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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/admin-guide/kernel-parameters.txt | 4 ++
arch/x86/Kconfig | 8 ++++
arch/x86/entry/common.c | 16 ++++++++-
arch/x86/entry/entry_64.S | 18 +++++++++--
arch/x86/entry/syscall_64.c | 39 ++++++++++++++++++++++++
arch/x86/include/asm/elf.h | 3 +
arch/x86/include/asm/syscall.h | 6 +++
7 files changed, 89 insertions(+), 5 deletions(-)
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -3829,6 +3829,10 @@
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
@@ -2757,6 +2757,14 @@ config COMPAT_32
select HAVE_UID16
select OLD_SIGSUSPEND3
+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
@@ -9,6 +9,7 @@
#include <asm/ptrace.h>
#include <asm/user.h>
#include <asm/auxvec.h>
+#include <asm/syscall.h>
typedef unsigned long elf_greg_t;
@@ -162,7 +163,7 @@ 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
@@ -190,8 +190,12 @@ 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
@@ -326,6 +330,16 @@ opportunistic_sysret_failed:
jmp restore_c_regs_and_iret
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
+
ENTRY(stub_ptregs_64)
/*
* Syscalls marked as needing ptregs land here.
--- 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/text-patching.h>
#define __SYSCALL_64_QUAL_(sym) sym
#define __SYSCALL_64_QUAL_ptregs(sym) ptregs_##sym
@@ -25,3 +31,36 @@ 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_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_mask_compare_end - system_call_mask_compare != 10);
+
+ if (x32_enabled) {
+ text_poke_early(system_call_fast_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
--- a/arch/x86/entry/common.c
+++ b/arch/x86/entry/common.c
@@ -264,6 +264,7 @@ __visible void do_syscall_64(struct pt_r
{
struct thread_info *ti = current_thread_info();
unsigned long nr = regs->orig_ax;
+ unsigned int syscall_mask, nr_syscalls_enabled;
enter_from_user_mode();
local_irq_enable();
@@ -276,8 +277,19 @@ __visible void do_syscall_64(struct pt_r
* table. The only functional difference is the x32 bit in
* regs->orig_ax, which changes the behavior of some syscalls.
*/
- if (likely((nr & __SYSCALL_MASK) < NR_syscalls)) {
- regs->ax = sys_call_table[nr & __SYSCALL_MASK](
+ if (__SYSCALL_MASK == ~0U || x32_enabled) {
+ syscall_mask = __SYSCALL_MASK;
+ nr_syscalls_enabled = NR_syscalls;
+ } else {
+ /*
+ * x32 syscalls present but not enabled. Don't mask out
+ * the x32 flag and don't enable any x32-specific calls.
+ */
+ syscall_mask = ~0U;
+ nr_syscalls_enabled = 512;
+ }
+ if (likely((nr & syscall_mask) < nr_syscalls_enabled)) {
+ regs->ax = sys_call_table[nr & syscall_mask](
regs->di, regs->si, regs->dx,
regs->r10, regs->r8, regs->r9);
}
--- a/arch/x86/include/asm/syscall.h
+++ b/arch/x86/include/asm/syscall.h
@@ -35,6 +35,12 @@ extern const sys_call_ptr_t sys_call_tab
extern const sys_call_ptr_t ia32_sys_call_table[];
#endif
+#if defined(CONFIG_X86_X32_ABI)
+extern bool x32_enabled;
+#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
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