linux-kernel - [tip: x86/cpu] x86/cpu: Add an LASS dependency on SMAP

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Message-ID: <176349412242.498.13461166073125191566.tip-bot2@tip-bot2>
Date: Tue, 18 Nov 2025 19:28:42 -0000
From: "tip-bot2 for Sohil Mehta" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: Sohil Mehta <sohil.mehta@...el.com>,
 Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
 linux-kernel@...r.kernel.org
Subject: [tip: x86/cpu] x86/cpu: Add an LASS dependency on SMAP

The following commit has been merged into the x86/cpu branch of tip:

Commit-ID:     e39c5387adebf2839aaf5779cdd09a3506963fc5
Gitweb:        https://git.kernel.org/tip/e39c5387adebf2839aaf5779cdd09a3506963fc5
Author:        Sohil Mehta <sohil.mehta@...el.com>
AuthorDate:    Tue, 18 Nov 2025 10:29:04 -08:00
Committer:     Dave Hansen <dave.hansen@...ux.intel.com>
CommitterDate: Tue, 18 Nov 2025 10:38:26 -08:00

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 <sohil.mehta@...el.com>
Signed-off-by: Dave Hansen <dave.hansen@...ux.intel.com>
Reviewed-by: Dave Hansen <dave.hansen@...ux.intel.com>
Link: https://patch.msgid.link/20251118182911.2983253-3-sohil.mehta%40intel.com
---
 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 46efcbd..98d0cdd 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      },
 	{}
 };