[PATCH v11 2/9] x86/cpu: Add an LASS dependency on SMAP

Sohil Mehta
2025-10-29T21:06:13Z

With LASS enabled, any kernel data access to userspace typically results
in a #GP, or a #SS in some stack-related cases. When the kernel needs to
access user memory, it can suspend LASS enforcement by toggling the
RFLAGS.AC bit. Most of these cases are already covered by the
stac()/clac() pairs used to avoid SMAP violations.

Even though LASS could potentially be enabled independently, it would be
very painful without SMAP and the related stac()/clac() calls. There is
no reason to support such a configuration because all future hardware
with LASS is expected to have SMAP as well. Also, the STAC/CLAC
instructions are architected to:
#UD - If CPUID.(EAX=07H, ECX=0H):EBX.SMAP[bit 20] = 0.

So, make LASS depend on SMAP to conveniently reuse the existing AC bit
toggling already in place.

Note: Additional STAC/CLAC would still be needed for accesses such as
text poking which are not flagged by SMAP. This is because such mappings
are in the lower half but do not have the _PAGE_USER bit set which SMAP
uses for enforcement.

Signed-off-by: Sohil Mehta <sohil.mehta@intel.com>
---
v11:
- New patch (split from patch 1).
---
arch/x86/kernel/cpu/cpuid-deps.c | 1 +
1 file changed, 1 insertion(+)

diff --git a/arch/x86/kernel/cpu/cpuid-deps.c b/arch/x86/kernel/cpu/cpuid-deps.c
index 46efcbd6afa4..98d0cdd82574 100644
--- a/arch/x86/kernel/cpu/cpuid-deps.c
+++ b/arch/x86/kernel/cpu/cpuid-deps.c
@@ -89,6 +89,7 @@ static const struct cpuid_dep cpuid_deps[] = {
{ X86_FEATURE_SHSTK, X86_FEATURE_XSAVES },
{ X86_FEATURE_FRED, X86_FEATURE_LKGS },
{ X86_FEATURE_SPEC_CTRL_SSBD, X86_FEATURE_SPEC_CTRL },
+ { X86_FEATURE_LASS, X86_FEATURE_SMAP },
{}
};

--
2.43.0