linux-kernel - Re: [PATCH v5 4/5] tpm: Allocate chip->auth in tpm2_start_auth

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Message-Id: <D4EPVO0KWRLK.2RQK9L93QM4VB@kernel.org>
Date: Tue, 24 Sep 2024 21:13:52 +0300
From: "Jarkko Sakkinen" <jarkko@...nel.org>
To: "James Bottomley" <James.Bottomley@...senPartnership.com>,
 <linux-integrity@...r.kernel.org>
Cc: <roberto.sassu@...wei.com>, <mapengyu@...il.com>,
 <stable@...r.kernel.org>, "Mimi Zohar" <zohar@...ux.ibm.com>, "David
 Howells" <dhowells@...hat.com>, "Paul Moore" <paul@...l-moore.com>, "James
 Morris" <jmorris@...ei.org>, "Serge E. Hallyn" <serge@...lyn.com>, "Peter
 Huewe" <peterhuewe@....de>, "Jason Gunthorpe" <jgg@...pe.ca>,
 <keyrings@...r.kernel.org>, <linux-security-module@...r.kernel.org>,
 <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 4/5] tpm: Allocate chip->auth in
 tpm2_start_auth_session()

On Tue Sep 24, 2024 at 4:33 PM EEST, James Bottomley wrote:
> On Sat, 2024-09-21 at 15:08 +0300, Jarkko Sakkinen wrote:
> > Move allocation of chip->auth to tpm2_start_auth_session() so that
> > the field can be used as flag to tell whether auth session is active
> > or not.
> > 
> > Cc: stable@...r.kernel.org # v6.10+
> > Fixes: 699e3efd6c64 ("tpm: Add HMAC session start and end functions")
> > Signed-off-by: Jarkko Sakkinen <jarkko@...nel.org>
> > ---
> > v5:
> > - No changes.
> > v4:
> > - Change to bug.
> > v3:
> > - No changes.
> > v2:
> > - A new patch.
> > ---
> >  drivers/char/tpm/tpm2-sessions.c | 43 +++++++++++++++++++-----------
> > --
> >  1 file changed, 25 insertions(+), 18 deletions(-)
> > 
> > diff --git a/drivers/char/tpm/tpm2-sessions.c
> > b/drivers/char/tpm/tpm2-sessions.c
> > index 1aef5b1f9c90..a8d3d5d52178 100644
> > --- a/drivers/char/tpm/tpm2-sessions.c
> > +++ b/drivers/char/tpm/tpm2-sessions.c
> > @@ -484,7 +484,8 @@ static void tpm2_KDFe(u8 z[EC_PT_SZ], const char
> > *str, u8 *pt_u, u8 *pt_v,
> >         sha256_final(&sctx, out);
> >  }
> >  
> > -static void tpm_buf_append_salt(struct tpm_buf *buf, struct tpm_chip
> > *chip)
> > +static void tpm_buf_append_salt(struct tpm_buf *buf, struct tpm_chip
> > *chip,
> > +                               struct tpm2_auth *auth)
>
> This addition of auth as an argument is a bit unnecessary.  You can set
> chip->auth before calling this and it will all function.  Since there's
> no error leg in tpm2_start_auth_session unless the session creation
> itself fails and the guarantee of the ops lock is single threading this
> chip->auth can be nulled again in that error leg.
>
> If you want to keep the flow proposed in the patch, the change from how
> it works now to how it works with this patch needs documenting in the
> change log

I checked this through and have to disagree with it. We don't want
to set chip->auth before the whole start auth session is successful

BR, Jarkko