Add randomized padding around shared library mappings.

Improve ASLR by increasing the randomly sized gaps between shared
library mappings, and keep them mapped PROT_NONE.

Bug: 158113540
Test: look at /proc/$$/maps
Change-Id: Ie72c84047fb624fe2ac8b7744b2a2d0d255ea974

5 files changed, 63 insertions, 8 deletions

diff --git a/linker/linker.cpp b/linker/linker.cpp
index edf0329c9..9301f8cb5 100644
--- a/linker/linker.cpp
+++ b/linker/linker.cpp
@@ -301,13 +301,14 @@ static void soinfo_free(soinfo* si) {
     return;
   }
 
-  if (si->base != 0 && si->size != 0) {
+  void* start = reinterpret_cast<void*>(si->has_min_version(6) ? si->get_map_start() : si->base);
+  size_t size = si->has_min_version(6) ? si->get_map_size() : si->size;
+  if (start != nullptr && size != 0) {
     if (!si->is_mapped_by_caller()) {
-      munmap(reinterpret_cast<void*>(si->base), si->size);
+      munmap(start, size);
     } else {
       // remap the region as PROT_NONE, MAP_ANONYMOUS | MAP_NORESERVE
-      mmap(reinterpret_cast<void*>(si->base), si->size, PROT_NONE,
-           MAP_FIXED | MAP_PRIVATE | MAP_ANONYMOUS | MAP_NORESERVE, -1, 0);
+      mmap(start, size, PROT_NONE, MAP_FIXED | MAP_PRIVATE | MAP_ANONYMOUS | MAP_NORESERVE, -1, 0);
     }
   }
 
@@ -599,6 +600,8 @@ class LoadTask {
     si_->load_bias = elf_reader.load_bias();
     si_->phnum = elf_reader.phdr_count();
     si_->phdr = elf_reader.loaded_phdr();
+    si_->set_map_start(elf_reader.map_start());
+    si_->set_map_size(elf_reader.map_size());
 
     return true;
   }
diff --git a/linker/linker_phdr.cpp b/linker/linker_phdr.cpp
index 9b7a461cd..74d7a3152 100644
--- a/linker/linker_phdr.cpp
+++ b/linker/linker_phdr.cpp
@@ -520,7 +520,8 @@ size_t phdr_table_get_load_size(const ElfW(Phdr)* phdr_table, size_t phdr_count,
 
 // Reserve a virtual address range such that if it's limits were extended to the next 2**align
 // boundary, it would not overlap with any existing mappings.
-static void* ReserveAligned(size_t size, size_t align) {
+static void* ReserveWithAlignmentPadding(size_t size, size_t align, void** map_start,
+                                         size_t* map_size) {
   int mmap_flags = MAP_PRIVATE | MAP_ANONYMOUS;
   if (align == PAGE_SIZE) {
     void* mmap_ptr = mmap(nullptr, size, PROT_NONE, mmap_flags, -1, 0);
@@ -532,7 +533,12 @@ static void* ReserveAligned(size_t size, size_t align) {
 
   // Allocate enough space so that the end of the desired region aligned up is still inside the
   // mapping.
-  size_t mmap_size = align_up(size, align) + align - PAGE_SIZE;
+#if defined(__LP64__)
+  constexpr size_t kLibraryPadding = 1ul << 24;
+#else
+  constexpr size_t kLibraryPadding = 0;
+#endif
+  size_t mmap_size = align_up(size + kLibraryPadding, align) + align - PAGE_SIZE;
   uint8_t* mmap_ptr =
       reinterpret_cast<uint8_t*>(mmap(nullptr, mmap_size, PROT_NONE, mmap_flags, -1, 0));
   if (mmap_ptr == MAP_FAILED) {
@@ -546,8 +552,18 @@ static void* ReserveAligned(size_t size, size_t align) {
   // created. Don't randomize then.
   size_t n = is_first_stage_init() ? 0 : arc4random_uniform((last - first) / PAGE_SIZE + 1);
   uint8_t* start = first + n * PAGE_SIZE;
+  // Unmap the extra space around the allocation.
+  // Keep it mapped PROT_NONE on 64-bit targets where address space is plentiful to make it harder
+  // to defeat ASLR by probing for readable memory mappings.
+#if defined(__LP64__)
+  *map_start = mmap_ptr;
+  *map_size = mmap_size;
+#else
   munmap(mmap_ptr, start - mmap_ptr);
   munmap(start + size, mmap_ptr + mmap_size - (start + size));
+  *map_start = start;
+  *map_size = size;
+#endif
   return start;
 }
 
