linux-kernel - Re: [PATCH] tpm: fix a race condition tpm2_unseal

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Message-ID: <20160720211332.GA32417@obsidianresearch.com>
Date:	Wed, 20 Jul 2016 15:13:32 -0600
From:	Jason Gunthorpe <jgunthorpe@...idianresearch.com>
To:	Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
Cc:	Peter Huewe <peterhuewe@....de>,
	linux-security-module@...r.kernel.org,
	Marcel Selhorst <tpmdd@...horst.net>,
	"moderated list:TPM DEVICE DRIVER" 
	<tpmdd-devel@...ts.sourceforge.net>,
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] tpm: fix a race condition tpm2_unseal_trusted()

On Wed, Jul 20, 2016 at 11:53:14PM +0300, Jarkko Sakkinen wrote:

> The only use cases I see at the moment for it work this way:
> 
> 1. Call tpm_try_get_ops.
> 2. Send a TPM command.
> 3. Call tpm_put_ops.

Right, but that is just a reflection of what the in kernel users are
doing today, not necessarily what they should be doing.

We should not break the put/get semantics..

> I did not find any other form of use. The only use is to make sure that
> there are no transactions running before the ops are cleared. Or did I
> overlook something perhaps?

The put/get is intended to allow a kapi user to hold a ref to tpm
without it geting destroyed. It is not intended to be an exclusive lock.

> Trusted key unseal operation with TPM2 is broken into two operations:
> 
> 1. Load the given key blob.
> 2. Unseal the data.
> 
> Without locking and unlocking mutex only once there is a race condition.

Well, the race condition is fundamentally because we don't have key
virtualization in the kernel :|

Those sorts of compound ops should hold the tpm_mutex manually, not
through the get_ops scheme.

Jason