From: Sohil Mehta Date: Tue, 18 Nov 2025 18:29:04 +0000 (-0800) Subject: x86/cpu: Add an LASS dependency on SMAP X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=e39c5387adebf2839aaf5779cdd09a3506963fc5;p=thirdparty%2Flinux.git x86/cpu: Add an LASS dependency on SMAP 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 Signed-off-by: Dave Hansen Reviewed-by: Dave Hansen Link: https://patch.msgid.link/20251118182911.2983253-3-sohil.mehta%40intel.com --- diff --git a/arch/x86/kernel/cpu/cpuid-deps.c b/arch/x86/kernel/cpu/cpuid-deps.c index 46efcbd6afa41..98d0cdd825747 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 }, {} };