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

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index a6fadef92d6d70db1426baa075648addc9526429..be36da42fe0656810d0a327d7fff8a8b283f5d70 100644 (file)
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -4020,6 +4020,10 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 
        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 bada636d1065c5451d3e42f54820757e4cf30455..1f871ee8cb5226444093f38f14c10ff47b7bcf42 100644 (file)
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -2721,6 +2721,14 @@ 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

diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
index bdd9cc59d20f7b514cf9ecb1f53ca49a5deb4d5b..e6a43278a826a99f5777a3bb1669e18f229dfc23 100644 (file)
--- a/arch/x86/entry/common.c
+++ b/arch/x86/entry/common.c
@@ -264,6 +264,7 @@ __visible void do_syscall_64(struct pt_regs *regs)
 {
        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_regs *regs)
         * 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);
        }

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index ef766a358b37dd355e8d5d75c10a61d47a83e04f..f5ea08f61b2dae34a9d77163e4978e193c0247f7 100644 (file)
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -195,8 +195,12 @@ 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
@@ -331,6 +335,16 @@ 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.

diff --git a/arch/x86/entry/syscall_64.c b/arch/x86/entry/syscall_64.c
index 9dbc5abb6162fa20581069499667a8c49b254868..753eeae0a72e0fac8fca10cc20112a686ad6edc0 100644 (file)
--- 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_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 */
+
+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

diff --git a/arch/x86/include/asm/elf.h b/arch/x86/include/asm/elf.h
index 94aad6364b4742deebb242f4e2469094501adacc..c688512ef6d61d9db3198c26e76f6e6bcdd15a36 100644 (file)
--- 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"

diff --git a/arch/x86/include/asm/syscall.h b/arch/x86/include/asm/syscall.h
index e3c95e8e61c5fca9808524ba47d539bd0377ece2..4cf8e121d874c52a5e37558255d24ee54c7c264e 100644 (file)
--- 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_table[];
 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