[PATCH v9 38/42] x86/cpufeatures: Enable CET CR4 bit for shadow stack

[Rick Edgecombe]

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.

Co-developed-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
Reviewed-by: Borislav Petkov (AMD) <bp@xxxxxxxxx>
Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
Acked-by: Mike Rapoport (IBM) <rppt@xxxxxxxxxx>
Tested-by: Pengfei Xu <pengfei.xu@xxxxxxxxx>
Tested-by: John Allen <john.allen@xxxxxxx>
Tested-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 arch/x86/kernel/cpu/common.c | 35 +++++++++++++++++++++++++++--------
 1 file changed, 27 insertions(+), 8 deletions(-)

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 80710a68ef7d..3ea06b0b4570 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -601,27 +601,43 @@ __noendbr void ibt_restore(u64 save)
 
 static __always_inline void setup_cet(struct cpuinfo_x86 *c)
 {
-	u64 msr = CET_ENDBR_EN;
+	bool user_shstk, kernel_ibt;
 
-	if (!HAS_KERNEL_IBT ||
-	    !cpu_feature_enabled(X86_FEATURE_IBT))
+	if (!IS_ENABLED(CONFIG_X86_CET))
 		return;
 
-	wrmsrl(MSR_IA32_S_CET, msr);
+	kernel_ibt = HAS_KERNEL_IBT && cpu_feature_enabled(X86_FEATURE_IBT);
+	user_shstk = cpu_feature_enabled(X86_FEATURE_SHSTK) &&
+		     IS_ENABLED(CONFIG_X86_USER_SHADOW_STACK);
+
+	if (!kernel_ibt && !user_shstk)
+		return;
+
+	if (user_shstk)
+		set_cpu_cap(c, X86_FEATURE_USER_SHSTK);
+
+	if (kernel_ibt)
+		wrmsrl(MSR_IA32_S_CET, CET_ENDBR_EN);
+	else
+		wrmsrl(MSR_IA32_S_CET, 0);
+
 	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;
 	}
 }
 
 __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);
 }
 
 /*
@@ -1483,6 +1499,9 @@ 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;
-- 
2.34.1