linux-kernel - [PATCH 3/3] x86: Disable running 32bit processes if ia32

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Message-Id: <20230607072936.3766231-4-nik.borisov@suse.com>
Date:   Wed,  7 Jun 2023 10:29:36 +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 3/3] x86: Disable running 32bit processes if ia32_disabled is passed

In addition to disabling 32bit syscall interface let's also disable the
ability to run 32bit processes altogether. This is achieved by setting
the GDT_ENTRY_DEFAULT_USER32_CS descriptor to not present which would
cause 32 bit processes to trap with a #NP exception. Furthermore,
forbid loading compat processes as well.

Signed-off-by: Nikolay Borisov <nik.borisov@...e.com>
---
 arch/x86/include/asm/elf.h   | 5 +++--
 arch/x86/kernel/cpu/common.c | 8 ++++++++
 2 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/elf.h b/arch/x86/include/asm/elf.h
index 18fd06f7936a..406245bc0fb0 100644
--- a/arch/x86/include/asm/elf.h
+++ b/arch/x86/include/asm/elf.h
@@ -148,9 +148,10 @@ do {						\
 #define elf_check_arch(x)			\
 	((x)->e_machine == EM_X86_64)
 
+extern bool ia32_disabled;
 #define compat_elf_check_arch(x)					\
-	(elf_check_arch_ia32(x) ||					\
-	 (IS_ENABLED(CONFIG_X86_X32_ABI) && (x)->e_machine == EM_X86_64))
+	(!ia32_disabled && (elf_check_arch_ia32(x) ||			\
+	 (IS_ENABLED(CONFIG_X86_X32_ABI) && (x)->e_machine == EM_X86_64)))
 
 static inline void elf_common_init(struct thread_struct *t,
 				   struct pt_regs *regs, const u16 ds)
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 71f8b55f70c9..ddc301c09419 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -2359,6 +2359,11 @@ void microcode_check(struct cpuinfo_x86 *prev_info)
 }
 #endif
 
+static void remove_user32cs_from_gdt(void * __unused)
+{
+	get_current_gdt_rw()[GDT_ENTRY_DEFAULT_USER32_CS].p = 0;
+}
+
 /*
  * Invoked from core CPU hotplug code after hotplug operations
  */
@@ -2368,4 +2373,7 @@ void arch_smt_update(void)
 	cpu_bugs_smt_update();
 	/* Check whether IPI broadcasting can be enabled */
 	apic_smt_update();
+	if (ia32_disabled)
+		on_each_cpu(remove_user32cs_from_gdt, NULL, 1);
+
 }
-- 
2.34.1