Kernel: Fix TOCTOU in syscall entry region validation

We were doing stack and syscall-origin region validations before
taking the big process lock. There was a window of time where those
regions could then be unmapped/remapped by another thread before we
proceed with our syscall.

This patch closes that window, and makes sys$get_stack_bounds() rely
on the fact that we now know the userspace stack pointer to be valid.

Thanks to @BenWiederhake for spotting this! :^)

Kernel/Syscall.cpp

@@ -169,6 +169,9 @@ void syscall_handler(TrapFrame* trap)
  PANIC("Syscall from process with IOPL != 0");
  }
 
+ // NOTE: We take the big process lock before inspecting memory regions.
+ process.big_lock().lock();
+
  if (!MM.validate_user_stack(process, VirtualAddress(regs.userspace_esp))) {
  dbgln("Invalid stack pointer: {:p}", regs.userspace_esp);
  handle_crash(regs, "Bad stack on syscall entry", SIGSTKFLT);
@@ -190,7 +193,6 @@ void syscall_handler(TrapFrame* trap)
  handle_crash(regs, "Syscall from non-syscall region", SIGSEGV);
  }
 
- process.big_lock().lock();
  u32 function = regs.eax;
  u32 arg1 = regs.edx;
  u32 arg2 = regs.ecx;

Kernel/Syscalls/get_stack_bounds.cpp

@@ -34,9 +34,9 @@ int Process::sys$get_stack_bounds(FlatPtr* user_stack_base, size_t* user_stack_s
 {
  FlatPtr stack_pointer = Thread::current()->get_register_dump_from_stack().userspace_esp;
  auto* stack_region = space().find_region_containing(Range { VirtualAddress(stack_pointer), 1 });
- if (!stack_region) {
-  PANIC("sys$get_stack_bounds: No stack region found for process");
- }
+
+ // The syscall handler should have killed us if we had an invalid stack pointer.
+ ASSERT(stack_region);
 
  FlatPtr stack_base = stack_region->range().base().get();
  size_t stack_size = stack_region->size();