[PATCH v4 04/39] x86/cpufeatures: Enable CET CR4 bit for shadow stack

[Rick Edgecombe]

From: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>

Setting CR4.CET is a prerequisite for utilizing any CET features, most of
which also require setting MSRs.

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.

Tested-by: Pengfei Xu <pengfei.xu@xxxxxxxxx>
Tested-by: John Allen <john.allen@xxxxxxx>
Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
Co-developed-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
---

v4:
 - Add back dedicated command line disable: "nousershtk" (Boris)

v3:
 - Remove stay new line (Boris)
 - Simplify commit log (Andrew Cooper)

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

 arch/x86/kernel/cpu/common.c | 37 ++++++++++++++++++++++++++++++------
 1 file changed, 31 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 73cc546e024d..579f10222432 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -597,29 +597,51 @@ __noendbr void ibt_restore(u64 save)
 
 #endif
 
+#ifdef CONFIG_X86_CET
 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;
+	u64 msr = 0;
+
+	/*
+	 * Enable user shadow stack only if the Linux defined user shadow stack
+	 * cap was not cleared by command line.
+	 */
+	user_shstk = cpu_feature_enabled(X86_FEATURE_SHSTK) &&
+		     IS_ENABLED(CONFIG_X86_USER_SHADOW_STACK);
 
-	if (!HAS_KERNEL_IBT ||
-	    !cpu_feature_enabled(X86_FEATURE_IBT))
+	if (!kernel_ibt && !user_shstk)
 		return;
 
+	if (user_shstk)
+		set_cpu_cap(c, X86_FEATURE_USER_SHSTK);
+
+	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");
 		wrmsrl(MSR_IA32_S_CET, 0);
 		setup_clear_cpu_cap(X86_FEATURE_IBT);
 		return;
 	}
 }
+#else /* CONFIG_X86_CET */
+static inline void setup_cet(struct cpuinfo_x86 *c) {}
+#endif
 
 __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);
 }
 
 /*
@@ -1470,6 +1492,9 @@ static void __init cpu_parse_early_param(void)
 	if (cmdline_find_option_bool(boot_command_line, "noxsaves"))
 		setup_clear_cpu_cap(X86_FEATURE_XSAVES);
 
+	if (cmdline_find_option_bool(boot_command_line, "nousershstk"))
+		setup_clear_cpu_cap(X86_FEATURE_USER_SHSTK);
+
 	arglen = cmdline_find_option(boot_command_line, "clearcpuid", arg, sizeof(arg));
 	if (arglen <= 0)
 		return;
-- 
2.17.1