From e47bffdc8c1b69614d24d135f2ce26dcd8e35ca1 Mon Sep 17 00:00:00 2001
From: Andreas Kling <kling@serenityos.org>
Date: Sun, 14 Feb 2021 11:47:25 +0100
Subject: [PATCH] Kernel: Add some bits of randomness to the userspace stack
 pointer

This patch adds a random offset between 0 and 4096 to the initial
stack pointer in new processes. Since the stack has to be 16-byte
aligned, the bottom bits can't be randomized.

Yet another thing to make things less predictable. :^)
---
 Kernel/Syscalls/execve.cpp | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/Kernel/Syscalls/execve.cpp b/Kernel/Syscalls/execve.cpp
index 033c8c1f0e..7301e853e8 100644
--- a/Kernel/Syscalls/execve.cpp
+++ b/Kernel/Syscalls/execve.cpp
@@ -90,7 +90,10 @@ static bool validate_stack_size(const Vector<String>& arguments, const Vector<St
 
 static KResultOr<FlatPtr> make_userspace_stack_for_main_thread(Region& region, Vector<String> arguments, Vector<String> environment, Vector<ELF::AuxiliaryValue> auxiliary_values)
 {
-    FlatPtr new_esp = region.vaddr().offset(Thread::default_userspace_stack_size).get();
+    FlatPtr new_esp = region.range().end().get();
+
+    // Add some bits of randomness to the user stack pointer.
+    new_esp -= round_up_to_power_of_two(get_fast_random<u32>() % 4096, 16);
 
     auto push_on_new_stack = [&new_esp](u32 value) {
         new_esp -= 4;