1 files changed, 62 insertions, 0 deletions

diff --git a/kernel/stackleak.c b/kernel/stackleak.c
new file mode 100644
index 000000000000..deba0d8992f9
--- /dev/null
+++ b/kernel/stackleak.c
@@ -0,0 +1,62 @@
+// 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>
+
+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);
+
+        /* 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);
+
+        /*
+         * 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;
+}
+