Re: [PATCH 5/8] x86, microcode, intel: don't check extsig entry checksum

Henrique de Moraes Holschuh
2014-10-31T17:14:51Z

On Thu, 30 Oct 2014, Borislav Petkov wrote:
> On Mon, Sep 08, 2014 at 02:37:51PM -0300, Henrique de Moraes Holschuh wrote:
> > The contents of the extended signature entries are already covered by
> > the extended table checksum, and the microcode driver should not be
> > attempting to check their internal checksum field.
> >
> > Unlike the main microcode checksum field and the extended signature
> > table checksum field, the checksum fields inside the extended signature
> > entries are not meant to be processed by a microcode update loader. The
> > extended signature entry checksum field's description in the Intel SDM,
> > vol 3A, table 9-6, page 9-30, reads in the first paragraph:
> >
> > "Used by utility software to decompose a microcode update into
> > multiple microcode updates where each of the new updates is
> > constructed without the optional Extended Processor Signature
> > Table."
> >
> > And the Linux microcode driver is not processing them correctly anyway.
> > The second paragraph of the signature entry checksum field's description
> > in the Intel SDM, vol 3A, table 9-6, page 9-30, reads:
> >
> > "To calculate the Checksum, substitute the Primary Processor
> > Signature entry and the Processor Flags entry with the corresponding
> > Extended Patch entry. Delete the Extended Processor Signature Table
> > entries. The Checksum is correct when the summation of all DWORDs
> > that comprise the created Extended Processor Patch results in
> > 00000000H."
> >
> > Deleting the extended signature table changes the Total Size field, and
> > the Intel SDM paragraph above makes it very clear that such a change must
> > be accounted for by the checksum. The current extended signature entry
> > checksum code in the Linux microcode driver, which has been in place
> > since 2003, will be thrown off by this and reject a valid microcode
> > update.
>
> I don't know where you come up with this but the code you're removing
> was added in 2013:
>
> e666dfa273db ("x86/microcode_intel_lib.c: Early update ucode on Intel's CPU")

Was it? As far as I know, it is "heavily based" (i.e. copied and adapted)
on code from the regular microcode driver, and not rewritten from scrach.

If I am correct, this really is code from 2003, changed by 11 years worth of
file renaming, file splits, and refactoring.

It took me about 30 minutes to track the origin of this code in the regular
Intel microcode driver. As far as I can tell, it was added to version 1.12
of the original driver, somewhere between v2.6.0-test7 and v2.6.0-test8.

The Intel engineers credited for the original change are: Nitin Kamble
<nitin.a.kamble@intel.com> and Jun Nakajima <jun.nakajima@intel.com>.
(information from the changelog at the top of the driver source).

You can see one of the earliest versions of the code here:
https://git.kernel.org/cgit/linux/kernel/git/history/history.git/tree/arch/i386/kernel/microcode.c?id=v2.6.0-test8

The extended table checksum gets calculated around line 323. The possibly
problematic logic is in line 341 and 345, and matches what the modern driver
does.

> and I'd strongly assume it was tested at the time. Now, the text in the
> SDM is confusing, as most of the time is so I would think the code is
> doing the right thing even if the text doesn't really say that clearly.

I am not so sure. Besides, it is entirely possible that the Intel SDM got
updated in that area after the september 2003 code drop. This is very hard
to verify outside of Intel. Or, maybe, the issue was detected internally
at Intel, and it was decided that the issue should be shelved until the day
extended microcode tables are actually required (which didn't happen yet).

> Fenghua, care to comment please?

I'd really appreciate that. I have asked about it in the past, but got no
answers.

I am in fact hoping for a "this feature is dead as currently documented,
please remove the code until further notice" reply :-) LoC reduction by
deleting something that was never used in 11 years, Yay!!

> Leaving in the rest for reference.
>
> > The microcode driver is better off by doing what the Intel SDM suggests,
> > and staying well clear of that checksum field. It has already checked
> > the whole extended signature table's checksum, anyway.
> >
> > Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
> > ---
> > arch/x86/kernel/cpu/microcode/intel_lib.c | 20 ++------------------
> > 1 file changed, 2 insertions(+), 18 deletions(-)
> >
> > diff --git a/arch/x86/kernel/cpu/microcode/intel_lib.c b/arch/x86/kernel/cpu/microcode/intel_lib.c
> > index 1cc6494..9200b83 100644
> > --- a/arch/x86/kernel/cpu/microcode/intel_lib.c
> > +++ b/arch/x86/kernel/cpu/microcode/intel_lib.c
> > @@ -49,8 +49,7 @@ int microcode_sanity_check(void *mc, int print_err)
> > unsigned long total_size, data_size, ext_table_size;
> > struct microcode_header_intel *mc_header = mc;
> > struct extended_sigtable *ext_header = NULL;
> > - int sum, orig_sum, ext_sigcount = 0, i;
> > - struct extended_signature *ext_sig;
> > + int orig_sum, i;
> >
> > total_size = get_totalsize(mc_header);
> > data_size = get_datasize(mc_header);
> > @@ -81,7 +80,6 @@ int microcode_sanity_check(void *mc, int print_err)
> > pr_err("error: bad exttable size in microcode data file\n");
> > return -EFAULT;
> > }
> > - ext_sigcount = ext_header->count;
> > }
> >
> > /*
> > @@ -129,21 +127,7 @@ int microcode_sanity_check(void *mc, int print_err)
> > pr_err("error: bad microcode update checksum\n");
> > return -EINVAL;
> > }
> > - if (!ext_table_size)
> > - return 0;
> > - /* check extended signature checksum */
> > - for (i = 0; i < ext_sigcount; i++) {
> > - ext_sig = (void *)ext_header + EXT_HEADER_SIZE +
> > - EXT_SIGNATURE_SIZE * i;
> > - sum = orig_sum
> > - - (mc_header->sig + mc_header->pf + mc_header->cksum)
> > - + (ext_sig->sig + ext_sig->pf + ext_sig->cksum);
> > - if (sum) {
> > - if (print_err)
> > - pr_err("error: bad extended signature checksum\n");
> > - return -EINVAL;
> > - }
> > - }
> > +
> > return 0;
> > }
> > EXPORT_SYMBOL_GPL(microcode_sanity_check);

--
"One disk to rule them all, One disk to find them. One disk to bring
them all and in the darkness grind them. In the Land of Redmond
where the shadows lie." -- The Silicon Valley Tarot
Henrique Holschuh