linux-kernel - [PATCH v8 02/26] x86/fpu/xstate: Refine CET user xstate bit enabling

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Message-Id: <20231221140239.4349-3-weijiang.yang@intel.com>
Date: Thu, 21 Dec 2023 09:02:15 -0500
From: Yang Weijiang <weijiang.yang@...el.com>
To: seanjc@...gle.com,
	pbonzini@...hat.com,
	dave.hansen@...el.com,
	kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Cc: peterz@...radead.org,
	chao.gao@...el.com,
	rick.p.edgecombe@...el.com,
	mlevitsk@...hat.com,
	john.allen@....com,
	weijiang.yang@...el.com
Subject: [PATCH v8 02/26] x86/fpu/xstate: Refine CET user xstate bit enabling

Remove XFEATURE_CET_USER entry from dependency array as the entry doesn't
reflect true dependency between CET features and the user xstate bit.
Enable the bit in fpu_kernel_cfg.max_features when either SHSTK or IBT is
available.

Both user mode shadow stack and indirect branch tracking features depend
on XFEATURE_CET_USER bit in XSS to automatically save/restore user mode
xstate registers, i.e., IA32_U_CET and IA32_PL3_SSP whenever necessary.

Note, the issue, i.e., CPUID only enumerates IBT but no SHSTK is resulted
from CET KVM series which synthesizes guest CPUIDs based on userspace
settings,in real world the case is rare. In other words, the existing
dependency check is correct when only user mode SHSTK is available.

Signed-off-by: Yang Weijiang <weijiang.yang@...el.com>
Reviewed-by: Rick Edgecombe <rick.p.edgecombe@...el.com>
Tested-by: Rick Edgecombe <rick.p.edgecombe@...el.com>
---
 arch/x86/kernel/fpu/xstate.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
index 07911532b108..f6b98693da59 100644
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c
@@ -73,7 +73,6 @@ static unsigned short xsave_cpuid_features[] __initdata = {
 	[XFEATURE_PT_UNIMPLEMENTED_SO_FAR]	= X86_FEATURE_INTEL_PT,
 	[XFEATURE_PKRU]				= X86_FEATURE_OSPKE,
 	[XFEATURE_PASID]			= X86_FEATURE_ENQCMD,
-	[XFEATURE_CET_USER]			= X86_FEATURE_SHSTK,
 	[XFEATURE_XTILE_CFG]			= X86_FEATURE_AMX_TILE,
 	[XFEATURE_XTILE_DATA]			= X86_FEATURE_AMX_TILE,
 };
@@ -798,6 +797,14 @@ void __init fpu__init_system_xstate(unsigned int legacy_size)
 			fpu_kernel_cfg.max_features &= ~BIT_ULL(i);
 	}
 
+	/*
+	 * CET user mode xstate bit has been cleared by above sanity check.
+	 * Now pick it up if either SHSTK or IBT is available. Either feature
+	 * depends on the xstate bit to save/restore user mode states.
+	 */
+	if (boot_cpu_has(X86_FEATURE_SHSTK) || boot_cpu_has(X86_FEATURE_IBT))
+		fpu_kernel_cfg.max_features |= BIT_ULL(XFEATURE_CET_USER);
+
 	if (!cpu_feature_enabled(X86_FEATURE_XFD))
 		fpu_kernel_cfg.max_features &= ~XFEATURE_MASK_USER_DYNAMIC;
 
-- 
2.39.3