linux-kernel - [tip: x86/cpu] x86/cpu: Don't write CSTAR MSR on Intel CPUs

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Date:   Wed, 24 Nov 2021 23:45:52 -0000
From:   "tip-bot2 for Andi Kleen" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Andi Kleen <ak@...ux.intel.com>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tony Luck <tony.luck@...el.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [tip: x86/cpu] x86/cpu: Don't write CSTAR MSR on Intel CPUs

The following commit has been merged into the x86/cpu branch of tip:

Commit-ID:     9c7e2634f647630db4e0719391dd80cd81132a66
Gitweb:        https://git.kernel.org/tip/9c7e2634f647630db4e0719391dd80cd81132a66
Author:        Andi Kleen <ak@...ux.intel.com>
AuthorDate:    Thu, 18 Nov 2021 19:58:03 -08:00
Committer:     Thomas Gleixner <tglx@...utronix.de>
CommitterDate: Thu, 25 Nov 2021 00:40:34 +01:00

x86/cpu: Don't write CSTAR MSR on Intel CPUs

Intel CPUs do not support SYSCALL in 32-bit mode, but the kernel
initializes MSR_CSTAR unconditionally. That MSR write is normally
ignored by the CPU, but in a TDX guest it raises a #VE trap.

Exclude Intel CPUs from the MSR_CSTAR initialization.

[ tglx: Fixed the subject line and removed the redundant comment. ]

Signed-off-by: Andi Kleen <ak@...ux.intel.com>
Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@...ux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Reviewed-by: Tony Luck <tony.luck@...el.com>
Link: https://lore.kernel.org/r/20211119035803.4012145-1-sathyanarayanan.kuppuswamy@linux.intel.com

---
 arch/x86/kernel/cpu/common.c | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 0083464..0663642 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1787,6 +1787,17 @@ EXPORT_PER_CPU_SYMBOL(__preempt_count);
 
 DEFINE_PER_CPU(unsigned long, cpu_current_top_of_stack) = TOP_OF_INIT_STACK;
 
+static void wrmsrl_cstar(unsigned long val)
+{
+	/*
+	 * Intel CPUs do not support 32-bit SYSCALL. Writing to MSR_CSTAR
+	 * is so far ignored by the CPU, but raises a #VE trap in a TDX
+	 * guest. Avoid the pointless write on all Intel CPUs.
+	 */
+	if (boot_cpu_data.x86_vendor != X86_VENDOR_INTEL)
+		wrmsrl(MSR_CSTAR, val);
+}
+
 /* May not be marked __init: used by software suspend */
 void syscall_init(void)
 {
@@ -1794,7 +1805,7 @@ void syscall_init(void)
 	wrmsrl(MSR_LSTAR, (unsigned long)entry_SYSCALL_64);
 
 #ifdef CONFIG_IA32_EMULATION
-	wrmsrl(MSR_CSTAR, (unsigned long)entry_SYSCALL_compat);
+	wrmsrl_cstar((unsigned long)entry_SYSCALL_compat);
 	/*
 	 * This only works on Intel CPUs.
 	 * On AMD CPUs these MSRs are 32-bit, CPU truncates MSR_IA32_SYSENTER_EIP.
@@ -1806,7 +1817,7 @@ void syscall_init(void)
 		    (unsigned long)(cpu_entry_stack(smp_processor_id()) + 1));
 	wrmsrl_safe(MSR_IA32_SYSENTER_EIP, (u64)entry_SYSENTER_compat);
 #else
-	wrmsrl(MSR_CSTAR, (unsigned long)ignore_sysret);
+	wrmsrl_cstar((unsigned long)ignore_sysret);
 	wrmsrl_safe(MSR_IA32_SYSENTER_CS, (u64)GDT_ENTRY_INVALID_SEG);
 	wrmsrl_safe(MSR_IA32_SYSENTER_ESP, 0ULL);
 	wrmsrl_safe(MSR_IA32_SYSENTER_EIP, 0ULL);