Kernel: Fix memory corruption when rolling back regions in execve

We need to free the regions before reverting the paging scope to the
original one when rolling back changes due to an error. This fixes
silent memory corruption.

Kernel/Syscalls/execve.cpp

@@ -157,7 +157,6 @@ KResultOr<Process::LoadResult> Process::load_elf_object(FileDescription& object_
     size_t master_tls_alignment = 0;
     FlatPtr load_base_address = 0;
 
-    MM.enter_process_paging_scope(*this);
     String elf_name = object_description.absolute_path();
     ASSERT(!Processor::current().in_critical());
 
@@ -293,15 +292,21 @@ KResultOr<Process::LoadResult> Process::load(NonnullRefPtr<FileDescription> main
         old_page_directory = move(m_page_directory);
         old_regions = move(m_regions);
         m_page_directory = page_directory.release_nonnull();
+        MM.enter_process_paging_scope(*this);
     }
 
     ArmedScopeGuard rollback_regions_guard([&]() {
         ASSERT(Process::current() == this);
         // Need to make sure we don't swap contexts in the middle
         ScopedCritical critical;
+        // Explicitly clear m_regions *before* restoring the page directory,
+        // otherwise we may silently corrupt memory!
+        m_regions.clear();
+        // Now that we freed the regions, revert to the original page directory
+        // and restore the original regions
         m_page_directory = move(old_page_directory);
-        m_regions = move(old_regions);
         MM.enter_process_paging_scope(*this);
+        m_regions = move(old_regions);
     });
 
     if (!interpreter_description) {