linux-kernel - Re: [Regression] "tpm: Require that all digests are present in TCG_PCR_EVENT2 structures" causes null pointer dereference

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Date:   Mon, 28 Sep 2020 20:12:39 +0300
From:   Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     Kai-Heng Feng <kai.heng.feng@...onical.com>,
        "Kenneth R. Crudup" <kenny@...ix.com>,
        Tyler Hicks <tyhicks@...ux.microsoft.com>,
        linux-integrity <linux-integrity@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        Matthew Garrett <mjg59@...gle.com>, tweek@...gle.com
Subject: Re: [Regression] "tpm: Require that all digests are present in
 TCG_PCR_EVENT2 structures" causes null pointer dereference

On Mon, Sep 28, 2020 at 06:15:00PM +0200, Ard Biesheuvel wrote:
> > > > It is possible but initially feels a bit weird:
> > > >
> > > > -                  sizeof(TCG_SPECID_SIG)) || count > efispecid->num_algs) {
> > > > +                  sizeof(TCG_SPECID_SIG)) ||
> > > > +           !efispecid->num_algs || count != efispecid->num_algs) {

^^^

> In tpm2_bios_measurements_start(), we dereference tpm_bios_log to
> access bios_event_log and bios_event_log_end without checking tpm_bios
> for NULL. This is where the crash seems to occur.

That's a good guess. Just a bit confused how that particular patch can
have the effect: it has two deferences to efispecid instead of one in
the same statement. Would be interesting to hear if the bug is triggered
in Kenneth's environment by the exact same commit.

/Jarkko