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 historically required it to be at a fixed offset 9 * from the percpu segment base. On x86_64, the offset is 40 bytes. 10 * 11 * The same segment is shared by percpu area and stack canary. On 12 * x86_64, percpu symbols are zero based and %gs (64-bit) points to the 13 * base of percpu area. The first occupant of the percpu area is always 14 * fixed_percpu_data which contains stack_canary at the approproate 15 * offset. On x86_32, the stack canary is just a regular percpu 16 * variable. 17 * 18 * Putting percpu data in %fs on 32-bit is a minor optimization compared to 19 * using %gs. Since 32-bit userspace normally has %fs == 0, we are likely 20 * to load 0 into %fs on exit to usermode, whereas with percpu data in 21 * %gs, we are likely to load a non-null %gs on return to user mode. 22 * 23 * Once we are willing to require GCC 8.1 or better for 64-bit stackprotector 24 * support, we can remove some of this complexity. 25 */ 26 27 #ifndef _ASM_STACKPROTECTOR_H 28 #define _ASM_STACKPROTECTOR_H 1 29 30 #ifdef CONFIG_STACKPROTECTOR 31 32 #include <asm/tsc.h> 33 #include <asm/processor.h> 34 #include <asm/percpu.h> 35 #include <asm/desc.h> 36 37 #include <linux/random.h> 38 #include <linux/sched.h> 39 40 /* 41 * Initialize the stackprotector canary value. 42 * 43 * NOTE: this must only be called from functions that never return 44 * and it must always be inlined. 45 * 46 * In addition, it should be called from a compilation unit for which 47 * stack protector is disabled. Alternatively, the caller should not end 48 * with a function call which gets tail-call optimized as that would 49 * lead to checking a modified canary value. 50 */ 51 static __always_inline void boot_init_stack_canary(void) 52 { 53 u64 canary; 54 u64 tsc; 55 56 #ifdef CONFIG_X86_64 57 BUILD_BUG_ON(offsetof(struct fixed_percpu_data, stack_canary) != 40); 58 #endif 59 /* 60 * We both use the random pool and the current TSC as a source 61 * of randomness. The TSC only matters for very early init, 62 * there it already has some randomness on most systems. Later 63 * on during the bootup the random pool has true entropy too. 64 */ 65 get_random_bytes(&canary, sizeof(canary)); 66 tsc = rdtsc(); 67 canary += tsc + (tsc << 32UL); 68 canary &= CANARY_MASK; 69 70 current->stack_canary = canary; 71 #ifdef CONFIG_X86_64 72 this_cpu_write(fixed_percpu_data.stack_canary, canary); 73 #else 74 this_cpu_write(__stack_chk_guard, canary); 75 #endif 76 } 77 78 static inline void cpu_init_stack_canary(int cpu, struct task_struct *idle) 79 { 80 #ifdef CONFIG_X86_64 81 per_cpu(fixed_percpu_data.stack_canary, cpu) = idle->stack_canary; 82 #else 83 per_cpu(__stack_chk_guard, cpu) = idle->stack_canary; 84 #endif 85 } 86 87 #else /* STACKPROTECTOR */ 88 89 /* dummy boot_init_stack_canary() is defined in linux/stackprotector.h */ 90 91 static inline void cpu_init_stack_canary(int cpu, struct task_struct *idle) 92 { } 93 94 #endif /* STACKPROTECTOR */ 95 #endif /* _ASM_STACKPROTECTOR_H */ 96