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

Borislav Petkov
2014-10-30T20:26:00Z

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")

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.

Fenghua, care to comment please?

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);
> --
> 1.7.10.4
>
>

--
Regards/Gruss,
Boris.

Sent from a fat crate under my desk. Formatting is fine.
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