linux-kernel - Re: [PATCH 4/6] tpm: sanity check the log version before using it

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Message-ID: <CAC_iWjKoptBngCj-W6axZ9bmJmhT11JMctn1m4maVvO4mzcENg@mail.gmail.com>
Date: Fri, 13 Sep 2024 16:39:19 +0300
From: Ilias Apalodimas <ilias.apalodimas@...aro.org>
To: Gregory Price <gourry@...rry.net>, Ard Biesheuvel <ardb@...nel.org>
Cc: linux-efi@...r.kernel.org, linux-kernel@...r.kernel.org, leitao@...ian.org, 
	usamaarif642@...il.com, sathyanarayanan.kuppuswamy@...ux.intel.com
Subject: Re: [PATCH 4/6] tpm: sanity check the log version before using it

On Fri, 13 Sept 2024 at 15:57, Gregory Price <gourry@...rry.net> wrote:
>
> On Fri, Sep 13, 2024 at 09:40:30AM +0300, Ilias Apalodimas wrote:
> > Hi Gregory,
> >
> > On Fri, 6 Sept 2024 at 23:28, Gregory Price <gourry@...rry.net> wrote:
> > >
> > > If the log version is not sane (0 or >2), don't attempt to use
> > > the rest of the log values for anything to avoid potential corruption.
> > >
> > > Signed-off-by: Gregory Price <gourry@...rry.net>
> > > ---
> > >  drivers/firmware/efi/tpm.c | 9 +++++++++
> > >  1 file changed, 9 insertions(+)
> > >
> > > diff --git a/drivers/firmware/efi/tpm.c b/drivers/firmware/efi/tpm.c
> > > index 6e03eed0dc6f..9a080887a3e0 100644
> > > --- a/drivers/firmware/efi/tpm.c
> > > +++ b/drivers/firmware/efi/tpm.c
> > > @@ -60,6 +60,15 @@ int __init efi_tpm_eventlog_init(void)
> > >                 return -ENOMEM;
> > >         }
> > >
> > > +       if (!log_tbl->version ||
> > > +           log_tbl->version > EFI_TCG2_EVENT_LOG_FORMAT_TCG_2) {
> > > +               pr_err(FW_BUG "TPM Events table version invalid (%x)\n",
> > > +                      log_tbl->version);
> > > +               early_memunmap(log_tbl, sizeof(*log_tbl));
> > > +               efi.tpm_log = EFI_INVALID_TABLE_ADDR;
> > > +               return -EINVAL;
> >
> > I don't think we need this check at all. Did you actually see this happening?
> > efi_retrieve_eventlog() that runs during the efistub tries to retrieve
> > the log and the EFI protocol itself explicitly says that the firmware
> > *must* return EFI_INVALID_PARAMETER if the event log is not in 1.2 or
> > 2.0 format. If the firmware does something wrong, we should report the
> > FW BUG in that function, instead of installing the config tables Linux
> > uses internally to handover the log and catching it late.
> >
> > Thanks
> > /Ilias
> >
>
> We saw this happen and discovered it was a disagreement between EFI/OS/kexec
> causing the table to be overwritten during kexec.  We've since found a fix for
> that.  So the result was that it appeared the firmware was doing something
> wrong. The sanity check at least allowed us to boot without immediately
> crashing - because the tables don't get reinstalled, they get re-used
> (at least that's by best understanding of the whole interaction).
>
> If the check seems superfluous, i can drop it.

Ok, that explains why it wasn't caught earlier at least. I would
prefer dropping it tbh, but I am going to defer to Ard for that.

If we agree that this needs to go in btw, I think you should refactor
it a bit. That function already defines an out: label, which unmaps
memory. So you can rewrite the above as

If(....) {
    ret = -EINVAL;
    efi.tpm_log = EFI_INVALID_TABLE_ADDR;
   goto out;
}

Cheers
/Ilias

>
> >
> >
> > > +       }
> > > +
> > >         tbl_size = sizeof(*log_tbl) + log_tbl->size;
> > >         if (memblock_reserve(efi.tpm_log, tbl_size)) {
> > >                 pr_err("TPM Event Log memblock reserve fails (0x%lx, 0x%x)\n",
> > > --
> > > 2.43.0
> > >