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SubjectRe: [PATCH 1/7] x86/cpu: Enumerate LASS CPUID and CR4 bits
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On 1/10/2023 4:13 PM, Dave Hansen wrote:
> On 1/10/23 12:14, Sohil Mehta wrote:
>> On 1/9/2023 9:51 PM, Yian Chen wrote:
>>> LASS (Linear Address Space Separation) is a CPU feature to
>>> prevent speculative address access in user/kernel mode.
>>
>> Would it be better to say?
>>
>> LASS (Linear Address Space Separation) is a security feature that
>> intends to prevent unintentional speculative address access across
>> user/kernel mode.
>
> It's more than that, though. The spec actually says this pretty nicely:
> >> Linear-address space separation (LASS) is an independent mechanism
>> that enforces the same mode-based protections as paging but without
>> traversing the paging structures. Because the protections enforced by
>> LASS are applied before paging, “probes” by malicious software will
>> provide no paging-based timing information
>
> So, it's not _just_ that it can prevent some speculative accesses. It
> completely short-circuits paging itself and *ALL* of the baggage that
> goes along with paging.
>
> The TLB, mid-level caches, the page walker itself, the data cache
> impact... all of it. Gone.
>
> *THAT* is the important part here, IMNHO.
sure, how about if I rewrite the 1st paragraph as follows:

LASS (Linear Address Space Separation) is an independent security
mechanism that enforces kernel and user mode-base protections, similar
to Intel SMAP/SMEP in kernel mode, but enhanced without traversing the
paging structures. Therefore, the protection will not leak paging-based
timing information and prevent malicious software from probing the info.
The LASS details have been published in Chapter 11 in
https://cdrdv2.intel.com/v1/dl/getContent/671368

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