gcc-plugins: Add STACKLEAK plugin for tracking the kernel stack

The STACKLEAK feature erases the kernel stack before returning from
syscalls. That reduces the information which kernel stack leak bugs can
reveal and blocks some uninitialized stack variable attacks.

This commit introduces the STACKLEAK gcc plugin. It is needed for
tracking the lowest border of the kernel stack, which is important
for the code erasing the used part of the kernel stack at the end
of syscalls (comes in a separate commit).

The STACKLEAK feature is ported from grsecurity/PaX. More information at:
  https://grsecurity.net/
  https://pax.grsecurity.net/

This code is modified from Brad Spengler/PaX Team's code in the last
public patch of grsecurity/PaX based on our understanding of the code.
Changes or omissions from the original code are ours and don't reflect
the original grsecurity/PaX code.

Signed-off-by: Alexander Popov <[email protected]>
Tested-by: Laura Abbott <[email protected]>
Signed-off-by: Kees Cook <[email protected]>

kernel/stackleak.c

@@ -60,3 +60,31 @@ asmlinkage void stackleak_erase(void)
 	current->lowest_stack = current_top_of_stack() - THREAD_SIZE/64;
 }
 
+void __used stackleak_track_stack(void)
+{
+	/*
+	 * N.B. stackleak_erase() fills the kernel stack with the poison value,
+	 * which has the register width. That code assumes that the value
+	 * of 'lowest_stack' is aligned on the register width boundary.
+	 *
+	 * That is true for x86 and x86_64 because of the kernel stack
+	 * alignment on these platforms (for details, see 'cc_stack_align' in
+	 * arch/x86/Makefile). Take care of that when you port STACKLEAK to
+	 * new platforms.
+	 */
+	unsigned long sp = (unsigned long)&sp;
+
+	/*
+	 * Having CONFIG_STACKLEAK_TRACK_MIN_SIZE larger than
+	 * STACKLEAK_SEARCH_DEPTH makes the poison search in
+	 * stackleak_erase() unreliable. Let's prevent that.
+	 */
+	BUILD_BUG_ON(CONFIG_STACKLEAK_TRACK_MIN_SIZE > STACKLEAK_SEARCH_DEPTH);
+
+	if (sp < current->lowest_stack &&
+	    sp >= (unsigned long)task_stack_page(current) +
+						sizeof(unsigned long)) {
+		current->lowest_stack = sp;
+	}
+}
+EXPORT_SYMBOL(stackleak_track_stack);

scripts/Makefile.gcc-plugins

@@ -26,6 +26,16 @@ gcc-plugin-cflags-$(CONFIG_GCC_PLUGIN_RANDSTRUCT)		\
 gcc-plugin-cflags-$(CONFIG_GCC_PLUGIN_RANDSTRUCT_PERFORMANCE)	\
 		+= -fplugin-arg-randomize_layout_plugin-performance-mode
 
+gcc-plugin-$(CONFIG_GCC_PLUGIN_STACKLEAK)	+= stackleak_plugin.so
+gcc-plugin-cflags-$(CONFIG_GCC_PLUGIN_STACKLEAK)		\
+		+= -DSTACKLEAK_PLUGIN
+gcc-plugin-cflags-$(CONFIG_GCC_PLUGIN_STACKLEAK)		\
+		+= -fplugin-arg-stackleak_plugin-track-min-size=$(CONFIG_STACKLEAK_TRACK_MIN_SIZE)
+ifdef CONFIG_GCC_PLUGIN_STACKLEAK
+    DISABLE_STACKLEAK_PLUGIN += -fplugin-arg-stackleak_plugin-disable
+endif
+export DISABLE_STACKLEAK_PLUGIN
+
 # All the plugin CFLAGS are collected here in case a build target needs to
 # filter them out of the KBUILD_CFLAGS.
 GCC_PLUGINS_CFLAGS := $(strip $(addprefix -fplugin=$(objtree)/scripts/gcc-plugins/, $(gcc-plugin-y)) $(gcc-plugin-cflags-y))

scripts/gcc-plugins/Kconfig

@@ -158,4 +158,16 @@ config GCC_PLUGIN_STACKLEAK
 	   * https://grsecurity.net/
 	   * https://pax.grsecurity.net/
 
+config STACKLEAK_TRACK_MIN_SIZE
+	int "Minimum stack frame size of functions tracked by STACKLEAK"
+	default 100
+	range 0 4096
+	depends on GCC_PLUGIN_STACKLEAK
+	help
+	  The STACKLEAK gcc plugin instruments the kernel code for tracking
+	  the lowest border of the kernel stack (and for some other purposes).
+	  It inserts the stackleak_track_stack() call for the functions with
+	  a stack frame size greater than or equal to this parameter.
+	  If unsure, leave the default value 100.
+
 endif