Messages in this thread | | | Date | Tue, 10 Jan 2023 16:13:18 -0800 | Subject | Re: [PATCH 1/7] x86/cpu: Enumerate LASS CPUID and CR4 bits | From | Dave Hansen <> |
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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.
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