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>
Gbp-Pq: Topic features/x86
Gbp-Pq: Name x86-make-x32-syscall-support-conditional.patch

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 830ad9f889fe04d63b97273583e64feb492e5b75..60c58a0d1170ab6153634138caa74ded3b463abb 100644 (file)
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -5631,6 +5631,10 @@
                        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.
+
        sysfs.deprecated=0|1 [KNL]
                        Enable/disable old style sysfs layout for old udev
                        on older distributions. When this option is enabled

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 5c2ccb85f2efb863fac8fca28a5bcff0f07ab900..f42ce4ea0bd7bdcd028150f29ac02a0e74e3d608 100644 (file)
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -2843,6 +2843,14 @@ 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

diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
index 6c2826417b3376cb70d53df8006f5998a0e33d9c..7c70efb87a8b2940d74d64980ec39a6715772516 100644 (file)
--- a/arch/x86/entry/common.c
+++ b/arch/x86/entry/common.c
@@ -62,7 +62,7 @@ static __always_inline bool do_syscall_x32(struct pt_regs *regs, int nr)
         */
        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;

diff --git a/arch/x86/entry/syscall_x32.c b/arch/x86/entry/syscall_x32.c
index bdd0e03a1265d23e474c5c45e1bd64e7b14b7b79..bdee000e87e7ad68ac325097b0105069807e1140 100644 (file)
--- 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);

diff --git a/arch/x86/include/asm/elf.h b/arch/x86/include/asm/elf.h
index 29fea180a6658e84bd0a094d4e767558cc91b4e1..cf3e4d179f7f7926555c4e9fbfa6b4ed5fccab41 100644 (file)
--- 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;
 
@@ -161,7 +164,8 @@ 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))
 
 #if __USER32_DS != __USER_DS
 # error "The following code assumes __USER32_DS == __USER_DS"

diff --git a/arch/x86/include/asm/syscall.h b/arch/x86/include/asm/syscall.h
index 5b85987a5e97c3d350636b2a3d72ed86f6e589e3..1a2ea0f2037665e5eef1a39dfd0af162aa34046b 100644 (file)
--- 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_call_table[];
 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