commit 3b3e59185dc1e9a319d8ce20ac19c30a966a5a9c
author Alec Mouri <alecmouri@google.com> Mon Sep 26 21:37:01 2022 +0000
committer Alec Mouri <alecmouri@google.com> Tue Sep 27 21:38:53 2022 +0000
tree 7e52357b1661beceba85520e3673e047be322a69
parent 59e3eb4a04ab5e8d5ef569f19d5ab71b2e5b472e

Fix use-after-free in SurfaceFlinger::doDump

SurfaceFlinger::doDump previously contained two layer traversals, one on
the main thread and one off the main thread. During concurrent dumpsys
commands, the layer traversals may race with each other, which causes
shared ownership of the underlying storage of a SortedVector containing
the z-ordered list of layers. Because the implementation of
SortedVector's STL iterators assumes that the underlying storage may be
edited, this can cause the storage to be copied whenever SortedVector::begin
and SortedVector::end are called, which means that SortedVector::begin()
+ SortedVector::size() == SortedVector::end() is not always true, which
causes invalid iteration.

In general, this use-after-free can happen as long as the off-main
thread traversal exists in doDump(), because the traversal can run in
parallel with any workload on the main thread that executes a layer
traversal. So, this patch moves the traversal for dumping out the list
of composition layers into the main thread.

A future patch could explore either fixing SortedVector to fix judicious
iterator invalidation, or building LayerVector on top of std::set, but
either option is an invasive data structure change.

Bug: 237291506
Test: Test script that calls dumpsys SurfaceFlinger on many threads
Change-Id: I0748396519c924dc1b84113d44259f22d0d7ebd6
(cherry picked from commit 6761733ad1dd775f011588c59d5a6d210175c546)
Merged-In: I0748396519c924dc1b84113d44259f22d0d7ebd6

services/surfaceflinger/SurfaceFlinger.cpp

diff --git a/services/surfaceflinger/SurfaceFlinger.cpp b/services/surfaceflinger/SurfaceFlinger.cpp
index db2447c..0e1acb4 100644
--- a/services/surfaceflinger/SurfaceFlinger.cpp
+++ b/services/surfaceflinger/SurfaceFlinger.cpp
@@ -5005,6 +5005,25 @@
 
         const auto flag = args.empty() ? ""s : std::string(String8(args[0]));
 
+        // Traversal of drawing state must happen on the main thread.
+        // Otherwise, SortedVector may have shared ownership during concurrent
+        // traversals, which can result in use-after-frees.
+        std::string compositionLayers;
+        mScheduler
+                ->schedule([&] {
+                    StringAppendF(&compositionLayers, "Composition layers\n");
+                    mDrawingState.traverseInZOrder([&](Layer* layer) {
+                        auto* compositionState = layer->getCompositionState();
+                        if (!compositionState || !compositionState->isVisible) return;
+
+                        android::base::StringAppendF(&compositionLayers, "* Layer %p (%s)\n", layer,
+                                                     layer->getDebugName() ? layer->getDebugName()
+                                                                           : "<unknown>");
+                        compositionState->dump(compositionLayers);
+                    });
+                })
+                .get();
+
         bool dumpLayers = true;
         {
             TimedLock lock(mStateLock, s2ns(1), __func__);
@@ -5017,7 +5036,7 @@
                 (it->second)(args, asProto, result);
                 dumpLayers = false;
             } else if (!asProto) {
-                dumpAllLocked(args, result);
+                dumpAllLocked(args, compositionLayers, result);
             }
         }
 
@@ -5316,7 +5335,8 @@
     result.append(future.get());
 }
 
-void SurfaceFlinger::dumpAllLocked(const DumpArgs& args, std::string& result) const {
+void SurfaceFlinger::dumpAllLocked(const DumpArgs& args, const std::string& compositionLayers,
+                                   std::string& result) const {
     const bool colorize = !args.empty() && args[0] == String16("--color");
     Colorizer colorizer(colorize);
 
@@ -5367,18 +5387,7 @@
     StringAppendF(&result, "Visible layers (count = %zu)\n", mNumLayers.load());
     colorizer.reset(result);
 
-    {
-        StringAppendF(&result, "Composition layers\n");
-        mDrawingState.traverseInZOrder([&](Layer* layer) {
-            auto* compositionState = layer->getCompositionState();
-            if (!compositionState || !compositionState->isVisible) return;
-
-            android::base::StringAppendF(&result, "* Layer %p (%s)\n", layer,
-                                         layer->getDebugName() ? layer->getDebugName()
-                                                               : "<unknown>");
-            compositionState->dump(result);
-        });
-    }
+    result.append(compositionLayers);
 
     colorizer.bold(result);
     StringAppendF(&result, "Displays (%zu entries)\n", mDisplays.size());

services/surfaceflinger/SurfaceFlinger.h

diff --git a/services/surfaceflinger/SurfaceFlinger.h b/services/surfaceflinger/SurfaceFlinger.h
index 83134a2e..9e0cee8 100644
--- a/services/surfaceflinger/SurfaceFlinger.h
+++ b/services/surfaceflinger/SurfaceFlinger.h
@@ -1085,7 +1085,8 @@
     /*
      * Debugging & dumpsys
      */
-    void dumpAllLocked(const DumpArgs& args, std::string& result) const REQUIRES(mStateLock);
+    void dumpAllLocked(const DumpArgs& args, const std::string& compositionLayers,
+                       std::string& result) const REQUIRES(mStateLock);
 
     void appendSfConfigString(std::string& result) const;
     void listLayersLocked(std::string& result) const;