From fa4c249425a65076ca04a3cb0c173d49472796fb Mon Sep 17 00:00:00 2001
From: Andreas Kling <kling@serenityos.org>
Date: Thu, 18 Feb 2021 18:43:20 +0100
Subject: [PATCH] LibELF+Userland: Enable RELRO for all userland executables
 :^)

The dynamic loader will now mark RELRO segments read-only after
performing relocations. This is pretty cool!

Note that this only applies to main executables so far,.
RELRO support for shared libraries will require some reorganizing
of the dynamic loader.
---
 CMakeLists.txt                              |  2 +-
 Userland/Libraries/LibELF/DynamicLoader.cpp | 25 ++++++++++++++++++++-
 Userland/Libraries/LibELF/DynamicLoader.h   |  4 ++++
 Userland/Libraries/LibELF/DynamicObject.cpp |  3 +++
 4 files changed, 32 insertions(+), 2 deletions(-)

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 868d5844807..80e946a3423 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -147,7 +147,7 @@ endforeach()
 set(CMAKE_INSTALL_NAME_TOOL "")
 set(CMAKE_SHARED_LIBRARY_SUFFIX ".so")
 set(CMAKE_SHARED_LIBRARY_CREATE_CXX_FLAGS "-shared -Wl,--hash-style=gnu")
-set(CMAKE_CXX_LINK_FLAGS "-Wl,--hash-style=gnu")
+set(CMAKE_CXX_LINK_FLAGS "-Wl,--hash-style=gnu,-z,relro,-z,now")
 
 # We disable it completely because it makes cmake very spammy.
 # This will need to be revisited when the Loader supports RPATH/RUN_PATH.
diff --git a/Userland/Libraries/LibELF/DynamicLoader.cpp b/Userland/Libraries/LibELF/DynamicLoader.cpp
index 8b08352a12f..af0c945fa0c 100644
--- a/Userland/Libraries/LibELF/DynamicLoader.cpp
+++ b/Userland/Libraries/LibELF/DynamicLoader.cpp
@@ -234,6 +234,20 @@ RefPtr<DynamicObject> DynamicLoader::load_stage_3(unsigned flags, size_t total_t
         return nullptr;
     }
 
+    if (m_relro_segment_size) {
+        if (mprotect(m_relro_segment_address.as_ptr(), m_relro_segment_size, PROT_READ) < 0) {
+            perror("mprotect .relro: PROT_READ");
+            return nullptr;
+        }
+
+#if __serenity__
+        if (set_mmap_name(m_relro_segment_address.as_ptr(), m_relro_segment_size, String::formatted("{}: .relro", m_filename).characters()) < 0) {
+            perror("set_mmap_name .relro");
+            return nullptr;
+        }
+#endif
+    }
+
     call_object_init_functions();
 
     dbgln_if(DYNAMIC_LOAD_DEBUG, "Loaded {}", m_filename);
@@ -255,11 +269,12 @@ void DynamicLoader::load_program_headers()
     Optional<ProgramHeaderRegion> text_region;
     Optional<ProgramHeaderRegion> data_region;
     Optional<ProgramHeaderRegion> tls_region;
+    Optional<ProgramHeaderRegion> relro_region;
 
     VirtualAddress dynamic_region_desired_vaddr;
 
     m_elf_image.for_each_program_header([&](const Image::ProgramHeader& program_header) {
-        ProgramHeaderRegion region;
+        ProgramHeaderRegion region {};
         region.set_program_header(program_header.raw_header());
         if (region.is_tls_template()) {
             ASSERT(!tls_region.has_value());
@@ -274,6 +289,9 @@ void DynamicLoader::load_program_headers()
             }
         } else if (region.is_dynamic()) {
             dynamic_region_desired_vaddr = region.desired_load_address();
+        } else if (region.is_relro()) {
+            ASSERT(!relro_region.has_value());
+            relro_region = region;
         }
         return IterationDecision::Continue;
     });
@@ -336,6 +354,11 @@ void DynamicLoader::load_program_headers()
     m_text_segment_size = text_segment_size;
     m_text_segment_load_address = VirtualAddress { (FlatPtr)text_segment_begin };
 
+    if (relro_region.has_value()) {
+        m_relro_segment_size = relro_region->size_in_memory();
+        m_relro_segment_address = m_text_segment_load_address.offset(relro_region->desired_load_address().get());
+    }
+
     if (m_elf_image.is_dynamic())
         m_dynamic_section_address = dynamic_region_desired_vaddr.offset(m_text_segment_load_address.get());
     else
diff --git a/Userland/Libraries/LibELF/DynamicLoader.h b/Userland/Libraries/LibELF/DynamicLoader.h
index 910dd9882e4..33f179d23a1 100644
--- a/Userland/Libraries/LibELF/DynamicLoader.h
+++ b/Userland/Libraries/LibELF/DynamicLoader.h
@@ -102,6 +102,7 @@ private:
         bool is_tls_template() const { return type() == PT_TLS; }
         bool is_load() const { return type() == PT_LOAD; }
         bool is_dynamic() const { return type() == PT_DYNAMIC; }
+        bool is_relro() const { return type() == PT_GNU_RELRO; }
 
     private:
         Elf32_Phdr m_program_header; // Explicitly a copy of the PHDR in the image
@@ -145,6 +146,9 @@ private:
     VirtualAddress m_text_segment_load_address;
     size_t m_text_segment_size { 0 };
 
+    VirtualAddress m_relro_segment_address;
+    size_t m_relro_segment_size { 0 };
+
     VirtualAddress m_tls_segment_address;
     VirtualAddress m_dynamic_section_address;
 
diff --git a/Userland/Libraries/LibELF/DynamicObject.cpp b/Userland/Libraries/LibELF/DynamicObject.cpp
index 999667aa126..14d4eda7517 100644
--- a/Userland/Libraries/LibELF/DynamicObject.cpp
+++ b/Userland/Libraries/LibELF/DynamicObject.cpp
@@ -160,6 +160,9 @@ void DynamicObject::parse()
             m_soname_index = entry.val();
             m_has_soname = true;
             break;
+        case DT_BIND_NOW:
+            m_dt_flags |= DF_BIND_NOW;
+            break;
         case DT_DEBUG:
             break;
         case DT_FLAGS_1: