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 appropriate 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/sched.h> 38 39 /* 40 * Initialize the stackprotector canary value. 41 * 42 * NOTE: this must only be called from functions that never return 43 * and it must always be inlined. 44 * 45 * In addition, it should be called from a compilation unit for which 46 * stack protector is disabled. Alternatively, the caller should not end 47 * with a function call which gets tail-call optimized as that would 48 * lead to checking a modified canary value. 49 */ 50 static __always_inline void boot_init_stack_canary(void) 51 { 52 unsigned long canary = get_random_canary(); 53 54 #ifdef CONFIG_X86_64 55 BUILD_BUG_ON(offsetof(struct fixed_percpu_data, stack_canary) != 40); 56 #endif 57 58 current->stack_canary = canary; 59 #ifdef CONFIG_X86_64 60 this_cpu_write(fixed_percpu_data.stack_canary, canary); 61 #else 62 this_cpu_write(__stack_chk_guard, canary); 63 #endif 64 } 65 66 static inline void cpu_init_stack_canary(int cpu, struct task_struct *idle) 67 { 68 #ifdef CONFIG_X86_64 69 per_cpu(fixed_percpu_data.stack_canary, cpu) = idle->stack_canary; 70 #else 71 per_cpu(__stack_chk_guard, cpu) = idle->stack_canary; 72 #endif 73 } 74 75 #else /* STACKPROTECTOR */ 76 77 /* dummy boot_init_stack_canary() is defined in linux/stackprotector.h */ 78 79 static inline void cpu_init_stack_canary(int cpu, struct task_struct *idle) 80 { } 81 82 #endif /* STACKPROTECTOR */ 83 #endif /* _ASM_STACKPROTECTOR_H */ 84