Re: [PATCH 1/7] x86/cpu: Enumerate LASS CPUID and CR4 bits

[Chen, Yian]

On 1/10/2023 12:14 PM, 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.
Sure, I will revise the statement precisely.

> > LASS partitions 64-bit virtual address space into two
> > halves, lower address (LA[63]=0) and upper address
> > (LA[63]=1). It stops any data access or code execution
> >      1. from upper half address space to any lower half address
> >      2, from lower half address space to any upper half address
> > and generates #GP fault for a violation.
>
> I am not sure if this is the best way to say it. The kernel already 
> partitions the address space this way. LASS takes what is already the 
> typical OS implementation and bakes it into the hardware architecture.
Yes, LASS by design matches the addressing usage in OS. I will try to 
include this in the statement.
> > In Linux, this means LASS does not allow both kernel code
> > to access any user space address and user code to access
> > any kernel space address.
>
> There is clearly an overlap between the protections provided by paging 
> and with SMAP and SMEP. It would be useful to paraphrase some of the 
> information mentioned in the spec regarding how LASS differs from them.
Yes, I will differentiate between LASS and SMAP more clearly.

> "With these mode-based protections, paging can prevent malicious 
> software from directly reading or writing memory inappropriately. To 
> enforce these protections, the processor must traverse the hierarchy of 
> paging structures in memory. Unprivileged software can use timing 
> information resulting from this traversal to determine details about the 
> paging structures, and these details may be used to determine the layout 
> of supervisor memory.
>
> 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."
Yes, I will also state the advantage of LASS.

> > Signed-off-by: Yian Chen <yian.chen@xxxxxxxxx>
> > Reviewed-by: Tony Luck <tony.luck@xxxxxxxxx>

Thanks,
Yian