[RFC PATCH 1/8] x86/fpu/xstate: Manually check and add XFEATURE_CET_USER xstate bit

[Yang Weijiang]

Remove XFEATURE_CET_USER entry from dependency array as the entry doesn't
reflect true dependency between CET features and the xstate bit, instead
manually check and add the bit back if either SHSTK or IBT is supported.

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.

Although in real world a platform with IBT but no SHSTK is rare, but in
virtualization world it's common, guest SHSTK and IBT can be controlled
independently via userspace app.

Signed-off-by: Yang Weijiang <weijiang.yang@xxxxxxxxx>
---
 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 cadf68737e6b..12c8cb278346 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);
 	}
 
+	/*
+	 * Manually add CET user mode xstate bit if either SHSTK or IBT is
+	 * available. Both features depend on the xstate bit to save/restore
+	 * CET user mode state.
+	 */
+	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;
 
-- 
2.27.0