[PATCH v2 04/39] x86/cpufeatures: Enable CET CR4 bit for shadow stack

[Rick Edgecombe <rick.p.edgecombe@intel.com>]

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

Utilizing CET features requires a CR4 bit to be enabled as well as bits
to be set in CET MSRs. Setting the CR4 bit does two things:
 1. Enables the usage of WRUSS instruction, which the kernel can use to
    write to userspace shadow stacks.
 2. Allows those individual aspects of CET to be enabled later via the MSR.
 3. Allows CET to be enabled in guests

While future patches will allow the MSR values to be saved and restored
per task, the CR4 bit will allow for WRUSS to be used regardless of if a
tasks CET MSRs have been restored.

Kernel IBT already enables the CET CR4 bit when it detects IBT HW support
and is configured with kernel IBT. However future patches that enable
userspace shadow stack support will need the bit set as well. So change
the logic to enable it in either case.

Clear MSR_IA32_U_CET in cet_disable() so that it can't live to see
userspace in a new kexec-ed kernel that has CR4.CET set from kernel IBT.

Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
Co-developed-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Cc: Kees Cook <keescook@chromium.org>

---

v2:
 - In the shadow stack case, go back to only setting CR4.CET if the
   kernel is compiled with user shadow stack support.
 - Clear MSR_IA32_U_CET as well. (PeterZ)

KVM refresh:
 - Set CR4.CET if SHSTK or IBT are supported by HW, so that KVM can
   support CET even if IBT is disabled.
 - Drop no_user_shstk (Dave Hansen)
 - Elaborate on what the CR4 bit does in the commit log
 - Integrate with Kernel IBT logic

v1:
 - Moved kernel-parameters.txt changes here from patch 1.

Yu-cheng v25:
 - Remove software-defined X86_FEATURE_CET.

 arch/x86/kernel/cpu/common.c | 22 ++++++++++++++++------
 1 file changed, 16 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 3e508f239098..d7415bb556b2 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -598,16 +598,21 @@ __noendbr void ibt_restore(u64 save)
 
 static __always_inline void setup_cet(struct cpuinfo_x86 *c)
 {
-	u64 msr = CET_ENDBR_EN;
+	bool kernel_ibt = HAS_KERNEL_IBT && cpu_feature_enabled(X86_FEATURE_IBT);
+	bool user_shstk = IS_ENABLED(CONFIG_X86_SHADOW_STACK) &&
+			  cpu_feature_enabled(X86_FEATURE_SHSTK);
+	u64 msr = 0;
 
-	if (!HAS_KERNEL_IBT ||
-	    !cpu_feature_enabled(X86_FEATURE_IBT))
+	if (!kernel_ibt && !user_shstk)
 		return;
 
+	if (kernel_ibt)
+		msr = CET_ENDBR_EN;
+
 	wrmsrl(MSR_IA32_S_CET, msr);
 	cr4_set_bits(X86_CR4_CET);
 
-	if (!ibt_selftest()) {
+	if (kernel_ibt && !ibt_selftest()) {
 		pr_err("IBT selftest: Failed!\n");
 		setup_clear_cpu_cap(X86_FEATURE_IBT);
 		return;
@@ -616,10 +621,15 @@ static __always_inline void setup_cet(struct cpuinfo_x86 *c)
 
 __noendbr void cet_disable(void)
 {
-	if (cpu_feature_enabled(X86_FEATURE_IBT))
-		wrmsrl(MSR_IA32_S_CET, 0);
+	if (!(cpu_feature_enabled(X86_FEATURE_IBT) ||
+	      cpu_feature_enabled(X86_FEATURE_SHSTK)))
+		return;
+
+	wrmsrl(MSR_IA32_S_CET, 0);
+	wrmsrl(MSR_IA32_U_CET, 0);
 }
 
+
 /*
  * Some CPU features depend on higher CPUID levels, which may not always
  * be available due to CPUID level capping or broken virtualization
-- 
2.17.1