Line data    Source code

       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 cpu_init_stack_canary(int cpu, struct task_struct *idle)
      94             : {
      95             : #ifdef CONFIG_X86_64
      96             :         per_cpu(fixed_percpu_data.stack_canary, cpu) = idle->stack_canary;
      97             : #else
      98             :         per_cpu(stack_canary.canary, cpu) = idle->stack_canary;
      99             : #endif
     100             : }
     101             : 
     102             : static inline void setup_stack_canary_segment(int cpu)
     103             : {
     104             : #ifdef CONFIG_X86_32
     105             :         unsigned long canary = (unsigned long)&per_cpu(stack_canary, cpu);
     106             :         struct desc_struct *gdt_table = get_cpu_gdt_rw(cpu);
     107             :         struct desc_struct desc;
     108             : 
     109             :         desc = gdt_table[GDT_ENTRY_STACK_CANARY];
     110             :         set_desc_base(&desc, canary);
     111             :         write_gdt_entry(gdt_table, GDT_ENTRY_STACK_CANARY, &desc, DESCTYPE_S);
     112             : #endif
     113             : }
     114             : 
     115             : static inline void load_stack_canary_segment(void)
     116             : {
     117             : #ifdef CONFIG_X86_32
     118             :         asm("mov %0, %%gs" : : "r" (__KERNEL_STACK_CANARY) : "memory");
     119             : #endif
     120             : }
     121             : 
     122             : #else   /* STACKPROTECTOR */
     123             : 
     124             : #define GDT_STACK_CANARY_INIT
     125             : 
     126             : /* dummy boot_init_stack_canary() is defined in linux/stackprotector.h */
     127             : 
     128           4 : static inline void setup_stack_canary_segment(int cpu)
     129           4 : { }
     130             : 
     131           3 : static inline void cpu_init_stack_canary(int cpu, struct task_struct *idle)
     132           3 : { }
     133             : 
     134           6 : static inline void load_stack_canary_segment(void)
     135             : {
     136             : #ifdef CONFIG_X86_32
     137             :         asm volatile ("mov %0, %%gs" : : "r" (0));
     138             : #endif
     139           6 : }
     140             : 
     141             : #endif  /* STACKPROTECTOR */
     142             : #endif  /* _ASM_STACKPROTECTOR_H */