linux-kernel - [PATCH v3 5/5] x86/entry: Make IA32 syscalls' availability depend on ia32

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Date:   Fri, 16 Jun 2023 15:57:30 +0300
From:   Nikolay Borisov <nik.borisov@...e.com>
To:     x86@...nel.org
Cc:     linux-kernel@...r.kernel.org, mhocko@...e.com, jslaby@...e.cz,
        Nikolay Borisov <nik.borisov@...e.com>
Subject: [PATCH v3 5/5] 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 are invoked either by
using the legacy int 0x80 call gate interface or via the newer sysenter
instruction.

Ensure that if ia32 emulation is disabled (either at compile time or
runtime) then those 2 syscall mechanisms are also disabled.

Signed-off-by: Nikolay Borisov <nik.borisov@...e.com>
---
 arch/x86/include/asm/proto.h |  3 +++
 arch/x86/kernel/cpu/common.c | 37 ++++++++++++++++++------------------
 arch/x86/kernel/idt.c        | 10 ++++++++++
 3 files changed, 32 insertions(+), 18 deletions(-)

diff --git a/arch/x86/include/asm/proto.h b/arch/x86/include/asm/proto.h
index 12ef86b19910..1241d27fc4a6 100644
--- a/arch/x86/include/asm/proto.h
+++ b/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);
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index b20774181e1a..aafb83d1b3a7 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -59,6 +59,7 @@
 #include <asm/intel-family.h>
 #include <asm/cpu_device_id.h>
 #include <asm/uv/uv.h>
+#include <asm/ia32.h>
 #include <asm/sigframe.h>
 #include <asm/traps.h>
 #include <asm/sev.h>
@@ -2053,24 +2054,24 @@ 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
diff --git a/arch/x86/kernel/idt.c b/arch/x86/kernel/idt.c
index a58c6bc1cd68..c76953656212 100644
--- a/arch/x86/kernel/idt.c
+++ b/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)
@@ -226,6 +230,12 @@ 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
-- 
2.34.1