1 /* SPDX-License-Identifier: GPL-2.0 */ 2 /* 3 * GCC stack protector support. 4 * 5 * Stack protector works by putting predefined pattern at the start of 6 * the stack frame and verifying that it hasn't been overwritten when 7 * returning from the function. The pattern is called stack canary 8 * and unfortunately gcc requires it to be at a fixed offset from %gs. 9 * On x86_64, the offset is 40 bytes and on x86_32 20 bytes. x86_64 10 * and x86_32 use segment registers differently and thus handles this 11 * requirement differently. 12 * 13 * On x86_64, %gs is shared by percpu area and stack canary. All 14 * percpu symbols are zero based and %gs points to the base of percpu 15 * area. The first occupant of the percpu area is always 16 * fixed_percpu_data which contains stack_canary at offset 40. Userland 17 * %gs is always saved and restored on kernel entry and exit using 18 * swapgs, so stack protector doesn't add any complexity there. 19 * 20 * On x86_32, it's slightly more complicated. As in x86_64, %gs is 21 * used for userland TLS. Unfortunately, some processors are much 22 * slower at loading segment registers with different value when 23 * entering and leaving the kernel, so the kernel uses %fs for percpu 24 * area and manages %gs lazily so that %gs is switched only when 25 * necessary, usually during task switch. 26 * 27 * As gcc requires the stack canary at %gs:20, %gs can't be managed 28 * lazily if stack protector is enabled, so the kernel saves and 29 * restores userland %gs on kernel entry and exit. This behavior is 30 * controlled by CONFIG_X86_32_LAZY_GS and accessors are defined in 31 * system.h to hide the details. 32 */ 33 34 #ifndef _ASM_STACKPROTECTOR_H 35 #define _ASM_STACKPROTECTOR_H 1 36 37 #ifdef CONFIG_STACKPROTECTOR 38 39 #include <asm/tsc.h> 40 #include <asm/processor.h> 41 #include <asm/percpu.h> 42 #include <asm/desc.h> 43 44 #include <linux/random.h> 45 #include <linux/sched.h> 46 47 /* 48 * 24 byte read-only segment initializer for stack canary. Linker 49 * can't handle the address bit shifting. Address will be set in 50 * head_32 for boot CPU and setup_per_cpu_areas() for others. 51 */ 52 #define GDT_STACK_CANARY_INIT \ 53 [GDT_ENTRY_STACK_CANARY] = GDT_ENTRY_INIT(0x4090, 0, 0x18), 54 55 /* 56 * Initialize the stackprotector canary value. 57 * 58 * NOTE: this must only be called from functions that never return, 59 * and it must always be inlined. 60 */ 61 static __always_inline void boot_init_stack_canary(void) 62 { 63 u64 canary; 64 u64 tsc; 65 66 #ifdef CONFIG_X86_64 67 BUILD_BUG_ON(offsetof(struct fixed_percpu_data, stack_canary) != 40); 68 #endif 69 /* 70 * We both use the random pool and the current TSC as a source 71 * of randomness. The TSC only matters for very early init, 72 * there it already has some randomness on most systems. Later 73 * on during the bootup the random pool has true entropy too. 74 */ 75 get_random_bytes(&canary, sizeof(canary)); 76 tsc = rdtsc(); 77 canary += tsc + (tsc << 32UL); 78 canary &= CANARY_MASK; 79 80 current->stack_canary = canary; 81 #ifdef CONFIG_X86_64 82 this_cpu_write(fixed_percpu_data.stack_canary, canary); 83 #else 84 this_cpu_write(stack_canary.canary, canary); 85 #endif 86 } 87 88 static inline void setup_stack_canary_segment(int cpu) 89 { 90 #ifdef CONFIG_X86_32 91 unsigned long canary = (unsigned long)&per_cpu(stack_canary, cpu); 92 struct desc_struct *gdt_table = get_cpu_gdt_rw(cpu); 93 struct desc_struct desc; 94 95 desc = gdt_table[GDT_ENTRY_STACK_CANARY]; 96 set_desc_base(&desc, canary); 97 write_gdt_entry(gdt_table, GDT_ENTRY_STACK_CANARY, &desc, DESCTYPE_S); 98 #endif 99 } 100 101 static inline void load_stack_canary_segment(void) 102 { 103 #ifdef CONFIG_X86_32 104 asm("mov %0, %%gs" : : "r" (__KERNEL_STACK_CANARY) : "memory"); 105 #endif 106 } 107 108 #else /* STACKPROTECTOR */ 109 110 #define GDT_STACK_CANARY_INIT 111 112 /* dummy boot_init_stack_canary() is defined in linux/stackprotector.h */ 113 114 static inline void setup_stack_canary_segment(int cpu) 115 { } 116 117 static inline void load_stack_canary_segment(void) 118 { 119 #ifdef CONFIG_X86_32 120 asm volatile ("mov %0, %%gs" : : "r" (0)); 121 #endif 122 } 123 124 #endif /* STACKPROTECTOR */ 125 #endif /* _ASM_STACKPROTECTOR_H */ 126