1 /* 2 * GCC stack protector support. 3 * 4 * Stack protector works by putting predefined pattern at the start of 5 * the stack frame and verifying that it hasn't been overwritten when 6 * returning from the function. The pattern is called stack canary 7 * and unfortunately gcc requires it to be at a fixed offset from %gs. 8 * On x86_64, the offset is 40 bytes and on x86_32 20 bytes. x86_64 9 * and x86_32 use segment registers differently and thus handles this 10 * requirement differently. 11 * 12 * On x86_64, %gs is shared by percpu area and stack canary. All 13 * percpu symbols are zero based and %gs points to the base of percpu 14 * area. The first occupant of the percpu area is always 15 * irq_stack_union which contains stack_canary at offset 40. Userland 16 * %gs is always saved and restored on kernel entry and exit using 17 * swapgs, so stack protector doesn't add any complexity there. 18 * 19 * On x86_32, it's slightly more complicated. As in x86_64, %gs is 20 * used for userland TLS. Unfortunately, some processors are much 21 * slower at loading segment registers with different value when 22 * entering and leaving the kernel, so the kernel uses %fs for percpu 23 * area and manages %gs lazily so that %gs is switched only when 24 * necessary, usually during task switch. 25 * 26 * As gcc requires the stack canary at %gs:20, %gs can't be managed 27 * lazily if stack protector is enabled, so the kernel saves and 28 * restores userland %gs on kernel entry and exit. This behavior is 29 * controlled by CONFIG_X86_32_LAZY_GS and accessors are defined in 30 * system.h to hide the details. 31 */ 32 33 #ifndef _ASM_STACKPROTECTOR_H 34 #define _ASM_STACKPROTECTOR_H 1 35 36 #ifdef CONFIG_CC_STACKPROTECTOR 37 38 #include <asm/tsc.h> 39 #include <asm/processor.h> 40 #include <asm/percpu.h> 41 #include <asm/desc.h> 42 43 #include <linux/random.h> 44 #include <linux/sched.h> 45 46 /* 47 * 24 byte read-only segment initializer for stack canary. Linker 48 * can't handle the address bit shifting. Address will be set in 49 * head_32 for boot CPU and setup_per_cpu_areas() for others. 50 */ 51 #define GDT_STACK_CANARY_INIT \ 52 [GDT_ENTRY_STACK_CANARY] = GDT_ENTRY_INIT(0x4090, 0, 0x18), 53 54 /* 55 * Initialize the stackprotector canary value. 56 * 57 * NOTE: this must only be called from functions that never return, 58 * and it must always be inlined. 59 */ 60 static __always_inline void boot_init_stack_canary(void) 61 { 62 u64 canary; 63 u64 tsc; 64 65 #ifdef CONFIG_X86_64 66 BUILD_BUG_ON(offsetof(union irq_stack_union, stack_canary) != 40); 67 #endif 68 /* 69 * We both use the random pool and the current TSC as a source 70 * of randomness. The TSC only matters for very early init, 71 * there it already has some randomness on most systems. Later 72 * on during the bootup the random pool has true entropy too. 73 */ 74 get_random_bytes(&canary, sizeof(canary)); 75 tsc = rdtsc(); 76 canary += tsc + (tsc << 32UL); 77 canary &= CANARY_MASK; 78 79 current->stack_canary = canary; 80 #ifdef CONFIG_X86_64 81 this_cpu_write(irq_stack_union.stack_canary, canary); 82 #else 83 this_cpu_write(stack_canary.canary, canary); 84 #endif 85 } 86 87 static inline void setup_stack_canary_segment(int cpu) 88 { 89 #ifdef CONFIG_X86_32 90 unsigned long canary = (unsigned long)&per_cpu(stack_canary, cpu); 91 struct desc_struct *gdt_table = get_cpu_gdt_rw(cpu); 92 struct desc_struct desc; 93 94 desc = gdt_table[GDT_ENTRY_STACK_CANARY]; 95 set_desc_base(&desc, canary); 96 write_gdt_entry(gdt_table, GDT_ENTRY_STACK_CANARY, &desc, DESCTYPE_S); 97 #endif 98 } 99 100 static inline void load_stack_canary_segment(void) 101 { 102 #ifdef CONFIG_X86_32 103 asm("mov %0, %%gs" : : "r" (__KERNEL_STACK_CANARY) : "memory"); 104 #endif 105 } 106 107 #else /* CC_STACKPROTECTOR */ 108 109 #define GDT_STACK_CANARY_INIT 110 111 /* dummy boot_init_stack_canary() is defined in linux/stackprotector.h */ 112 113 static inline void setup_stack_canary_segment(int cpu) 114 { } 115 116 static inline void load_stack_canary_segment(void) 117 { 118 #ifdef CONFIG_X86_32 119 asm volatile ("mov %0, %%gs" : : "r" (0)); 120 #endif 121 } 122 123 #endif /* CC_STACKPROTECTOR */ 124 #endif /* _ASM_STACKPROTECTOR_H */ 125