Re-exec TSan with no ASLR if memory layout is incompatible

TSan's shadow mappings only support 30-bits of ASLR entropy on x86, and
it is not practical to support the maximum of 32-bits (due to pointer
compression and the overhead of shadow mappings). Instead, this patch
changes TSan to re-exec without ASLR if it encounters an incompatible
memory layout, as suggested by Dmitry in google/sanitizers#1716.
If ASLR is already disabled, it will abort.

This patch involves a bit of refactoring, because the old code is:
    InitializePlatformEarly()
    InitializeAllocator()
    InitializePlatform(): CheckAndProtect()
but it may already segfault during InitializeAllocator() if the memory
layout is incompatible, before we get a chance to check in
CheckAndProtect.

This patch adds CheckAndProtect during InitializePlatformEarly(), before
the allocator is initialized. Naturally, it is necessary to ensure that
CheckAndProtect does *not* allow the heap regions to be occupied there,
hence we generalize CheckAndProtect to optionally check the heap
regions. We keep the original behavior of CheckAndProtect() in InitializePlatform()
as a last line of defense.

We need to careful not to prematurely abort if ASLR is disabled but TSan was going to re-exec
for other reasons (e.g., unlimited stack size); we implement this by
moving all the re-exec logic into ReExecIfNeeded().

Author: thurstond

compiler-rt/lib/tsan/rtl/tsan_platform.h

@@ -1024,7 +1024,7 @@ inline uptr RestoreAddr(uptr addr) {
 
 void InitializePlatform();
 void InitializePlatformEarly();
-void CheckAndProtect();
+bool CheckAndProtect(bool protect, bool ignore_heap, bool print_warnings);
 void InitializeShadowMemoryPlatform();
 void WriteMemoryProfile(char *buf, uptr buf_size, u64 uptime_ns);
 int ExtractResolvFDs(void *state, int *fds, int nfd);

compiler-rt/lib/tsan/rtl/tsan_platform_linux.cpp

@@ -214,6 +214,88 @@ void InitializeShadowMemoryPlatform() {
 
 #endif  // #if !SANITIZER_GO
 
+#  if !SANITIZER_GO
+static void ReExecIfNeeded() {
+  // Go maps shadow memory lazily and works fine with limited address space.
+  // Unlimited stack is not a problem as well, because the executable
+  // is not compiled with -pie.
+  {
+    bool reexec = false;
+    // TSan doesn't play well with unlimited stack size (as stack
+    // overlaps with shadow memory). If we detect unlimited stack size,
+    // we re-exec the program with limited stack size as a best effort.
+    if (StackSizeIsUnlimited()) {
+      const uptr kMaxStackSize = 32 * 1024 * 1024;
+      VReport(1,
+              "Program is run with unlimited stack size, which wouldn't "
+              "work with ThreadSanitizer.\n"
+              "Re-execing with stack size limited to %zd bytes.\n",
+              kMaxStackSize);
+      SetStackSizeLimitInBytes(kMaxStackSize);
+      reexec = true;
+    }
+
+    if (!AddressSpaceIsUnlimited()) {
+      Report(
+          "WARNING: Program is run with limited virtual address space,"
+          " which wouldn't work with ThreadSanitizer.\n");
+      Report("Re-execing with unlimited virtual address space.\n");
+      SetAddressSpaceUnlimited();
+      reexec = true;
+    }
+
+    // ASLR personality check.
+    int old_personality = personality(0xffffffff);
+    bool aslr_on =
+        (old_personality != -1) && ((old_personality & ADDR_NO_RANDOMIZE) == 0);
+
+#    if SANITIZER_ANDROID && (defined(__aarch64__) || defined(__x86_64__))
+    // After patch "arm64: mm: support ARCH_MMAP_RND_BITS." is introduced in
+    // linux kernel, the random gap between stack and mapped area is increased
+    // from 128M to 36G on 39-bit aarch64. As it is almost impossible to cover
+    // this big range, we should disable randomized virtual space on aarch64.
+    if (aslr_on) {
+      VReport(1,
+              "WARNING: Program is run with randomized virtual address "
+              "space, which wouldn't work with ThreadSanitizer on Android.\n"
+              "Re-execing with fixed virtual address space.\n");
+      CHECK_NE(personality(old_personality | ADDR_NO_RANDOMIZE), -1);
+      reexec = true;
+    }
+#    endif
+
+    if (reexec) {
+      // Don't check the address space since we're going to re-exec anyway.
+    } else if (!CheckAndProtect(false, false, false)) {
+      if (aslr_on) {
+        // Disable ASLR if the memory layout was incompatible.
+        // Alternatively, we could just keep re-execing until we get lucky
+        // with a compatible randomized layout, but the risk is that if it's
+        // not an ASLR-related issue, we will be stuck in an infinite loop of
+        // re-execing (unless we change ReExec to pass a parameter of the
+        // number of retries allowed.)
+        VReport(1,
+                "WARNING: ThreadSanitizer: memory layout is incompatible, "
+                "possibly due to high-entropy ASLR.\n"
+                "Re-execing with fixed virtual address space.\n"
+                "N.B. reducing ASLR entropy is preferable.\n");
+        CHECK_NE(personality(old_personality | ADDR_NO_RANDOMIZE), -1);
+        reexec = true;
+      } else {
+        VReport(1,
+                "FATAL: ThreadSanitizer: memory layout is incompatible, "
+                "even though ASLR is disabled.\n"
+                "Please file a bug.\n");
+        Die();
+      }
+    }
+
+    if (reexec)
+      ReExec();
+  }
+}
+#  endif
+
 void InitializePlatformEarly() {
   vmaSize =
     (MostSignificantSetBitIndex(GET_CURRENT_FRAME()) + 1);
@@ -284,6 +366,10 @@ void InitializePlatformEarly() {
   }
 #    endif
 #  endif
+
+#  if !SANITIZER_GO
+  ReExecIfNeeded();
+#  endif
 }
 
