From: Ben Hutchings <ben@decadent.org.uk>
Date: Fri, 25 Jul 2014 01:16:15 +0100
Subject: x86: Make x32 syscall support conditional on a kernel parameter
Bug-Debian: https://bugs.debian.org/708070
Forwarded: http://mid.gmane.org/1415245982.3398.53.camel@decadent.org.uk

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 two instructions (mask out
the x32 flag and compare the syscall number).

So pad the standard comparison with a nop and add a kernel parameter
"syscall.x32" which controls whether this is replaced with the x32
version at boot time.  Add a Kconfig parameter to set the default.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
 Documentation/kernel-parameters.txt |  4 ++++
 arch/x86/Kconfig                    |  8 +++++++
 arch/x86/include/asm/elf.h          |  8 ++++++-
 arch/x86/entry/entry_64.S           | 36 ++++++++++++++++++++++---------
 arch/x86/entry/syscall_64.c         | 43 +++++++++++++++++++++++++++++++++++++
 5 files changed, 88 insertions(+), 11 deletions(-)

--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -3580,6 +3580,10 @@ bytes respectively. Such letter suffixes
 
 	switches=	[HW,M68k]
 
+	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.
+
 	sysfs.deprecated=0|1 [KNL]
 			Enable/disable old style sysfs layout for old udev
 			on older distributions. When this option is enabled
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -2547,6 +2547,14 @@ config X86_X32
 	  elf32_x86_64 support enabled to compile a kernel with this
 	  option set.
 
+config X86_X32_DISABLED
+	bool "x32 ABI disabled by default"
+	depends on X86_X32
+	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
--- a/arch/x86/include/asm/elf.h
+++ b/arch/x86/include/asm/elf.h
@@ -154,6 +154,12 @@ do {						\
 
 #else /* CONFIG_X86_32 */
 
+#ifdef CONFIG_X86_X32_ABI
+extern bool x32_enabled;
+#else
+#define x32_enabled 0
+#endif
+
 /*
  * This is used to ensure we don't load something for the wrong architecture.
  */
@@ -162,7 +168,7 @@ do {						\
 
 #define compat_elf_check_arch(x)					\
 	(elf_check_arch_ia32(x) ||					\
-	 (IS_ENABLED(CONFIG_X86_X32_ABI) && (x)->e_machine == EM_X86_64))
+	 (x32_enabled && (x)->e_machine == EM_X86_64))
 
 #if __USER32_DS != __USER_DS
 # error "The following code assumes __USER32_DS == __USER_DS"
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -178,8 +178,12 @@ entry_SYSCALL_64_fastpath:
 #if __SYSCALL_MASK == ~0
 	cmpq	$__NR_syscall_max, %rax
 #else
-	andl	$__SYSCALL_MASK, %eax
-	cmpl	$__NR_syscall_max, %eax
+.global system_call_fast_compare
+.global system_call_fast_compare_end
+system_call_fast_compare:
+	cmpq	$511, %rax			/* x32 syscalls start at 512 */
+	.byte	P6_NOP4
+system_call_fast_compare_end:
 #endif
 	ja	1f				/* return -ENOSYS (already in pt_regs->ax) */
 	movq	%r10, %rcx
@@ -257,8 +261,12 @@ tracesys_phase2:
 #if __SYSCALL_MASK == ~0
 	cmpq	$__NR_syscall_max, %rax
 #else
-	andl	$__SYSCALL_MASK, %eax
-	cmpl	$__NR_syscall_max, %eax
+.global system_call_trace_compare
+.global system_call_trace_compare_end
+system_call_trace_compare:
+	cmpq	$511, %rax			/* x32 syscalls start at 512 */
+	.byte	P6_NOP4
+system_call_trace_compare_end:
 #endif
 	ja	1f				/* return -ENOSYS (already in pt_regs->ax) */
 	movq	%r10, %rcx			/* fixup for C */
@@ -410,6 +418,16 @@ opportunistic_sysret_failed:
 END(entry_SYSCALL_64)
 
 
+#if __SYSCALL_MASK != ~0
+	/* This replaces the usual comparisons if syscall.x32 is set */
+.global system_call_mask_compare
+.global system_call_mask_compare_end
+system_call_mask_compare:
+	andl	$__SYSCALL_MASK, %eax
+	cmpl	$__NR_syscall_max, %eax
+system_call_mask_compare_end:
+#endif
+
 	.macro FORK_LIKE func
 ENTRY(stub_\func)
 	SAVE_EXTRA_REGS 8
--- a/arch/x86/entry/syscall_64.c
+++ b/arch/x86/entry/syscall_64.c
@@ -3,8 +3,14 @@
 #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/bug.h>
+#include <linux/init.h>
 #include <asm/asm-offsets.h>
 #include <asm/syscall.h>
+#include <asm/alternative.h>
 
 #define __SYSCALL_COMMON(nr, sym, compat) __SYSCALL_64(nr, sym, compat)
 
@@ -30,3 +36,40 @@ asmlinkage const sys_call_ptr_t sys_call
 	[0 ... __NR_syscall_max] = &sys_ni_syscall,
 #include <asm/syscalls_64.h>
 };
+
+#ifdef CONFIG_X86_X32_ABI
+
+/* Maybe enable x32 syscalls */
+
+bool x32_enabled = !IS_ENABLED(CONFIG_X86_X32_DISABLED);
+module_param_named(x32, x32_enabled, bool, 0444);
+
+extern char system_call_fast_compare_end[], system_call_fast_compare[],
+	system_call_trace_compare_end[], system_call_trace_compare[],
+	system_call_mask_compare_end[], system_call_mask_compare[];
+
+static int __init x32_enable(void)
+{
+	BUG_ON(system_call_fast_compare_end - system_call_fast_compare != 10);
+	BUG_ON(system_call_trace_compare_end - system_call_trace_compare != 10);
+	BUG_ON(system_call_mask_compare_end - system_call_mask_compare != 10);
+
+	if (x32_enabled) {
+		text_poke_early(system_call_fast_compare,
+				system_call_mask_compare, 10);
+		text_poke_early(system_call_trace_compare,
+				system_call_mask_compare, 10);
+#ifdef CONFIG_X86_X32_DISABLED
+		pr_info("Enabled x32 syscalls\n");
+#endif
+	}
+#ifndef CONFIG_X86_X32_DISABLED
+	else
+		pr_info("Disabled x32 syscalls\n");
+#endif
+
+	return 0;
+}
+late_initcall(x32_enable);
+
+#endif
