linux-kernel - [PATCH 2/3] x86/entry: Disable IA32 syscalls in the presence of ia32

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Message-Id: <20230607072936.3766231-3-nik.borisov@suse.com>
Date:   Wed,  7 Jun 2023 10:29:35 +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 2/3] x86/entry: Disable IA32 syscalls in the presence of ia32_disabled

First stage of disabling ia32 compat layer is to disable 32bit syscall
entry points. Legacy int 0x80 vector is disabled by setting its gate
descriptor to "not present" and the sysenter vector is disabled by
re-using the existing code in case IA32_EMULATION is disabled.

Signed-off-by: Nikolay Borisov <nik.borisov@...e.com>
---
 arch/x86/entry/entry_64.S    |  2 --
 arch/x86/include/asm/desc.h  |  5 +++++
 arch/x86/kernel/cpu/common.c | 29 ++++++++++++++++++-----------
 3 files changed, 23 insertions(+), 13 deletions(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index f31e286c2977..5e0e8a5e05ca 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -1514,7 +1514,6 @@ SYM_CODE_START(asm_exc_nmi)
 	iretq
 SYM_CODE_END(asm_exc_nmi)
 
-#ifndef CONFIG_IA32_EMULATION
 /*
  * This handles SYSCALL from 32-bit code.  There is no way to program
  * MSRs to fully disable 32-bit SYSCALL.
@@ -1525,7 +1524,6 @@ SYM_CODE_START(ignore_sysret)
 	mov	$-ENOSYS, %eax
 	sysretl
 SYM_CODE_END(ignore_sysret)
-#endif
 
 .pushsection .text, "ax"
 	__FUNC_ALIGN
diff --git a/arch/x86/include/asm/desc.h b/arch/x86/include/asm/desc.h
index ab97b22ac04a..618b428586d1 100644
--- a/arch/x86/include/asm/desc.h
+++ b/arch/x86/include/asm/desc.h
@@ -8,6 +8,7 @@
 #include <asm/fixmap.h>
 #include <asm/irq_vectors.h>
 #include <asm/cpu_entry_area.h>
+#include <asm/traps.h>
 
 #include <linux/debug_locks.h>
 #include <linux/smp.h>
@@ -429,6 +430,10 @@ static inline void idt_init_desc(gate_desc *gate, const struct idt_data *d)
 	gate->offset_high	= (u32) (addr >> 32);
 	gate->reserved		= 0;
 #endif
+#ifdef CONFIG_IA32_EMULATION
+	if (ia32_disabled && d->vector == IA32_SYSCALL_VECTOR)
+		gate->bits.p = 0;
+#endif
 }
 
 extern unsigned long system_vectors[];
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 80710a68ef7d..71f8b55f70c9 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -2054,17 +2054,24 @@ void syscall_init(void)
 	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);
+	if (ia32_disabled) {
+		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);
+	} else {
+		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)ignore_sysret);
 	wrmsrl_safe(MSR_IA32_SYSENTER_CS, (u64)GDT_ENTRY_INVALID_SEG);
-- 
2.34.1