x86: Make x32 syscall support conditional on a kernel parameter

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 04f14df03aed8ea144cd392a758b177d504fc6a9..b1a373cb8883ee2ee26e9554accc4ae607267d1b 100644 (file)
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -4608,6 +4608,10 @@
 
        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

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 2bbbd4d1ba31de5c0431393247ac4279878eb13d..c041428a96b995fa8357637795136d49ef1ea807 100644 (file)
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -2850,6 +2850,14 @@ 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 2418804e66b4d8ec686b13c480de04c8c3ad141f..efcd0edf4d0326baeb63c0a0a697ab2090025b2e 100644 (file)
--- a/arch/x86/entry/common.c
+++ b/arch/x86/entry/common.c
@@ -295,12 +295,21 @@ __visible void do_syscall_64(unsigned long nr, struct pt_regs *regs)
         * table.  The only functional difference is the x32 bit in
         * regs->orig_ax, which changes the behavior of some syscalls.
         */
-       nr &= __SYSCALL_MASK;
-       if (likely(nr < NR_syscalls)) {
+       if (x32_enabled) {
+               nr &= ~__X32_SYSCALL_BIT;
+               if (unlikely(nr >= NR_syscalls))
+                       goto bad;
                nr = array_index_nospec(nr, NR_syscalls);
+               goto good;
+       } else {
+               nr &= ~0U;
+               if (unlikely(nr >= NR_non_x32_syscalls))
+                       goto bad;
+               nr = array_index_nospec(nr, NR_non_x32_syscalls);
+good:
                regs->ax = sys_call_table[nr](regs);
        }
-
+bad:
        syscall_return_slowpath(regs);
 }
 #endif

diff --git a/arch/x86/entry/syscall_64.c b/arch/x86/entry/syscall_64.c
index d5252bc1e38058ebc2d071d9bd3b3bd3cc1a976a..5a9040771275b83785fab06a9c08696873d262f6 100644 (file)
--- a/arch/x86/entry/syscall_64.c
+++ b/arch/x86/entry/syscall_64.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 <asm/asm-offsets.h>
 #include <asm/syscall.h>
 
@@ -23,3 +26,50 @@ asmlinkage const sys_call_ptr_t sys_call_table[__NR_syscall_max+1] = {
        [0 ... __NR_syscall_max] = &sys_ni_syscall,
 #include <asm/syscalls_64.h>
 };
+
+#ifdef CONFIG_X86_X32_ABI
+
+/* 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);
+
+#endif

diff --git a/arch/x86/include/asm/elf.h b/arch/x86/include/asm/elf.h
index 69c0f892e310a37a9f8bd543227a73104616b5df..4cfc1795e2c48c37aa5e97ef8cea8de0cfc841f8 100644 (file)
--- a/arch/x86/include/asm/elf.h
+++ b/arch/x86/include/asm/elf.h
@@ -11,6 +11,7 @@
 #include <asm/user.h>
 #include <asm/auxvec.h>
 #include <asm/fsgsbase.h>
+#include <asm/syscall.h>
 
 typedef unsigned long elf_greg_t;
 
@@ -163,7 +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 2dc4a021beea4e163bf56ae367404cfda79304bc..ea05631030973f3e50bc49e60e9702f305f9b572 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/asm-offsets.h>   /* For NR_syscalls */
 #include <asm/thread_info.h>   /* for TS_COMPAT */
 #include <asm/unistd.h>
@@ -36,6 +37,18 @@ extern const sys_call_ptr_t sys_call_table[];
 extern const sys_call_ptr_t ia32_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

diff --git a/arch/x86/include/asm/unistd.h b/arch/x86/include/asm/unistd.h
index 146859efd83c60b0ef4b6e08b80a0054b6a3ada4..45ff6ef8bed2822cd35fd565c93d2809417b7ef9 100644 (file)
--- a/arch/x86/include/asm/unistd.h
+++ b/arch/x86/include/asm/unistd.h
@@ -6,9 +6,9 @@
 
 
 # ifdef CONFIG_X86_X32_ABI
-#  define __SYSCALL_MASK (~(__X32_SYSCALL_BIT))
+#  define NR_non_x32_syscalls 512
 # else
-#  define __SYSCALL_MASK (~0)
+#  define NR_non_x32_syscalls NR_syscalls
 # endif
 
 # ifdef CONFIG_X86_32