Re: [PATCH 8/8] x86, microcode, intel: defend apply_microcode_intel with BUG_ON

Henrique de Moraes Holschuh
2014-11-07T22:56:47Z

On Fri, 07 Nov 2014, Borislav Petkov wrote:
> On Mon, Sep 08, 2014 at 02:37:54PM -0300, Henrique de Moraes Holschuh wrote:
> > Microcode updates that requires an unknown loader should never reach the
> > apply_* functions (the code should have rejected it earlier). Likewise
> > for an unknown microcode header layout.
> >
> > Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
> > ---
> > arch/x86/kernel/cpu/microcode/intel.c | 2 ++
> > arch/x86/kernel/cpu/microcode/intel_early.c | 2 ++
> > 2 files changed, 4 insertions(+)
> >
> > diff --git a/arch/x86/kernel/cpu/microcode/intel.c b/arch/x86/kernel/cpu/microcode/intel.c
> > index 40caef1..439681f 100644
> > --- a/arch/x86/kernel/cpu/microcode/intel.c
> > +++ b/arch/x86/kernel/cpu/microcode/intel.c
> > @@ -157,6 +157,8 @@ static int apply_microcode_intel(int cpu)
> > if (mc_intel == NULL)
> > return 0;
> >
> > + BUG_ON(mc_intel->hdr.hdrver != 1 || mc_intel->hdr.ldrver != 1);
> > +
> > /* Intel SDM vol 3A section 9.11.6, page 9-34 */
> > if (WARN_ONCE((unsigned long)(mc_intel->bits) % 16,
> > "microcode data incorrectly aligned"))
> > diff --git a/arch/x86/kernel/cpu/microcode/intel_early.c b/arch/x86/kernel/cpu/microcode/intel_early.c
> > index 994c59b..095db11 100644
> > --- a/arch/x86/kernel/cpu/microcode/intel_early.c
> > +++ b/arch/x86/kernel/cpu/microcode/intel_early.c
> > @@ -671,6 +671,8 @@ static int apply_microcode_early(struct mc_saved_data *mc_saved_data,
> > if (mc_intel == NULL)
> > return 0;
> >
> > + BUG_ON(mc_intel->hdr.hdrver != 1 || mc_intel->hdr.ldrver != 1);
> > +
> > mcu_data = mc_intel->bits;
> > aligned_mcu_data = mc_intel->bits;
>
> Both not needed, because we're running all microcode through
> microcode_sanity_check() first which already does that check.

Yeah, that's why it is BUG_ON().

But if you feel this is too defensive, I will just drop this patch.

--
"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