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/* SPDX-License-Identifier: GPL-2.0 */ /* * 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 * fixed_percpu_data 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 #ifdef CONFIG_STACKPROTECTOR #include <asm/tsc.h> #include <asm/processor.h> #include <asm/percpu.h> #include <asm/desc.h> #include <linux/random.h> #include <linux/sched.h> /* * 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] = GDT_ENTRY_INIT(0x4090, 0, 0x18), /* * Initialize the stackprotector canary value. * * NOTE: this must only be called from functions that never return * and it must always be inlined. * * In addition, it should be called from a compilation unit for which * stack protector is disabled. Alternatively, the caller should not end * with a function call which gets tail-call optimized as that would * lead to checking a modified canary value. */ static __always_inline void boot_init_stack_canary(void) { u64 canary; u64 tsc; #ifdef CONFIG_X86_64 BUILD_BUG_ON(offsetof(struct fixed_percpu_data, 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, * there it already has some randomness on most systems. Later * on during the bootup the random pool has true entropy too. */ get_random_bytes(&canary, sizeof(canary)); tsc = rdtsc(); canary += tsc + (tsc << 32UL); canary &= CANARY_MASK; current->stack_canary = canary; #ifdef CONFIG_X86_64 this_cpu_write(fixed_percpu_data.stack_canary, canary); #else this_cpu_write(stack_canary.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_rw(cpu); struct desc_struct desc; desc = gdt_table[GDT_ENTRY_STACK_CANARY]; set_desc_base(&desc, canary); 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 /* 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 /* STACKPROTECTOR */ #endif /* _ASM_STACKPROTECTOR_H */
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