On Mon, Oct 18, 2021, Borislav Petkov wrote:
> On Wed, Oct 13, 2021 at 03:22:30PM +0100, Jane Malalane wrote:
> Totally untested of course.
>
> static void early_probe_null_seg_clearing_base(struct cpuinfo_x86 *c)
> {
> /*
> * A hypervisor may sythesize the bit, but may also hide it
> * for migration safety, so do not probe for model-specific
> * behaviour when virtualised.
> */
> if (cpu_has(c, X86_FEATURE_HYPERVISOR))
This isn't correct. When running as a guest, the intended behavior is to fully
trust the CPUID.0x80000021 bit. If bit 6 is set, yay, the hypervisor has told
the kernel that it will only ever run on hardware without the bug. If bit 6 is
clear and HYPERVISOR is true, then the FMS crud can't be trusted because the
kernel _may_ run on affected hardware in the future even if the current underlying
hardware is not affected.
> return;
>
> /* Zen3 CPUs advertise Null Selector Clears Base in CPUID. */
> if (c->extended_cpuid_level >= 0x80000021 && cpuid_eax(0x80000021) & BIT(6))
> return;
>
> /* Zen2 CPUs also have this behaviour, but no CPUID bit. */
> if (c->x86 == 0x17 && check_null_seg_clears_base(c))
> return;
>
> /* All the remaining ones are affected */
> set_cpu_bug(c, X86_BUG_NULL_SEG);
> }
...
> > @@ -1457,8 +1457,6 @@ static void generic_identify(struct cpuinfo_x86 *c)
> >
> > get_model_name(c); /* Default name */
> >
> > - detect_null_seg_behavior(c);
> > -
> > /*
> > * ESPFIX is a strange bug. All real CPUs have it. Paravirt
> > * systems that run Linux at CPL > 0 may or may not have the
>
> So this function is called on all x86 CPUs. Are you sure others besides
> AMD and Hygon do not have the same issue?
>
> IOW, I wouldn't remove that call here.
I agree. If the argument for this patch is that the kernel can be migrated to
older hardware, then it stands to reason that the kernel could also be migrated
to a different CPU vendor entirely. E.g. start on Intel, migrate to Zen1, kaboom.
