Kernel+LibELF: Validate PT_LOAD and PT_TLS offsets before memcpy()'ing

Before this, you could make the kernel copy memory from anywhere by
setting up an ELF executable with a program header specifying file
offsets outside the file.

Since ELFImage didn't even know how large it was, we had no clue that
we were copying things from outside the ELF.

Fix this by adding a size field to ELFImage and validating program
header ranges before memcpy()'ing to them.

The ELF code is definitely going to need more validation and checking.

Kernel/Process.cpp

@@ -49,6 +49,7 @@
 //#define DEBUG_IO
 //#define TASK_DEBUG
 //#define FORK_DEBUG
+//#define EXEC_DEBUG
 //#define SIGNAL_DEBUG
 //#define SHARED_BUFFER_DEBUG
 
@@ -655,7 +656,7 @@ int Process::do_exec(String path, Vector<String> arguments, Vector<String> envir
         // Okay, here comes the sleight of hand, pay close attention..
         auto old_regions = move(m_regions);
         m_regions.append(move(executable_region));
-        loader = make<ELFLoader>(region->vaddr().as_ptr());
+        loader = make<ELFLoader>(region->vaddr().as_ptr(), metadata.size);
         loader->map_section_hook = [&](VirtualAddress vaddr, size_t size, size_t alignment, size_t offset_in_image, bool is_readable, bool is_writable, bool is_executable, const String& name) -> u8* {
             ASSERT(size);
             ASSERT(alignment == PAGE_SIZE);
@@ -703,6 +704,11 @@ int Process::do_exec(String path, Vector<String> arguments, Vector<String> envir
 
         // NOTE: At this point, we've committed to the new executable.
         entry_eip = loader->entry().get();
+
+#ifdef EXEC_DEBUG
+        kprintf("Memory layout after ELF load:");
+        dump_regions();
+#endif
     }
 
     region->set_user_accessible(false);
@@ -3818,8 +3824,7 @@ int Process::sys$module_load(const char* user_path, size_t path_length)
     memcpy(storage.data(), payload.data(), payload.size());
     payload.clear();
 
-    // FIXME: ELFImage should really be taking a size argument as well...
-    auto elf_image = make<ELFImage>(storage.data());
+    auto elf_image = make<ELFImage>(storage.data(), storage.size());
     if (!elf_image->parse())
         return -ENOEXEC;
 

Libraries/LibELF/ELFDynamicLoader.cpp

@@ -56,7 +56,7 @@ void* ELFDynamicLoader::symbol_for_name(const char* name)
 
 bool ELFDynamicLoader::load_from_image(unsigned flags)
 {
-    ELFImage elf_image((u8*)m_file_mapping);
+    ELFImage elf_image((u8*)m_file_mapping, m_file_size);
 
     m_valid = elf_image.is_valid() && elf_image.is_dynamic();
 

Libraries/LibELF/ELFImage.cpp

@@ -2,8 +2,9 @@
 #include <AK/kstdio.h>
 #include <LibELF/ELFImage.h>
 
-ELFImage::ELFImage(const u8* buffer)
+ELFImage::ELFImage(const u8* buffer, size_t size)
     : m_buffer(buffer)
+    , m_size(size)
 {
     m_valid = parse();
 }
@@ -59,6 +60,8 @@ void ELFImage::dump() const
     dbgprintf("    entry:   %x\n", header().e_entry);
     dbgprintf("    shoff:   %u\n", header().e_shoff);
     dbgprintf("    shnum:   %u\n", header().e_shnum);
+    dbgprintf("    phoff:   %u\n", header().e_phoff);
+    dbgprintf("    phnum:   %u\n", header().e_phnum);
     dbgprintf(" shstrndx:   %u\n", header().e_shstrndx);
 
     for (unsigned i = 0; i < header().e_shnum; ++i) {

Libraries/LibELF/ELFImage.h

@@ -8,12 +8,21 @@
 
 class ELFImage {
 public:
-    explicit ELFImage(const u8*);
+    explicit ELFImage(const u8*, size_t);
     ~ELFImage();
     void dump() const;
     bool is_valid() const { return m_valid; }
     bool parse();
 
+    bool is_within_image(const void* address, size_t size)
+    {
+        if (address < m_buffer)
+            return false;
+        if (((const u8*)address + size) > m_buffer + m_size)
+            return false;
+        return true;
+    }
+
     class Section;
     class RelocationSection;
     class Symbol;
@@ -190,6 +199,7 @@ private:
     const char* section_index_to_string(unsigned index) const;
 
     const u8* m_buffer { nullptr };
+    size_t m_size { 0 };
     HashMap<String, unsigned> m_sections;
     bool m_valid { false };
     unsigned m_symbol_table_section_index { 0 };

Libraries/LibELF/ELFLoader.cpp

@@ -9,8 +9,8 @@
 
 //#define ELFLOADER_DEBUG
 
-ELFLoader::ELFLoader(const u8* buffer)
-    : m_image(buffer)
+ELFLoader::ELFLoader(const u8* buffer, size_t size)
+    : m_image(buffer, size)
 {
 }
 
@@ -43,6 +43,11 @@ bool ELFLoader::layout()
                 failed = true;
                 return;
             }
+            if (!m_image.is_within_image(program_header.raw_data(), program_header.size_in_image())) {
+                dbg() << "Shenanigans! ELF PT_TLS header sneaks outside of executable.";
+                failed = true;
+                return;
+            }
             memcpy(tls_image, program_header.raw_data(), program_header.size_in_image());
 #endif
             return;
@@ -65,6 +70,11 @@ bool ELFLoader::layout()
                 failed = true;
                 return;
             }
+            if (!m_image.is_within_image(program_header.raw_data(), program_header.size_in_image())) {
+                dbg() << "Shenanigans! Writable ELF PT_LOAD header sneaks outside of executable.";
+                failed = true;
+                return;
+            }
             memcpy(program_header.vaddr().as_ptr(), program_header.raw_data(), program_header.size_in_image());
         } else {
             auto* mapped_section = map_section_hook(

Libraries/LibELF/ELFLoader.h

@@ -13,7 +13,7 @@ class Region;
 
 class ELFLoader {
 public:
-    explicit ELFLoader(const u8*);
+    explicit ELFLoader(const u8*, size_t);
     ~ELFLoader();
 
     bool load();