Fix issue #27252896: Security Vulnerability -- weak binder

Sending transaction to freed BBinder through weak handle
can cause use of a (mostly) freed object.  We need to try to
safely promote to a strong reference first.

Change-Id: Ic9c6940fa824980472e94ed2dfeca52a6b0fd342
(cherry picked from commit c11146106f94e07016e8e26e4f8628f9a0c73199)

1 files changed, 10 insertions, 2 deletions

diff --git a/libs/binder/IPCThreadState.cpp b/libs/binder/IPCThreadState.cpp
index ef88181..af18e11 100644
--- a/libs/binder/IPCThreadState.cpp
+++ b/libs/binder/IPCThreadState.cpp
@@ -1083,8 +1083,16 @@ status_t IPCThreadState::executeCommand(int32_t cmd)
                     << reinterpret_cast<const size_t*>(tr.data.ptr.offsets) << endl;
             }
             if (tr.target.ptr) {
-                sp<BBinder> b((BBinder*)tr.cookie);
-                error = b->transact(tr.code, buffer, &reply, tr.flags);
+                // We only have a weak reference on the target object, so we must first try to
+                // safely acquire a strong reference before doing anything else with it.
+                if (reinterpret_cast<RefBase::weakref_type*>(
+                        tr.target.ptr)->attemptIncStrong(this)) {
+                    error = reinterpret_cast<BBinder*>(tr.cookie)->transact(tr.code, buffer,
+                            &reply, tr.flags);
+                    reinterpret_cast<BBinder*>(tr.cookie)->decStrong(this);
+                } else {
+                    error = UNKNOWN_TRANSACTION;
+                }
 
             } else {
                 error = the_context_object->transact(tr.code, buffer, &reply, tr.flags);