include/linux/randomize_kstack.h

   1 /* SPDX-License-Identifier: GPL-2.0-only */
   2 #ifndef _LINUX_RANDOMIZE_KSTACK_H
   3 #define _LINUX_RANDOMIZE_KSTACK_H
   4
   5 #ifdef CONFIG_RANDOMIZE_KSTACK_OFFSET
   6 #include <linux/kernel.h>
   7 #include <linux/jump_label.h>
   8 #include <linux/percpu-defs.h>
   9
  10 DECLARE_STATIC_KEY_MAYBE(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT,
  11                          randomize_kstack_offset);
  12 DECLARE_PER_CPU(u32, kstack_offset);
  13
  14 /*
  15  * Do not use this anywhere else in the kernel. This is used here because
  16  * it provides an arch-agnostic way to grow the stack with correct
  17  * alignment. Also, since this use is being explicitly masked to a max of
  18  * 10 bits, stack-clash style attacks are unlikely. For more details see
  19  * "VLAs" in Documentation/process/deprecated.rst
  20  *
  21  * The normal __builtin_alloca() is initialized with INIT_STACK_ALL (currently
  22  * only with Clang and not GCC). Initializing the unused area on each syscall
  23  * entry is expensive, and generating an implicit call to memset() may also be
  24  * problematic (such as in noinstr functions). Therefore, if the compiler
  25  * supports it (which it should if it initializes allocas), always use the
  26  * "uninitialized" variant of the builtin.
  27  */
  28 #if __has_builtin(__builtin_alloca_uninitialized)
  29 #define __kstack_alloca __builtin_alloca_uninitialized
  30 #else
  31 #define __kstack_alloca __builtin_alloca
  32 #endif
  33
  34 /*
  35  * Use, at most, 6 bits of entropy (on 64-bit; 8 on 32-bit). This cap is
  36  * to keep the "VLA" from being unbounded (see above). Additionally clear
  37  * the bottom 4 bits (on 64-bit systems, 2 for 32-bit), since stack
  38  * alignment will always be at least word size. This makes the compiler
  39  * code gen better when it is applying the actual per-arch alignment to
  40  * the final offset. The resulting randomness is reasonable without overly
  41  * constraining usable stack space.
  42  */
  43 #ifdef CONFIG_64BIT
  44 #define KSTACK_OFFSET_MAX(x)    ((x) & 0b1111110000)
  45 #else
  46 #define KSTACK_OFFSET_MAX(x)    ((x) & 0b1111111100)
  47 #endif
  48
  49 /**
  50  * add_random_kstack_offset - Increase stack utilization by previously
  51  *                            chosen random offset
  52  *
  53  * This should be used in the syscall entry path when interrupts and
  54  * preempt are disabled, and after user registers have been stored to
  55  * the stack. For testing the resulting entropy, please see:
  56  * tools/testing/selftests/lkdtm/stack-entropy.sh
  57  */
  58 #define add_random_kstack_offset() do {                                 \
  59         if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, \
  60                                 &randomize_kstack_offset)) {            \
  61                 u32 offset = raw_cpu_read(kstack_offset);               \
  62                 u8 *ptr = __kstack_alloca(KSTACK_OFFSET_MAX(offset));   \
  63                 /* Keep allocation even after "ptr" loses scope. */     \
  64                 asm volatile("" :: "r"(ptr) : "memory");                \
  65         }                                                               \
  66 } while (0)
  67
  68 /**
  69  * choose_random_kstack_offset - Choose the random offset for the next
  70  *                               add_random_kstack_offset()
  71  *
  72  * This should only be used during syscall exit when interrupts and
  73  * preempt are disabled. This position in the syscall flow is done to
  74  * frustrate attacks from userspace attempting to learn the next offset:
  75  * - Maximize the timing uncertainty visible from userspace: if the
  76  *   offset is chosen at syscall entry, userspace has much more control
  77  *   over the timing between choosing offsets. "How long will we be in
  78  *   kernel mode?" tends to be more difficult to predict than "how long
  79  *   will we be in user mode?"
  80  * - Reduce the lifetime of the new offset sitting in memory during
  81  *   kernel mode execution. Exposure of "thread-local" memory content
  82  *   (e.g. current, percpu, etc) tends to be easier than arbitrary
  83  *   location memory exposure.
  84  */
  85 #define choose_random_kstack_offset(rand) do {                          \
  86         if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, \
  87                                 &randomize_kstack_offset)) {            \
  88                 u32 offset = raw_cpu_read(kstack_offset);               \
  89                 offset = ror32(offset, 5) ^ (rand);                     \
  90                 raw_cpu_write(kstack_offset, offset);                   \
  91         }                                                               \
  92 } while (0)
  93 #else /* CONFIG_RANDOMIZE_KSTACK_OFFSET */
  94 #define add_random_kstack_offset()              do { } while (0)
  95 #define choose_random_kstack_offset(rand)       do { } while (0)
  96 #endif /* CONFIG_RANDOMIZE_KSTACK_OFFSET */
  97
  98 #endif