diff options
Diffstat (limited to 'arch/x86/include/asm/stackprotector.h')
| -rw-r--r-- | arch/x86/include/asm/stackprotector.h | 79 |
1 files changed, 16 insertions, 63 deletions
diff --git a/arch/x86/include/asm/stackprotector.h b/arch/x86/include/asm/stackprotector.h index 7fb482f0f..b6ffe58c7 100644 --- a/arch/x86/include/asm/stackprotector.h +++ b/arch/x86/include/asm/stackprotector.h @@ -5,30 +5,23 @@ * 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. + * and unfortunately gcc historically required it to be at a fixed offset + * from the percpu segment base. On x86_64, the offset is 40 bytes. * - * 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. + * The same segment is shared by percpu area and stack canary. On + * x86_64, percpu symbols are zero based and %gs (64-bit) points to the + * base of percpu area. The first occupant of the percpu area is always + * fixed_percpu_data which contains stack_canary at the approproate + * offset. On x86_32, the stack canary is just a regular percpu + * variable. * - * 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. + * Putting percpu data in %fs on 32-bit is a minor optimization compared to + * using %gs. Since 32-bit userspace normally has %fs == 0, we are likely + * to load 0 into %fs on exit to usermode, whereas with percpu data in + * %gs, we are likely to load a non-null %gs on return to user mode. * - * 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. + * Once we are willing to require GCC 8.1 or better for 64-bit stackprotector + * support, we can remove some of this complexity. */ #ifndef _ASM_STACKPROTECTOR_H @@ -45,14 +38,6 @@ #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 @@ -86,7 +71,7 @@ static __always_inline void boot_init_stack_canary(void) #ifdef CONFIG_X86_64 this_cpu_write(fixed_percpu_data.stack_canary, canary); #else - this_cpu_write(stack_canary.canary, canary); + this_cpu_write(__stack_chk_guard, canary); #endif } @@ -95,48 +80,16 @@ static inline void cpu_init_stack_canary(int cpu, struct task_struct *idle) #ifdef CONFIG_X86_64 per_cpu(fixed_percpu_data.stack_canary, cpu) = idle->stack_canary; #else - per_cpu(stack_canary.canary, cpu) = idle->stack_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"); + per_cpu(__stack_chk_guard, cpu) = idle->stack_canary; #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 cpu_init_stack_canary(int cpu, struct task_struct *idle) { } -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 */ |