patch adds support for partial reads.

You should solve this in user space, the kernel API requires a full
sized buffer here, the tss library should always provide such an
internal buffer and then implement whatever scheme TSS wants by
memcpying from that buffer..

Jason