linux-kernel - [PATCH 02/19] x86/cpufeatures: Add CPU feature flags for shadow stacks

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Date:   Thu, 16 Jun 2022 04:46:26 -0400
From:   Yang Weijiang <weijiang.yang@...el.com>
To:     pbonzini@...hat.com, seanjc@...gle.com, x86@...nel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        rick.p.edgecombe@...el.com
Cc:     weijiang.yang@...el.com, Yu-cheng Yu <yu-cheng.yu@...el.com>,
        Kees Cook <keescook@...omium.org>
Subject: [PATCH 02/19] x86/cpufeatures: Add CPU feature flags for shadow stacks

From: Yu-cheng Yu <yu-cheng.yu@...el.com>

The Control-Flow Enforcement Technology contains two related features,
one of which is Shadow Stacks. Future patches will utilize this feature
for shadow stack support in KVM, so add a CPU feature flags for Shadow
Stacks (CPUID.(EAX=7,ECX=0):ECX[bit 7]).

To protect shadow stack state from malicious modification, the registers
are only accessible in supervisor mode. This implementation
context-switches the registers with XSAVES. Make X86_FEATURE_SHSTK depend
on XSAVES.

Signed-off-by: Yu-cheng Yu <yu-cheng.yu@...el.com>
Co-developed-by: Rick Edgecombe <rick.p.edgecombe@...el.com>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@...el.com>
Signed-off-by: Yang Weijiang <weijiang.yang@...el.com>
Cc: Kees Cook <keescook@...omium.org>

---
v2:
 - Remove IBT reference in commit log (Kees)
 - Describe xsaves dependency using text from (Dave)

v1:
 - Remove IBT, can be added in a follow on IBT series.

Yu-cheng v25:
 - Make X86_FEATURE_IBT depend on X86_FEATURE_SHSTK.

Yu-cheng v24:
 - Update for splitting CONFIG_X86_CET to CONFIG_X86_SHADOW_STACK and
   CONFIG_X86_IBT.
 - Move DISABLE_IBT definition to the IBT series.

 arch/x86/include/asm/cpufeatures.h       | 1 +
 arch/x86/include/asm/disabled-features.h | 8 +++++++-
 arch/x86/kernel/cpu/cpuid-deps.c         | 1 +
 3 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
index 393f2bbb5e3a..2a3aaf5e1052 100644
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -355,6 +355,7 @@
 #define X86_FEATURE_OSPKE		(16*32+ 4) /* OS Protection Keys Enable */
 #define X86_FEATURE_WAITPKG		(16*32+ 5) /* UMONITOR/UMWAIT/TPAUSE Instructions */
 #define X86_FEATURE_AVX512_VBMI2	(16*32+ 6) /* Additional AVX512 Vector Bit Manipulation Instructions */
+#define X86_FEATURE_SHSTK		(16*32+ 7) /* Shadow Stack */
 #define X86_FEATURE_GFNI		(16*32+ 8) /* Galois Field New Instructions */
 #define X86_FEATURE_VAES		(16*32+ 9) /* Vector AES */
 #define X86_FEATURE_VPCLMULQDQ		(16*32+10) /* Carry-Less Multiplication Double Quadword */
diff --git a/arch/x86/include/asm/disabled-features.h b/arch/x86/include/asm/disabled-features.h
index 36369e76cc63..c61c65bbc58d 100644
--- a/arch/x86/include/asm/disabled-features.h
+++ b/arch/x86/include/asm/disabled-features.h
@@ -68,6 +68,12 @@
 # define DISABLE_TDX_GUEST	(1 << (X86_FEATURE_TDX_GUEST & 31))
 #endif
 
+#ifdef CONFIG_X86_SHADOW_STACK
+#define DISABLE_SHSTK	0
+#else
+#define DISABLE_SHSTK	(1 << (X86_FEATURE_SHSTK & 31))
+#endif
+
 /*
  * Make sure to add features to the correct mask
  */
@@ -88,7 +94,7 @@
 #define DISABLED_MASK14	0
 #define DISABLED_MASK15	0
 #define DISABLED_MASK16	(DISABLE_PKU|DISABLE_OSPKE|DISABLE_LA57|DISABLE_UMIP| \
-			 DISABLE_ENQCMD)
+			 DISABLE_ENQCMD|DISABLE_SHSTK)
 #define DISABLED_MASK17	0
 #define DISABLED_MASK18	0
 #define DISABLED_MASK19	0
diff --git a/arch/x86/kernel/cpu/cpuid-deps.c b/arch/x86/kernel/cpu/cpuid-deps.c
index c881bcafba7d..bf1b55a1ba21 100644
--- a/arch/x86/kernel/cpu/cpuid-deps.c
+++ b/arch/x86/kernel/cpu/cpuid-deps.c
@@ -78,6 +78,7 @@ static const struct cpuid_dep cpuid_deps[] = {
 	{ X86_FEATURE_XFD,			X86_FEATURE_XSAVES    },
 	{ X86_FEATURE_XFD,			X86_FEATURE_XGETBV1   },
 	{ X86_FEATURE_AMX_TILE,			X86_FEATURE_XFD       },
+	{ X86_FEATURE_SHSTK,			X86_FEATURE_XSAVES    },
 	{}
 };
 
-- 
2.27.0