stackleak: Allow runtime disabling of kernel stack erasing

Introduce CONFIG_STACKLEAK_RUNTIME_DISABLE option, which provides
'stack_erasing' sysctl. It can be used in runtime to control kernel
stack erasing for kernels built with CONFIG_GCC_PLUGIN_STACKLEAK.

Suggested-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Alexander Popov <alex.popov@linux.com>
Tested-by: Laura Abbott <labbott@redhat.com>
Signed-off-by: Kees Cook <keescook@chromium.org>

kernel/stackleak.c

@@ -12,6 +12,41 @@
 
 #include <linux/stackleak.h>
 
+#ifdef CONFIG_STACKLEAK_RUNTIME_DISABLE
+#include <linux/jump_label.h>
+#include <linux/sysctl.h>
+
+static DEFINE_STATIC_KEY_FALSE(stack_erasing_bypass);
+
+int stack_erasing_sysctl(struct ctl_table *table, int write,
+			void __user *buffer, size_t *lenp, loff_t *ppos)
+{
+	int ret = 0;
+	int state = !static_branch_unlikely(&stack_erasing_bypass);
+	int prev_state = state;
+
+	table->data = &state;
+	table->maxlen = sizeof(int);
+	ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
+	state = !!state;
+	if (ret || !write || state == prev_state)
+		return ret;
+
+	if (state)
+		static_branch_disable(&stack_erasing_bypass);
+	else
+		static_branch_enable(&stack_erasing_bypass);
+
+	pr_warn("stackleak: kernel stack erasing is %s\n",
+					state ? "enabled" : "disabled");
+	return ret;
+}
+
+#define skip_erasing()	static_branch_unlikely(&stack_erasing_bypass)
+#else
+#define skip_erasing()	false
+#endif /* CONFIG_STACKLEAK_RUNTIME_DISABLE */
+
 asmlinkage void stackleak_erase(void)
 {
 	/* It would be nice not to have 'kstack_ptr' and 'boundary' on stack */
@@ -20,6 +55,9 @@ asmlinkage void stackleak_erase(void)
 	unsigned int poison_count = 0;
 	const unsigned int depth = STACKLEAK_SEARCH_DEPTH / sizeof(unsigned long);
 
+	if (skip_erasing())
+		return;
+
 	/* Check that 'lowest_stack' value is sane */
 	if (unlikely(kstack_ptr - boundary >= THREAD_SIZE))
 		kstack_ptr = boundary;

kernel/sysctl.c

@@ -91,7 +91,9 @@
 #ifdef CONFIG_CHR_DEV_SG
 #include <scsi/sg.h>
 #endif
-
+#ifdef CONFIG_STACKLEAK_RUNTIME_DISABLE
+#include <linux/stackleak.h>
+#endif
 #ifdef CONFIG_LOCKUP_DETECTOR
 #include <linux/nmi.h>
 #endif
@@ -1232,6 +1234,17 @@ static struct ctl_table kern_table[] = {
 		.extra1		= &zero,
 		.extra2		= &one,
 	},
+#endif
+#ifdef CONFIG_STACKLEAK_RUNTIME_DISABLE
+	{
+		.procname	= "stack_erasing",
+		.data		= NULL,
+		.maxlen		= sizeof(int),
+		.mode		= 0600,
+		.proc_handler	= stack_erasing_sysctl,
+		.extra1		= &zero,
+		.extra2		= &one,
+	},
 #endif
 	{ }
 };