Kernel: Implement some basic stack pointer validation

VM regions can now be marked as stack regions, which is then validated
on syscall, and on page fault.

If a thread is caught with its stack pointer pointing into anything
that's *not* a Region with its stack bit set, we'll crash the whole
process with SIGSTKFLT.

Userspace must now allocate custom stacks by using mmap() with the new
MAP_STACK flag. This mechanism was first introduced in OpenBSD, and now
we have it too, yay! :^)

1 files changed, 2 insertions, 0 deletions

diff --git a/Kernel/Thread.cpp b/Kernel/Thread.cpp
index 4e4f8fae14..f1034e61e4 100644
--- a/Kernel/Thread.cpp
+++ b/Kernel/Thread.cpp
@@ -569,6 +569,7 @@ void Thread::make_userspace_stack_for_main_thread(Vector<String> arguments, Vect
 {
     auto* region = m_process.allocate_region(VirtualAddress(), default_userspace_stack_size, "Stack (Main thread)", PROT_READ | PROT_WRITE, false);
     ASSERT(region);
+    region->set_stack(true);
     m_tss.esp = region->vaddr().offset(default_userspace_stack_size).get();
 
     char* stack_base = (char*)region->vaddr().get();
@@ -604,6 +605,7 @@ void Thread::make_userspace_stack_for_secondary_thread(void* argument)
 {
     m_userspace_stack_region = m_process.allocate_region(VirtualAddress(), default_userspace_stack_size, String::format("Stack (Thread %d)", tid()), PROT_READ | PROT_WRITE, false);
     ASSERT(m_userspace_stack_region);
+    m_userspace_stack_region->set_stack(true);
     m_tss.esp = m_userspace_stack_region->vaddr().offset(default_userspace_stack_size).get();
 
     // NOTE: The stack needs to be 16-byte aligned.