1 files changed, 132 insertions, 0 deletions
diff --git a/kernel/stackleak.c b/kernel/stackleak.c
new file mode 100644
index 000000000000..e42892926244
--- /dev/null
+++ b/kernel/stackleak.c
@@ -0,0 +1,132 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * This code fills the used part of the kernel stack with a poison value
+ * before returning to userspace. It's part of the STACKLEAK feature
+ * ported from grsecurity/PaX.
+ *
+ * Author: Alexander Popov <alex.popov@linux.com>
+ *
+ * STACKLEAK reduces the information which kernel stack leak bugs can
+ * reveal and blocks some uninitialized stack variable attacks.
+ */
+
+#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 */
+	unsigned long kstack_ptr = current->lowest_stack;
+	unsigned long boundary = (unsigned long)end_of_stack(current);
+	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;
+
+	/* Search for the poison value in the kernel stack */
+	while (kstack_ptr > boundary && poison_count <= depth) {
+		if (*(unsigned long *)kstack_ptr == STACKLEAK_POISON)
+			poison_count++;
+		else
+			poison_count = 0;
+
+		kstack_ptr -= sizeof(unsigned long);
+	}
+
+	/*
+	 * One 'long int' at the bottom of the thread stack is reserved and
+	 * should not be poisoned (see CONFIG_SCHED_STACK_END_CHECK=y).
+	 */
+	if (kstack_ptr == boundary)
+		kstack_ptr += sizeof(unsigned long);
+
+#ifdef CONFIG_STACKLEAK_METRICS
+	current->prev_lowest_stack = kstack_ptr;
+#endif
+
+	/*
+	 * Now write the poison value to the kernel stack. Start from
+	 * 'kstack_ptr' and move up till the new 'boundary'. We assume that
+	 * the stack pointer doesn't change when we write poison.
+	 */
+	if (on_thread_stack())
+		boundary = current_stack_pointer;
+	else
+		boundary = current_top_of_stack();
+
+	while (kstack_ptr < boundary) {
+		*(unsigned long *)kstack_ptr = STACKLEAK_POISON;
+		kstack_ptr += sizeof(unsigned long);
+	}
+
+	/* Reset the 'lowest_stack' value for the next syscall */
+	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);