Re: [PATCHv6 00/16] x86: Enable Linear Address Space Separation support

H. Peter Anvin
2025-06-20T22:44:05Z

On June 20, 2025 3:04:53 PM PDT, Andrew Cooper <andrew.cooper3@citrix.com> wrote:
>> Most of the cover letter here reads like an Intel whitepaper. That's not
>> the worst thing in the world, but I think it glosses over one very
>> important point:
>>
>> Had it been available, LASS alone would have mitigated Meltdown.
>>
>> Could we say this up front in a prominent place, please?
>
>I'm going to nitpick. :)
>
>Yes, LASS would have made Meltdown a far less major problem than it was,
>but I don't think that phrasing is fair.  As I recall, LASS was
>literally invented as a "what would have been useful?" exercise in the
>wake of Meltdown.
>
>However, a less well known/researched area of Meltdown, which would not
>be addressed by LASS, is the ability to pend stores to read-only memory
>and proceed with the expectation that they'll success.
>
>Other things which would have helped would have been PKS (and this *was*
>asked for ahead of Skylake launching...)
>
>The other important thing about LASS is that it does cut off a whole
>class of sidechannels.  This halts definitely-rogue speculation, but is
>useful for non-speculative security too.
>
>~Andrew

To some degree, the best way to describe LASS is "KPTI in hardware." No, it isn't an exact analogy – we don't switch the entire page table structure – but it does make the supervisor page table structure protected from user space *while still in memory*.

It is coarser-grained than software KPTI, but on the other hand the separation is absolute; there is to keep things like the kernel stack, descriptor tables and entry/exit code unprotected.