 void InitializePlatform() {
@@ -294,52 +380,22 @@ void InitializePlatform() {
   // is not compiled with -pie.
 #if !SANITIZER_GO
   {
-    bool reexec = false;
-    // TSan doesn't play well with unlimited stack size (as stack
-    // overlaps with shadow memory). If we detect unlimited stack size,
-    // we re-exec the program with limited stack size as a best effort.
-    if (StackSizeIsUnlimited()) {
-      const uptr kMaxStackSize = 32 * 1024 * 1024;
-      VReport(1, "Program is run with unlimited stack size, which wouldn't "
-                 "work with ThreadSanitizer.\n"
-                 "Re-execing with stack size limited to %zd bytes.\n",
-              kMaxStackSize);
-      SetStackSizeLimitInBytes(kMaxStackSize);
-      reexec = true;
-    }
-
-    if (!AddressSpaceIsUnlimited()) {
-      Report("WARNING: Program is run with limited virtual address space,"
-             " which wouldn't work with ThreadSanitizer.\n");
-      Report("Re-execing with unlimited virtual address space.\n");
-      SetAddressSpaceUnlimited();
-      reexec = true;
-    }
-#if SANITIZER_ANDROID && (defined(__aarch64__) || defined(__x86_64__))
-    // After patch "arm64: mm: support ARCH_MMAP_RND_BITS." is introduced in
-    // linux kernel, the random gap between stack and mapped area is increased
-    // from 128M to 36G on 39-bit aarch64. As it is almost impossible to cover
-    // this big range, we should disable randomized virtual space on aarch64.
-    // ASLR personality check.
-    int old_personality = personality(0xffffffff);
-    if (old_personality != -1 && (old_personality & ADDR_NO_RANDOMIZE) == 0) {
-      VReport(1, "WARNING: Program is run with randomized virtual address "
-              "space, which wouldn't work with ThreadSanitizer.\n"
-              "Re-execing with fixed virtual address space.\n");
-      CHECK_NE(personality(old_personality | ADDR_NO_RANDOMIZE), -1);
-      reexec = true;
-    }
-
-#endif
-#if SANITIZER_LINUX && (defined(__aarch64__) || defined(__loongarch_lp64))
+#    if SANITIZER_LINUX && (defined(__aarch64__) || defined(__loongarch_lp64))
     // Initialize the xor key used in {sig}{set,long}jump.
     InitializeLongjmpXorKey();
-#endif
-    if (reexec)
-      ReExec();
+#    endif
+  }
+
+  // Earlier initialization steps already re-exec'ed until we got a compatible
+  // memory layout, so we don't expect any more issues here.
+  if (!CheckAndProtect(true, true, true)) {
+    Printf(
+        "FATAL: ThreadSanitizer: unexpectedly found incompatible memory "
+        "layout.\n");
+    Printf("FATAL: Please file a bug.\n");
+    Die();
   }
 
-  CheckAndProtect();
   InitTlsSize();
 #endif  // !SANITIZER_GO
 }

compiler-rt/lib/tsan/rtl/tsan_platform_posix.cpp

@@ -94,22 +94,51 @@ static void ProtectRange(uptr beg, uptr end) {
   }
 }
 
-void CheckAndProtect() {
+// CheckAndProtect will check if the memory layout is compatible with TSan.
+// Optionally (if 'protect' is true), it will set the memory regions between
+// app memory to be inaccessible.
+// 'ignore_heap' means it will not consider heap memory allocations to be a
+// conflict. Set this based on whether we are calling CheckAndProtect before
+// or after the allocator has initialized the heap.
+bool CheckAndProtect(bool protect, bool ignore_heap, bool print_warnings) {
   // Ensure that the binary is indeed compiled with -pie.
   MemoryMappingLayout proc_maps(true);
   MemoryMappedSegment segment;
   while (proc_maps.Next(&segment)) {
-    if (IsAppMem(segment.start)) continue;
+    if (segment.start >= HeapMemBeg() && segment.end <= HeapEnd()) {
+      if (ignore_heap) {
+        continue;
+      } else {
+        return false;
+      }
+    }
+
+    // Note: IsAppMem includes if it is heap memory, hence we must
+    // put this check after the heap bounds check.
+    if (IsAppMem(segment.start) && IsAppMem(segment.end - 1))
+      continue;
+
+    // Guard page after the heap end
     if (segment.start >= HeapMemEnd() && segment.start < HeapEnd()) continue;
+
     if (segment.protection == 0)  // Zero page or mprotected.
       continue;
+
     if (segment.start >= VdsoBeg())  // vdso
       break;
-    Printf("FATAL: ThreadSanitizer: unexpected memory mapping 0x%zx-0x%zx\n",
-           segment.start, segment.end);
-    Die();
+
+    // Debug output can break tests. Suppress this message in most cases.
+    if (print_warnings)
+      Printf(
+          "WARNING: ThreadSanitizer: unexpected memory mapping 0x%zx-0x%zx\n",
+          segment.start, segment.end);
+
+    return false;
   }
 
+  if (!protect)
+    return true;
+
 #    if SANITIZER_IOS && !SANITIZER_IOSSIM
   ProtectRange(HeapMemEnd(), ShadowBeg());
   ProtectRange(ShadowEnd(), MetaShadowBeg());
@@ -135,8 +164,10 @@ void CheckAndProtect() {
   // Older s390x kernels may not support 5-level page tables.
   TryProtectRange(user_addr_max_l4, user_addr_max_l5);
 #endif
+
+  return true;
 }
-#endif
+#  endif
 
 }  // namespace __tsan