From: Ben Hutchings <ben@decadent.org.uk>
Date: Mon, 12 Feb 2018 23:59:26 +0000
Subject: x86: Make x32 syscall support conditional on a kernel parameter
Bug-Debian: https://bugs.debian.org/708070
Forwarded: https://lore.kernel.org/lkml/1415245982.3398.53.camel@decadent.org.uk/T/#u

Enabling x32 in the standard amd64 kernel would increase its attack
surface while provide no benefit to the vast majority of its users.
No-one seems interested in regularly checking for vulnerabilities
specific to x32 (at least no-one with a white hat).

Still, adding another flavour just to turn on x32 seems wasteful.  And
the only differences on syscall entry are a few instructions that mask
out the x32 flag and compare the syscall number.

Use a static key to control whether x32 syscalls are really enabled, a
Kconfig parameter to set its default value and a kernel parameter
"syscall.x32" to change it at boot time.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
 .../admin-guide/kernel-parameters.txt         |  4 ++
 arch/x86/Kconfig                              |  8 ++++
 arch/x86/entry/common.c                       |  3 +-
 arch/x86/entry/syscall_64.c                   | 46 +++++++++++++++++++
 arch/x86/include/asm/elf.h                    |  6 ++-
 arch/x86/include/asm/syscall.h                | 13 ++++++
 6 files changed, 78 insertions(+), 2 deletions(-)

--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -6172,6 +6172,10 @@
 			later by a loaded module cannot be set this way.
 			Example: sysctl.vm.swappiness=40
 
+	syscall.x32=	[KNL,x86_64] Enable/disable use of x32 syscalls on
+			an x86_64 kernel where CONFIG_X86_X32 is enabled.
+			Default depends on CONFIG_X86_X32_DISABLED.
+
 	sysrq_always_enabled
 			[KNL]
 			Ignore sysrq setting - this boot parameter will
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -3058,6 +3058,14 @@ config COMPAT_32
 	select HAVE_UID16
 	select OLD_SIGSUSPEND3
 
+config X86_X32_DISABLED
+	bool "x32 ABI disabled by default"
+	depends on X86_X32_ABI
+	default n
+	help
+	  Disable the x32 ABI unless explicitly enabled using the
+	  kernel paramter "syscall.x32=y".
+
 config COMPAT
 	def_bool y
 	depends on IA32_EMULATION || X86_X32_ABI
--- a/arch/x86/entry/common.c
+++ b/arch/x86/entry/common.c
@@ -62,7 +62,7 @@ static __always_inline bool do_syscall_x
 	 */
 	unsigned int xnr = nr - __X32_SYSCALL_BIT;
 
-	if (IS_ENABLED(CONFIG_X86_X32_ABI) && likely(xnr < X32_NR_syscalls)) {
+	if (IS_ENABLED(CONFIG_X86_X32_ABI) && unlikely(x32_enabled) && likely(xnr < X32_NR_syscalls)) {
 		xnr = array_index_nospec(xnr, X32_NR_syscalls);
 		regs->ax = x32_sys_call_table[xnr](regs);
 		return true;
--- a/arch/x86/entry/syscall_x32.c
+++ b/arch/x86/entry/syscall_x32.c
@@ -4,6 +4,9 @@
 #include <linux/linkage.h>
 #include <linux/sys.h>
 #include <linux/cache.h>
+#include <linux/moduleparam.h>
+#undef MODULE_PARAM_PREFIX
+#define MODULE_PARAM_PREFIX "syscall."
 #include <linux/syscalls.h>
 #include <asm/syscall.h>
 
@@ -16,3 +19,46 @@
 asmlinkage const sys_call_ptr_t x32_sys_call_table[] = {
 #include <asm/syscalls_x32.h>
 };
+
+/* Maybe enable x32 syscalls */
+
+#if defined(CONFIG_X86_X32_DISABLED)
+DEFINE_STATIC_KEY_FALSE(x32_enabled_skey);
+#else
+DEFINE_STATIC_KEY_TRUE(x32_enabled_skey);
+#endif
+
+static int __init x32_param_set(const char *val, const struct kernel_param *p)
+{
+	bool enabled;
+	int ret;
+
+	ret = kstrtobool(val, &enabled);
+	if (ret)
+		return ret;
+	if (IS_ENABLED(CONFIG_X86_X32_DISABLED)) {
+		if (enabled) {
+			static_key_enable(&x32_enabled_skey.key);
+			pr_info("Enabled x32 syscalls\n");
+		}
+	} else {
+		if (!enabled) {
+			static_key_disable(&x32_enabled_skey.key);
+			pr_info("Disabled x32 syscalls\n");
+		}
+	}
+	return 0;
+}
+
+static int x32_param_get(char *buffer, const struct kernel_param *p)
+{
+	return sprintf(buffer, "%c\n",
+		       static_key_enabled(&x32_enabled_skey) ? 'Y' : 'N');
+}
+
+static const struct kernel_param_ops x32_param_ops = {
+	.set = x32_param_set,
+	.get = x32_param_get,
+};
+
+arch_param_cb(x32, &x32_param_ops, NULL, 0444);
--- a/arch/x86/include/asm/elf.h
+++ b/arch/x86/include/asm/elf.h
@@ -11,6 +11,9 @@
 #include <asm/user.h>
 #include <asm/auxvec.h>
 #include <asm/fsgsbase.h>
+#ifndef COMPILE_OFFSETS /* avoid a circular dependency on asm-offsets.h */
+#include <asm/syscall.h>
+#endif
 
 typedef unsigned long elf_greg_t;
 
@@ -150,7 +153,8 @@ do {						\
 
 #define compat_elf_check_arch(x)					\
 	(elf_check_arch_ia32(x) ||					\
-	 (IS_ENABLED(CONFIG_X86_X32_ABI) && (x)->e_machine == EM_X86_64))
+	 (IS_ENABLED(CONFIG_X86_X32_ABI) && x32_enabled &&		\
+	  (x)->e_machine == EM_X86_64))
 
 static inline void elf_common_init(struct thread_struct *t,
 				   struct pt_regs *regs, const u16 ds)
--- a/arch/x86/include/asm/syscall.h
+++ b/arch/x86/include/asm/syscall.h
@@ -13,6 +13,7 @@
 #include <uapi/linux/audit.h>
 #include <linux/sched.h>
 #include <linux/err.h>
+#include <linux/jump_label.h>
 #include <asm/thread_info.h>	/* for TS_COMPAT */
 #include <asm/unistd.h>
 
@@ -30,6 +31,18 @@ extern const sys_call_ptr_t ia32_sys_cal
 extern const sys_call_ptr_t x32_sys_call_table[];
 #endif
 
+#if defined(CONFIG_X86_X32_ABI)
+#if defined(CONFIG_X86_X32_DISABLED)
+DECLARE_STATIC_KEY_FALSE(x32_enabled_skey);
+#define x32_enabled static_branch_unlikely(&x32_enabled_skey)
+#else
+DECLARE_STATIC_KEY_TRUE(x32_enabled_skey);
+#define x32_enabled static_branch_likely(&x32_enabled_skey)
+#endif
+#else
+#define x32_enabled 0
+#endif
+
 /*
  * Only the low 32 bits of orig_ax are meaningful, so we return int.
  * This importantly ignores the high bits on 64-bit, so comparisons
