From 60a5317ff0f42dd313094b88f809f63041568b08 Mon Sep 17 00:00:00 2001 From: Tejun Heo Date: Mon, 9 Feb 2009 22:17:40 +0900 Subject: x86: implement x86_32 stack protector Impact: stack protector for x86_32 Implement stack protector for x86_32. GDT entry 28 is used for it. It's set to point to stack_canary-20 and have the length of 24 bytes. CONFIG_CC_STACKPROTECTOR turns off CONFIG_X86_32_LAZY_GS and sets %gs to the stack canary segment on entry. As %gs is otherwise unused by the kernel, the canary can be anywhere. It's defined as a percpu variable. x86_32 exception handlers take register frame on stack directly as struct pt_regs. With -fstack-protector turned on, gcc copies the whole structure after the stack canary and (of course) doesn't copy back on return thus losing all changed. For now, -fno-stack-protector is added to all files which contain those functions. We definitely need something better. Signed-off-by: Tejun Heo Signed-off-by: Ingo Molnar --- arch/x86/include/asm/processor.h | 4 ++ arch/x86/include/asm/segment.h | 9 +++- arch/x86/include/asm/stackprotector.h | 91 +++++++++++++++++++++++++++++++++-- arch/x86/include/asm/system.h | 21 ++++++++ 4 files changed, 119 insertions(+), 6 deletions(-) (limited to 'arch/x86/include') diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h index 9763eb70013..5a947210425 100644 --- a/arch/x86/include/asm/processor.h +++ b/arch/x86/include/asm/processor.h @@ -396,7 +396,11 @@ DECLARE_PER_CPU(union irq_stack_union, irq_stack_union); DECLARE_INIT_PER_CPU(irq_stack_union); DECLARE_PER_CPU(char *, irq_stack_ptr); +#else /* X86_64 */ +#ifdef CONFIG_CC_STACKPROTECTOR +DECLARE_PER_CPU(unsigned long, stack_canary); #endif +#endif /* X86_64 */ extern void print_cpu_info(struct cpuinfo_x86 *); extern unsigned int xstate_size; diff --git a/arch/x86/include/asm/segment.h b/arch/x86/include/asm/segment.h index 1dc1b51ac62..14e0ed86a6f 100644 --- a/arch/x86/include/asm/segment.h +++ b/arch/x86/include/asm/segment.h @@ -61,7 +61,7 @@ * * 26 - ESPFIX small SS * 27 - per-cpu [ offset to per-cpu data area ] - * 28 - unused + * 28 - stack_canary-20 [ for stack protector ] * 29 - unused * 30 - unused * 31 - TSS for double fault handler @@ -95,6 +95,13 @@ #define __KERNEL_PERCPU 0 #endif +#define GDT_ENTRY_STACK_CANARY (GDT_ENTRY_KERNEL_BASE + 16) +#ifdef CONFIG_CC_STACKPROTECTOR +#define __KERNEL_STACK_CANARY (GDT_ENTRY_STACK_CANARY * 8) +#else +#define __KERNEL_STACK_CANARY 0 +#endif + #define GDT_ENTRY_DOUBLEFAULT_TSS 31 /* diff --git a/arch/x86/include/asm/stackprotector.h b/arch/x86/include/asm/stackprotector.h index ee275e9f48a..fa7e5bd6fbe 100644 --- a/arch/x86/include/asm/stackprotector.h +++ b/arch/x86/include/asm/stackprotector.h @@ -1,3 +1,35 @@ +/* + * GCC stack protector support. + * + * Stack protector works by putting predefined pattern at the start of + * the stack frame and verifying that it hasn't been overwritten when + * returning from the function. The pattern is called stack canary + * and unfortunately gcc requires it to be at a fixed offset from %gs. + * On x86_64, the offset is 40 bytes and on x86_32 20 bytes. x86_64 + * and x86_32 use segment registers differently and thus handles this + * requirement differently. + * + * On x86_64, %gs is shared by percpu area and stack canary. All + * percpu symbols are zero based and %gs points to the base of percpu + * area. The first occupant of the percpu area is always + * irq_stack_union which contains stack_canary at offset 40. Userland + * %gs is always saved and restored on kernel entry and exit using + * swapgs, so stack protector doesn't add any complexity there. + * + * On x86_32, it's slightly more complicated. As in x86_64, %gs is + * used for userland TLS. Unfortunately, some processors are much + * slower at loading segment registers with different value when + * entering and leaving the kernel, so the kernel uses %fs for percpu + * area and manages %gs lazily so that %gs is switched only when + * necessary, usually during task switch. + * + * As gcc requires the stack canary at %gs:20, %gs can't be managed + * lazily if stack protector is enabled, so the kernel saves and + * restores userland %gs on kernel entry and exit. This behavior is + * controlled by CONFIG_X86_32_LAZY_GS and accessors are defined in + * system.h to hide the details. + */ + #ifndef _ASM_STACKPROTECTOR_H #define _ASM_STACKPROTECTOR_H 1 @@ -6,8 +38,18 @@ #include #include #include +#include +#include #include +/* + * 24 byte read-only segment initializer for stack canary. Linker + * can't handle the address bit shifting. Address will be set in + * head_32 for boot CPU and setup_per_cpu_areas() for others. + */ +#define GDT_STACK_CANARY_INIT \ + [GDT_ENTRY_STACK_CANARY] = { { { 0x00000018, 0x00409000 } } }, + /* * Initialize the stackprotector canary value. * @@ -19,12 +61,9 @@ static __always_inline void boot_init_stack_canary(void) u64 canary; u64 tsc; - /* - * Build time only check to make sure the stack_canary is at - * offset 40 in the pda; this is a gcc ABI requirement - */ +#ifdef CONFIG_X86_64 BUILD_BUG_ON(offsetof(union irq_stack_union, stack_canary) != 40); - +#endif /* * We both use the random pool and the current TSC as a source * of randomness. The TSC only matters for very early init, @@ -36,7 +75,49 @@ static __always_inline void boot_init_stack_canary(void) canary += tsc + (tsc << 32UL); current->stack_canary = canary; +#ifdef CONFIG_X86_64 percpu_write(irq_stack_union.stack_canary, canary); +#else + percpu_write(stack_canary, canary); +#endif +} + +static inline void setup_stack_canary_segment(int cpu) +{ +#ifdef CONFIG_X86_32 + unsigned long canary = (unsigned long)&per_cpu(stack_canary, cpu); + struct desc_struct *gdt_table = get_cpu_gdt_table(cpu); + struct desc_struct desc; + + desc = gdt_table[GDT_ENTRY_STACK_CANARY]; + desc.base0 = canary & 0xffff; + desc.base1 = (canary >> 16) & 0xff; + desc.base2 = (canary >> 24) & 0xff; + write_gdt_entry(gdt_table, GDT_ENTRY_STACK_CANARY, &desc, DESCTYPE_S); +#endif +} + +static inline void load_stack_canary_segment(void) +{ +#ifdef CONFIG_X86_32 + asm("mov %0, %%gs" : : "r" (__KERNEL_STACK_CANARY) : "memory"); +#endif +} + +#else /* CC_STACKPROTECTOR */ + +#define GDT_STACK_CANARY_INIT + +/* dummy boot_init_stack_canary() is defined in linux/stackprotector.h */ + +static inline void setup_stack_canary_segment(int cpu) +{ } + +static inline void load_stack_canary_segment(void) +{ +#ifdef CONFIG_X86_32 + asm volatile ("mov %0, %%gs" : : "r" (0)); +#endif } #endif /* CC_STACKPROTECTOR */ diff --git a/arch/x86/include/asm/system.h b/arch/x86/include/asm/system.h index 79b98e5b96f..2692ee8ef03 100644 --- a/arch/x86/include/asm/system.h +++ b/arch/x86/include/asm/system.h @@ -23,6 +23,22 @@ struct task_struct *__switch_to(struct task_struct *prev, #ifdef CONFIG_X86_32 +#ifdef CONFIG_CC_STACKPROTECTOR +#define __switch_canary \ + "movl "__percpu_arg([current_task])",%%ebx\n\t" \ + "movl %P[task_canary](%%ebx),%%ebx\n\t" \ + "movl %%ebx,"__percpu_arg([stack_canary])"\n\t" +#define __switch_canary_oparam \ + , [stack_canary] "=m" (per_cpu_var(stack_canary)) +#define __switch_canary_iparam \ + , [current_task] "m" (per_cpu_var(current_task)) \ + , [task_canary] "i" (offsetof(struct task_struct, stack_canary)) +#else /* CC_STACKPROTECTOR */ +#define __switch_canary +#define __switch_canary_oparam +#define __switch_canary_iparam +#endif /* CC_STACKPROTECTOR */ + /* * Saving eflags is important. It switches not only IOPL between tasks, * it also protects other tasks from NT leaking through sysenter etc. @@ -46,6 +62,7 @@ do { \ "pushl %[next_ip]\n\t" /* restore EIP */ \ "jmp __switch_to\n" /* regparm call */ \ "1:\t" \ + __switch_canary \ "popl %%ebp\n\t" /* restore EBP */ \ "popfl\n" /* restore flags */ \ \ @@ -58,6 +75,8 @@ do { \ "=b" (ebx), "=c" (ecx), "=d" (edx), \ "=S" (esi), "=D" (edi) \ \ + __switch_canary_oparam \ + \ /* input parameters: */ \ : [next_sp] "m" (next->thread.sp), \ [next_ip] "m" (next->thread.ip), \ @@ -66,6 +85,8 @@ do { \ [prev] "a" (prev), \ [next] "d" (next) \ \ + __switch_canary_iparam \ + \ : /* reloaded segment registers */ \ "memory"); \ } while (0) -- cgit v1.2.3