Re: [PATCH v6 3/6] security: keys: trusted: fix TPM2 authorizations

[Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>]

On Tue, Mar 03, 2020 at 03:39:08PM -0500, James Bottomley wrote:
> On Tue, 2020-03-03 at 21:33 +0200, Jarkko Sakkinen wrote:
> > On Mon, Mar 02, 2020 at 07:27:56AM -0500, James Bottomley wrote:
> > > In TPM 1.2 an authorization was a 20 byte number.  The spec
> > > actually recommended you to hash variable length passwords and use
> > > the sha1 hash as the authorization.  Because the spec doesn't
> > > require this hashing, the current authorization for trusted keys is
> > > a 40 digit hex number.  For TPM 2.0 the spec allows the passing in
> > > of variable length passwords and passphrases directly, so we should
> > > allow that in trusted keys for ease of use.  Update the 'blobauth'
> > > parameter to take this into account, so we can now use plain text
> > > passwords for the keys.
> > > 
> > > so before
> > > 
> > > keyctl add trusted kmk "new 32
> > > blobauth=f572d396fae9206628714fb2ce00f72e94f2258f"
> > > 
> > > after we will accept both the old hex sha1 form as well as a new
> > > directly supplied password:
> > > 
> > > keyctl add trusted kmk "new 32 blobauth=hello keyhandle=81000001"
> > > 
> > > Since a sha1 hex code must be exactly 40 bytes long and a direct
> > > password must be 20 or less, we use the length as the discriminator
> > > for which form is input.
> > > 
> > > Note this is both and enhancement and a potential bug fix.  The TPM
> > > 2.0 spec requires us to strip leading zeros, meaning empyty
> > > authorization is a zero length HMAC whereas we're currently passing
> > > in
> > > 20 bytes of zeros.  A lot of TPMs simply accept this as OK, but the
> > > Microsoft TPM emulator rejects it with TPM_RC_BAD_AUTH, so this
> > > patch
> > > makes the Microsoft TPM emulator work with trusted keys.
> > 
> > The commit message does not mention it but there limitation that you
> > cannot have this as a *password*:
> > 
> >   f572d396fae9206628714fb2ce00f72e94f2258f
> > 
> > The commit message should explicitly state this.
> 
> Well, that's impossible anyway: the password can be at most
> TPM_DIGEST_SIZE characters and the above is twice that, so the
> discriminator is fairly simple: if the string size is less than or
> equal to TPM_DIGEST_SIZE, then it's a plain password, if it's exactly
> 2xTPM_DIGEST_SIZE it must be a hex value and if it's anything else,
> it's illegal.  I thought the sentence
> 
>    Since a sha1 hex code must be exactly 40 bytes long and a direct
>    password must be 20 or less, we use the length as the discriminator
>    for which form is input.
> 
> Was the explanation for this, but I can update it.

Thanks! No need to update. I missed that part somehow.

/Jarkko