Kernel: Unmap non-readable pages

This was caught by running all crash tests with "crash -A".
Basically, non-readable pages need to not be mapped *at all* so that
a "page not present" exception is provoked on access.

Unfortunately x86 does not support write-only mappings, so this is
the best we can do.

Fixes #1336.

1 files changed, 5 insertions, 2 deletions

diff --git a/Kernel/VM/Region.cpp b/Kernel/VM/Region.cpp
index d498ca910a..a64c66a7f9 100644
--- a/Kernel/VM/Region.cpp
+++ b/Kernel/VM/Region.cpp
@@ -213,7 +213,7 @@ void Region::map_individual_page_impl(size_t page_index)
     auto page_vaddr = vaddr().offset(page_index * PAGE_SIZE);
     auto& pte = MM.ensure_pte(*m_page_directory, page_vaddr);
     auto& physical_page = vmobject().physical_pages()[first_page_index() + page_index];
-    if (!physical_page) {
+    if (!physical_page || !is_readable()) {
         pte.clear();
     } else {
         pte.set_cache_disabled(!m_cacheable);
@@ -291,7 +291,10 @@ PageFaultResponse Region::handle_fault(const PageFault& fault)
             dbg() << "NP(non-readable) fault in Region{" << this << "}[" << page_index_in_region << "]";
             return PageFaultResponse::ShouldCrash;
         }
-
+        if (fault.is_write() && !is_writable()) {
+            dbg() << "NP(non-writable) write fault in Region{" << this << "}[" << page_index_in_region << "] at " << fault.vaddr();
+            return PageFaultResponse::ShouldCrash;
+        }
         if (vmobject().is_inode()) {
 #ifdef PAGE_FAULT_DEBUG
             dbg() << "NP(inode) fault in Region{" << this << "}[" << page_index_in_region << "]";