x86/entry: Make IA32 syscalls' availability depend on ia32_enabled()

Another major aspect of supporting running of 32bit processes is the
ability to access 32bit syscalls. Such syscalls can be invoked by
using the legacy int 0x80 handler and  sysenter/syscall instructions.

If IA32 emulation is disabled ensure that each of those 3 distinct
mechanisms are also disabled. For int 0x80 a #GP exception would be
generated since the respective descriptor is not going to be loaded at
all. Invoking sysenter will also result in a #GP since IA32_SYSENTER_CS
contains an invalid segment. Finally, syscall instruction cannot really
be disabled so it's configured to execute a minimal handler.

Signed-off-by: Nikolay Borisov <nik.borisov@suse.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20230623111409.3047467-6-nik.borisov@suse.com

Author: Nikolay Borisov

arch/x86/include/asm/proto.h

@@ -36,6 +36,9 @@ void entry_INT80_compat(void);
 #ifdef CONFIG_XEN_PV
 void xen_entry_INT80_compat(void);
 #endif
+#else /* !CONFIG_IA32_EMULATION */
+#define entry_SYSCALL_compat NULL
+#define entry_SYSENTER_compat NULL
 #endif
 
 void x86_configure_nx(void);

arch/x86/kernel/cpu/common.c

@@ -62,6 +62,7 @@
 #include <asm/intel-family.h>
 #include <asm/cpu_device_id.h>
 #include <asm/uv/uv.h>
+#include <asm/ia32.h>
 #include <asm/set_memory.h>
 #include <asm/traps.h>
 #include <asm/sev.h>
@@ -2074,24 +2075,24 @@ void syscall_init(void)
 	wrmsr(MSR_STAR, 0, (__USER32_CS << 16) | __KERNEL_CS);
 	wrmsrl(MSR_LSTAR, (unsigned long)entry_SYSCALL_64);
 
-#ifdef CONFIG_IA32_EMULATION
-	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.
-	 * This does not cause SYSENTER to jump to the wrong location, because
-	 * AMD doesn't allow SYSENTER in long mode (either 32- or 64-bit).
-	 */
-	wrmsrl_safe(MSR_IA32_SYSENTER_CS, (u64)__KERNEL_CS);
-	wrmsrl_safe(MSR_IA32_SYSENTER_ESP,
-		    (unsigned long)(cpu_entry_stack(smp_processor_id()) + 1));
-	wrmsrl_safe(MSR_IA32_SYSENTER_EIP, (u64)entry_SYSENTER_compat);
-#else
-	wrmsrl_cstar((unsigned long)entry_SYSCALL32_ignore);
-	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);
-#endif
+	if (ia32_enabled()) {
+		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.
+		 * This does not cause SYSENTER to jump to the wrong location, because
+		 * AMD doesn't allow SYSENTER in long mode (either 32- or 64-bit).
+		 */
+		wrmsrl_safe(MSR_IA32_SYSENTER_CS, (u64)__KERNEL_CS);
+		wrmsrl_safe(MSR_IA32_SYSENTER_ESP,
+			    (unsigned long)(cpu_entry_stack(smp_processor_id()) + 1));
+		wrmsrl_safe(MSR_IA32_SYSENTER_EIP, (u64)entry_SYSENTER_compat);
+	} else {
+		wrmsrl_cstar((unsigned long)entry_SYSCALL32_ignore);
+		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);
+	}
 
 	/*
 	 * Flags to clear on syscall; clear as much as possible

arch/x86/kernel/idt.c

@@ -10,6 +10,7 @@
 #include <asm/proto.h>
 #include <asm/desc.h>
 #include <asm/hw_irq.h>
+#include <asm/ia32.h>
 #include <asm/idtentry.h>
 
 #define DPL0		0x0
@@ -116,6 +117,9 @@ static const __initconst struct idt_data def_idts[] = {
 #endif
 
 	SYSG(X86_TRAP_OF,		asm_exc_overflow),
+};
+
+static const struct idt_data ia32_idt[] __initconst = {
 #if defined(CONFIG_IA32_EMULATION)
 	SYSG(IA32_SYSCALL_VECTOR,	entry_INT80_compat),
 #elif defined(CONFIG_X86_32)
@@ -225,6 +229,9 @@ void __init idt_setup_early_traps(void)
 void __init idt_setup_traps(void)
 {
 	idt_setup_from_table(idt_table, def_idts, ARRAY_SIZE(def_idts), true);
+
+	if (ia32_enabled())
+		idt_setup_from_table(idt_table, ia32_idt, ARRAY_SIZE(ia32_idt), true);
 }
 
 #ifdef CONFIG_X86_64