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       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                 :         21 : static __always_inline void boot_init_stack_canary(void)
      62                 :            : {
      63                 :         21 :         u64 canary;
      64                 :         21 :         u64 tsc;
      65                 :            : 
      66                 :            : #ifdef CONFIG_X86_64
      67                 :         21 :         BUILD_BUG_ON(offsetof(struct fixed_percpu_data, stack_canary) != 40);
      68                 :            : #endif
      69                 :            :         /*
      70                 :            :          * We both use the random pool and the current TSC as a source
      71                 :            :          * of randomness. The TSC only matters for very early init,
      72                 :            :          * there it already has some randomness on most systems. Later
      73                 :            :          * on during the bootup the random pool has true entropy too.
      74                 :            :          */
      75                 :         21 :         get_random_bytes(&canary, sizeof(canary));
      76                 :         21 :         tsc = rdtsc();
      77                 :         21 :         canary += tsc + (tsc << 32UL);
      78                 :         21 :         canary &= CANARY_MASK;
      79                 :            : 
      80                 :         21 :         current->stack_canary = canary;
      81                 :            : #ifdef CONFIG_X86_64
      82                 :         21 :         this_cpu_write(fixed_percpu_data.stack_canary, canary);
      83                 :            : #else
      84                 :            :         this_cpu_write(stack_canary.canary, canary);
      85                 :            : #endif
      86                 :            : }
      87                 :            : 
      88                 :         21 : static inline void setup_stack_canary_segment(int cpu)
      89                 :            : {
      90                 :            : #ifdef CONFIG_X86_32
      91                 :            :         unsigned long canary = (unsigned long)&per_cpu(stack_canary, cpu);
      92                 :            :         struct desc_struct *gdt_table = get_cpu_gdt_rw(cpu);
      93                 :            :         struct desc_struct desc;
      94                 :            : 
      95                 :            :         desc = gdt_table[GDT_ENTRY_STACK_CANARY];
      96                 :            :         set_desc_base(&desc, canary);
      97                 :            :         write_gdt_entry(gdt_table, GDT_ENTRY_STACK_CANARY, &desc, DESCTYPE_S);
      98                 :            : #endif
      99         [ +  - ]:         21 : }
     100                 :            : 
     101                 :         63 : static inline void load_stack_canary_segment(void)
     102                 :            : {
     103                 :            : #ifdef CONFIG_X86_32
     104                 :            :         asm("mov %0, %%gs" : : "r" (__KERNEL_STACK_CANARY) : "memory");
     105                 :            : #endif
     106                 :         63 : }
     107                 :            : 
     108                 :            : #else   /* STACKPROTECTOR */
     109                 :            : 
     110                 :            : #define GDT_STACK_CANARY_INIT
     111                 :            : 
     112                 :            : /* dummy boot_init_stack_canary() is defined in linux/stackprotector.h */
     113                 :            : 
     114                 :            : static inline void setup_stack_canary_segment(int cpu)
     115                 :            : { }
     116                 :            : 
     117                 :            : static inline void load_stack_canary_segment(void)
     118                 :            : {
     119                 :            : #ifdef CONFIG_X86_32
     120                 :            :         asm volatile ("mov %0, %%gs" : : "r" (0));
     121                 :            : #endif
     122                 :            : }
     123                 :            : 
     124                 :            : #endif  /* STACKPROTECTOR */
     125                 :            : #endif  /* _ASM_STACKPROTECTOR_H */