@@ -571,13 +587,15 @@ bool ElfReader::ReserveAddressSpace(address_space_params* address_space) {
              load_size_ - address_space->reserved_size, load_size_, name_.c_str());
       return false;
     }
-    start = ReserveAligned(load_size_, kLibraryAlignment);
+    start = ReserveWithAlignmentPadding(load_size_, kLibraryAlignment, &map_start_, &map_size_);
     if (start == nullptr) {
       DL_ERR("couldn't reserve %zd bytes of address space for \"%s\"", load_size_, name_.c_str());
       return false;
     }
   } else {
     start = address_space->start_addr;
+    map_start_ = start;
+    map_size_ = load_size_;
     mapped_by_caller_ = true;
 
     // Update the reserved address space to subtract the space used by this library.
diff --git a/linker/linker_phdr.h b/linker/linker_phdr.h
index 5d1cfc2bd..18881cad9 100644
--- a/linker/linker_phdr.h
+++ b/linker/linker_phdr.h
@@ -49,6 +49,8 @@ class ElfReader {
   size_t phdr_count() const { return phdr_num_; }
   ElfW(Addr) load_start() const { return reinterpret_cast<ElfW(Addr)>(load_start_); }
   size_t load_size() const { return load_size_; }
+  ElfW(Addr) map_start() const { return reinterpret_cast<ElfW(Addr)>(map_start_); }
+  size_t map_size() const { return map_size_; }
   ElfW(Addr) load_bias() const { return load_bias_; }
   const ElfW(Phdr)* loaded_phdr() const { return loaded_phdr_; }
   const ElfW(Dyn)* dynamic() const { return dynamic_; }
@@ -96,6 +98,10 @@ class ElfReader {
   void* load_start_;
   // Size in bytes of reserved address space.
   size_t load_size_;
+  // First page of reserved address space including randomized padding.
+  void* map_start_;
+  // Size in bytes of reserved address space including randomized padding.
+  size_t map_size_;
   // Load bias.
   ElfW(Addr) load_bias_;
 
diff --git a/linker/linker_soinfo.cpp b/linker/linker_soinfo.cpp
index 4f67003b8..088c17dfc 100644
--- a/linker/linker_soinfo.cpp
+++ b/linker/linker_soinfo.cpp
@@ -900,6 +900,24 @@ void soinfo::generate_handle() {
   g_soinfo_handles_map[handle_] = this;
 }
 
+void soinfo::set_map_start(ElfW(Addr) map_start) {
+  CHECK(has_min_version(6));
+  map_start_ = map_start;
+}
+ElfW(Addr) soinfo::get_map_start() const {
+  CHECK(has_min_version(6));
+  return map_start_;
+}
+
+void soinfo::set_map_size(size_t map_size) {
+  CHECK(has_min_version(6));
+  map_size_ = map_size;
+}
+size_t soinfo::get_map_size() const {
+  CHECK(has_min_version(6));
+  return map_size_;
+}
+
 // TODO(dimitry): Move SymbolName methods to a separate file.
 
 uint32_t calculate_elf_hash(const char* name) {
diff --git a/linker/linker_soinfo.h b/linker/linker_soinfo.h
index e1a3c303d..0ecf8d852 100644
--- a/linker/linker_soinfo.h
+++ b/linker/linker_soinfo.h
@@ -66,7 +66,7 @@
 #define FLAG_PRELINKED        0x00000400 // prelink_image has successfully processed this soinfo
 #define FLAG_NEW_SOINFO       0x40000000 // new soinfo format
 
-#define SOINFO_VERSION 5
+#define SOINFO_VERSION 6
 
 ElfW(Addr) call_ifunc_resolver(ElfW(Addr) resolver_addr);
 
@@ -345,6 +345,12 @@ struct soinfo {
 
   SymbolLookupLib get_lookup_lib();
 
+  void set_map_start(ElfW(Addr) map_start);
+  ElfW(Addr) get_map_start() const;
+
+  void set_map_size(size_t map_size);
+  size_t get_map_size() const;
+
  private:
   bool is_image_linked() const;
   void set_image_linked();
@@ -423,6 +429,10 @@ struct soinfo {
   // version >= 5
   std::unique_ptr<soinfo_tls> tls_;
   std::vector<TlsDynamicResolverArg> tlsdesc_args_;
+
+  // version >= 6
+  ElfW(Addr) map_start_;
+  size_t map_size_;
 };
 
 // This function is used by dlvsym() to calculate hash of sym_ver