From: Ben Hutchings Date: Fri, 25 Jul 2014 00:16:15 +0000 (+0100) Subject: x86: Make x32 syscall support conditional on a kernel parameter X-Git-Tag: archive/raspbian/4.9.13-1+rpi1~11^2~77 X-Git-Url: https://dgit.raspbian.org/%22http://www.example.com/cgi/%22/%22http:/www.example.com/cgi/%22?a=commitdiff_plain;h=7283b9f8290f13f34974fa9a6388e5bc9bd35c1e;p=linux-4.9.git 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 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 65b05ba6ef98..ac0abe5687b3 100644 --- a/Documentation/kernel-parameters.txt +++ b/Documentation/kernel-parameters.txt @@ -4010,6 +4010,10 @@ 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 bada636d1065..1f871ee8cb52 100644 --- 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 bdd9cc59d20f..e6a43278a826 100644 --- 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 ef766a358b37..f5ea08f61b2d 100644 --- 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 9dbc5abb6162..753eeae0a72e 100644 --- a/arch/x86/entry/syscall_64.c +++ b/arch/x86/entry/syscall_64.c @@ -3,8 +3,14 @@ #include #include #include +#include +#undef MODULE_PARAM_PREFIX +#define MODULE_PARAM_PREFIX "syscall." +#include +#include #include #include +#include #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 }; + +#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 e7f155c3045e..ce6342cadea8 100644 --- a/arch/x86/include/asm/elf.h +++ b/arch/x86/include/asm/elf.h @@ -9,6 +9,7 @@ #include #include #include +#include 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 e3c95e8e61c5..4cf8e121d874 100644 --- 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