Re: [PATCH v2] x86/cpu: Fix migration safety with X86_BUG_NULL_SEL

[Greg KH]

On Wed, Oct 13, 2021 at 03:22:30PM +0100, Jane Malalane wrote:
> Currently, Linux probes for X86_BUG_NULL_SEL unconditionally which
> makes it unsafe to migrate in a virtualised environment as the
> properties across the migration pool might differ.
> 
> To be specific, the case which goes wrong is:
> 
> 1. Zen1 (or earlier) and Zen2 (or later) in a migration pool
> 2. Linux boots on Zen2, probes and finds the absence of X86_BUG_NULL_SEL
> 3. Linux is then migrated to Zen1
> 
> Linux is now running on a X86_BUG_NULL_SEL-impacted CPU while believing
> that the bug is fixed.
> 
> The only way to address the problem is to fully trust the "no longer
> affected" CPUID bit when virtualised, because in the above case it would
> be clear deliberately to indicate the fact "you might migrate to
> somewhere which has this behaviour".
> 
> Zen3 adds the NullSelectorClearsBase bit to indicate that loading
> a NULL segment selector zeroes the base and limit fields, as well as
> just attributes. Zen2 also has this behaviour but doesn't have the
> NSCB bit.
> 
> Signed-off-by: Jane Malalane <jane.malalane@xxxxxxxxxx>
> ---
> CC: <x86@xxxxxxxxxx>
> CC: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> CC: Ingo Molnar <mingo@xxxxxxxxxx>
> CC: Borislav Petkov <bp@xxxxxxxxx>
> CC: "H. Peter Anvin" <hpa@xxxxxxxxx>
> CC: Pu Wen <puwen@xxxxxxxx>
> CC: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> CC: Sean Christopherson <seanjc@xxxxxxxxxx>
> CC: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> CC: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> CC: Yazen Ghannam <Yazen.Ghannam@xxxxxxx>
> CC: Brijesh Singh <brijesh.singh@xxxxxxx>
> CC: Huang Rui <ray.huang@xxxxxxx>
> CC: Andy Lutomirski <luto@xxxxxxxxxx>
> CC: Kim Phillips <kim.phillips@xxxxxxx>
> CC: <stable@xxxxxxxxxxxxxxx>

These need to go above the --- line, otherwise they are cut off when
the patch is applied and you will loose the cc: stable@ tag.

thanks,

greg k-h