From: Ben Hutchings Date: Mon, 12 Feb 2018 23:59:26 +0000 Subject: 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 --- .../admin-guide/kernel-parameters.txt | 4 ++ arch/x86/Kconfig | 8 ++++ arch/x86/entry/common.c | 3 +- arch/x86/entry/syscall_64.c | 46 +++++++++++++++++++ arch/x86/include/asm/elf.h | 6 ++- arch/x86/include/asm/syscall.h | 13 ++++++ 6 files changed, 78 insertions(+), 2 deletions(-) --- a/Documentation/admin-guide/kernel-parameters.txt +++ b/Documentation/admin-guide/kernel-parameters.txt @@ -6160,6 +6160,10 @@ 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 --- a/arch/x86/Kconfig +++ b/arch/x86/Kconfig @@ -2997,6 +2997,14 @@ config COMPAT_32 select HAVE_UID16 select OLD_SIGSUSPEND3 +config X86_X32_DISABLED + bool "x32 ABI disabled by default" + depends on X86_X32_ABI + 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_ABI --- a/arch/x86/entry/common.c +++ b/arch/x86/entry/common.c @@ -63,7 +63,7 @@ static __always_inline bool do_syscall_x */ 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(regs, xnr); return true; --- a/arch/x86/entry/syscall_x32.c +++ b/arch/x86/entry/syscall_x32.c @@ -4,6 +4,9 @@ #include #include #include +#include +#undef MODULE_PARAM_PREFIX +#define MODULE_PARAM_PREFIX "syscall." #include #include @@ -20,3 +23,46 @@ long x32_sys_call(const struct pt_regs * default: return __x64_sys_ni_syscall(regs); } }; + +/* 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); --- a/arch/x86/include/asm/elf.h +++ b/arch/x86/include/asm/elf.h @@ -11,6 +11,9 @@ #include #include #include +#ifndef COMPILE_OFFSETS /* avoid a circular dependency on asm-offsets.h */ +#include +#endif typedef unsigned long elf_greg_t; @@ -150,7 +153,8 @@ 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" --- a/arch/x86/include/asm/syscall.h +++ b/arch/x86/include/asm/syscall.h @@ -13,6 +13,7 @@ #include #include #include +#include #include /* for TS_COMPAT */ #include @@ -28,6 +29,18 @@ extern long ia32_sys_call(const struct p extern long x32_sys_call(const struct pt_regs *, unsigned int nr); extern long x64_sys_call(const struct pt_regs *, unsigned int nr); +#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