Re: [PATCHv7 01/16] x86/cpu: Enumerate the LASS feature bits

Borislav Petkov
2025-06-26T15:23:15Z

On Wed, Jun 25, 2025 at 03:50:53PM +0300, Kirill A. Shutemov wrote:
> From: Sohil Mehta <sohil.mehta@intel.com>
>
> Linear Address Space Separation (LASS) is a security feature that
> intends to prevent malicious virtual address space accesses across
> user/kernel mode.
>
> Such mode based access protection already exists today with paging and
> features such as SMEP and SMAP. However, to enforce these protections,
> the processor must traverse the paging structures in memory. Malicious
> software can use timing information resulting from this traversal to
> determine details about the paging structures, and these details may
> also be used to determine the layout of the kernel memory.
>
> The LASS mechanism provides the same mode-based protections as paging
> but without traversing the paging structures. Because the protections
> enforced by LASS are applied before paging, software will not be able to
> derive paging-based timing information from the various caching
> structures such as the TLBs, mid-level caches, page walker, data caches,
> etc.
>
> LASS enforcement relies on the typical kernel implementation to divide
> the 64-bit virtual address space into two halves:
> Addr[63]=0 -> User address space
> Addr[63]=1 -> Kernel address space
>
> Any data access or code execution across address spaces typically
> results in a #GP fault.
>
> The LASS enforcement for kernel data access is dependent on CR4.SMAP
> being set. The enforcement can be disabled by toggling the RFLAGS.AC bit
> similar to SMAP.
>
> Define the CPU feature bits to enumerate this feature and include
> feature dependencies to reflect the same.
>
> LASS provides protection against a class of speculative attacks, such as
> SLAM[1]. Add the "lass" flag to /proc/cpuinfo to indicate that the feature
> is supported by hardware and enabled by the kernel. This allows userspace
> to determine if the setup is secure against such attacks.
>
> [1] https://download.vusec.net/papers/slam_sp24.pdf
>
> Co-developed-by: Yian Chen <yian.chen@intel.com>
> Signed-off-by: Yian Chen <yian.chen@intel.com>
> Signed-off-by: Sohil Mehta <sohil.mehta@intel.com>
> Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> ---
> arch/x86/Kconfig.cpufeatures | 4 ++++
> arch/x86/include/asm/cpufeatures.h | 1 +
> arch/x86/include/uapi/asm/processor-flags.h | 2 ++
> arch/x86/kernel/cpu/cpuid-deps.c | 1 +
> tools/arch/x86/include/asm/cpufeatures.h | 1 +
> 5 files changed, 9 insertions(+)

Reviewed-by: Borislav Petkov (AMD) <bp@alien8.de>

--
Regards/Gruss,
Boris.