stack: Introduce CONFIG_RANDOMIZE_KSTACK_OFFSET

The randomize_kstack_offset feature is unconditionally compiled in when
the architecture supports it.

To add constraints on compiler versions, we require a dedicated Kconfig
variable. Therefore, introduce RANDOMIZE_KSTACK_OFFSET.

Furthermore, this option is now also configurable by EXPERT kernels:
while the feature is supposed to have zero performance overhead when
disabled, due to its use of static branches, there are few cases where
giving a distribution the option to disable the feature entirely makes
sense. For example, in very resource constrained environments, which
would never enable the feature to begin with, in which case the
additional kernel code size increase would be redundant.

Signed-off-by: Marco Elver <[e-mail]>
Reviewed-by: Nathan Chancellor <[e-mail]>
Acked-by: Peter Zijlstra (Intel) <[e-mail]>
Acked-by: Kees Cook <[e-mail]>
Signed-off-by: Kees Cook <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

arch/Kconfig

···
 	  to the compiler, so it will attempt to add canary checks regardless
 	  of the static branch state.
 
-config RANDOMIZE_KSTACK_OFFSET_DEFAULT
-	bool "Randomize kernel stack offset on syscall entry"
+config RANDOMIZE_KSTACK_OFFSET
+	bool "Support for randomizing kernel stack offset on syscall entry" if EXPERT
+	default y
 	depends on HAVE_ARCH_RANDOMIZE_KSTACK_OFFSET
 	help
 	  The kernel stack offset can be randomized (after pt_regs) by
 	  roughly 5 bits of entropy, frustrating memory corruption
 	  attacks that depend on stack address determinism or
-	  cross-syscall address exposures. This feature is controlled
-	  by kernel boot param "randomize_kstack_offset=on/off", and this
-	  config chooses the default boot state.
+	  cross-syscall address exposures.
+
+	  The feature is controlled via the "randomize_kstack_offset=on/off"
+	  kernel boot param, and if turned off has zero overhead due to its use
+	  of static branches (see JUMP_LABEL).
+
+	  If unsure, say Y.
+
+config RANDOMIZE_KSTACK_OFFSET_DEFAULT
+	bool "Default state of kernel stack offset randomization"
+	depends on RANDOMIZE_KSTACK_OFFSET
+	help
+	  Kernel stack offset randomization is controlled by kernel boot param
+	  "randomize_kstack_offset=on/off", and this config chooses the default
+	  boot state.
 
 config ARCH_OPTIONAL_KERNEL_RWX
 	def_bool n

include/linux/randomize_kstack.h

···
 #ifndef _LINUX_RANDOMIZE_KSTACK_H
 #define _LINUX_RANDOMIZE_KSTACK_H
 
+#ifdef CONFIG_RANDOMIZE_KSTACK_OFFSET
 #include <linux/kernel.h>
 #include <linux/jump_label.h>
 #include <linux/percpu-defs.h>
···
 		raw_cpu_write(kstack_offset, offset);			\
 	}								\
 } while (0)
+#else /* CONFIG_RANDOMIZE_KSTACK_OFFSET */
+#define add_random_kstack_offset()		do { } while (0)
+#define choose_random_kstack_offset(rand)	do { } while (0)
+#endif /* CONFIG_RANDOMIZE_KSTACK_OFFSET */
 
 #endif

init/main.c

···
 	pti_init();
 }
 
-#ifdef CONFIG_HAVE_ARCH_RANDOMIZE_KSTACK_OFFSET
+#ifdef CONFIG_RANDOMIZE_KSTACK_OFFSET
 DEFINE_STATIC_KEY_MAYBE_RO(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT,
 			   randomize_kstack_offset);
 DEFINE_PER_CPU(u32, kstack_offset);