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/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 520fdec15bbb5b1b8f01719bf6714090152d0cea..e33293aea1ec9828e111598b98d69e521866515b 100644 (file)
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -4018,6 +4018,10 @@
 
        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 592c974d4558873653b2486db2a24949ed324096..a09ccab348fcad3004d9c1641b2a1b03ba737ad8 100644 (file)
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -2833,6 +2833,14 @@ 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 03505ffbe1b68d49982db71dfad0c83b04e45246..48be8a5d850334e3751098be32642f7b39c7f90b 100644 (file)
--- a/arch/x86/entry/common.c
+++ b/arch/x86/entry/common.c
@@ -271,6 +271,7 @@ __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();
@@ -283,8 +284,19 @@ __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 dd696b966e58355e725dc1cb72b373afee92d2d5..6b6370f3e4267eb0bd0989c2aa64788e38864131 100644 (file)
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -255,8 +255,12 @@ 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
@@ -414,6 +418,16 @@ syscall_return_via_sysret:
        USERGS_SYSRET64
 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 9c09775e589d67ab574b6412b02a1899f26a4a0d..f821b675613776e3e7840a1b3d1c6e754a15ac95 100644 (file)
--- a/arch/x86/entry/syscall_64.c
+++ b/arch/x86/entry/syscall_64.c
@@ -4,8 +4,14 @@
 #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
@@ -26,3 +32,36 @@ 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 3a091cea36c5a118d953fd25c897989270c6f0e4..fe6ccdb65413cd4ab13f61803cbe3c8f63b141c5 100644 (file)
--- a/arch/x86/include/asm/elf.h
+++ b/arch/x86/include/asm/elf.h
@@ -10,6 +10,7 @@
 #include <asm/ptrace.h>
 #include <asm/user.h>
 #include <asm/auxvec.h>
+#include <asm/syscall.h>
 
 typedef unsigned long elf_greg_t;
 
@@ -163,7 +164,7 @@ 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