arch/x86/include/asm/stackprotector.h

   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  * In addition, it should be called from a compilation unit for which
  62  * stack protector is disabled. Alternatively, the caller should not end
  63  * with a function call which gets tail-call optimized as that would
  64  * lead to checking a modified canary value.
  65  */
  66 static __always_inline void boot_init_stack_canary(void)
  67 {
  68         u64 canary;
  69         u64 tsc;
  70
  71 #ifdef CONFIG_X86_64
  72         BUILD_BUG_ON(offsetof(struct fixed_percpu_data, stack_canary) != 40);
  73 #endif
  74         /*
  75          * We both use the random pool and the current TSC as a source
  76          * of randomness. The TSC only matters for very early init,
  77          * there it already has some randomness on most systems. Later
  78          * on during the bootup the random pool has true entropy too.
  79          */
  80         get_random_bytes(&canary, sizeof(canary));
  81         tsc = rdtsc();
  82         canary += tsc + (tsc << 32UL);
  83         canary &= CANARY_MASK;
  84
  85         current->stack_canary = canary;
  86 #ifdef CONFIG_X86_64
  87         this_cpu_write(fixed_percpu_data.stack_canary, canary);
  88 #else
  89         this_cpu_write(stack_canary.canary, canary);
  90 #endif
  91 }
  92
  93 static inline void setup_stack_canary_segment(int cpu)
  94 {
  95 #ifdef CONFIG_X86_32
  96         unsigned long canary = (unsigned long)&per_cpu(stack_canary, cpu);
  97         struct desc_struct *gdt_table = get_cpu_gdt_rw(cpu);
  98         struct desc_struct desc;
  99
 100         desc = gdt_table[GDT_ENTRY_STACK_CANARY];
 101         set_desc_base(&desc, canary);
 102         write_gdt_entry(gdt_table, GDT_ENTRY_STACK_CANARY, &desc, DESCTYPE_S);
 103 #endif
 104 }
 105
 106 static inline void load_stack_canary_segment(void)
 107 {
 108 #ifdef CONFIG_X86_32
 109         asm("mov %0, %%gs" : : "r" (__KERNEL_STACK_CANARY) : "memory");
 110 #endif
 111 }
 112
 113 #else   /* STACKPROTECTOR */
 114
 115 #define GDT_STACK_CANARY_INIT
 116
 117 /* dummy boot_init_stack_canary() is defined in linux/stackprotector.h */
 118
 119 static inline void setup_stack_canary_segment(int cpu)
 120 { }
 121
 122 static inline void load_stack_canary_segment(void)
 123 {
 124 #ifdef CONFIG_X86_32
 125         asm volatile ("mov %0, %%gs" : : "r" (0));
 126 #endif
 127 }
 128
 129 #endif  /* STACKPROTECTOR */
 130 #endif  /* _ASM_STACKPROTECTOR_H */