The following lldb command “process attach –name xpcproxy_sim –waitfor” allows us to attach xpcproxy_sim then set a breakpoint on posix_spawn once it’s launched. The “xpcproxy_sim” launches target processes with a posix_spawn system call, which gives us an opportunity to inject environment variables into the target process, in this case “.previewUI”. The process “launchd_sim” launches Simulator XPC services with a trampoline process called “xpcproxy_sim”. In this case we cannot simply add an environment variable with the command line since the previewUI launches on clicking the PDF which does not launch from the terminal, we need to inject libgmalloc before the launch. Subsequent attempts to access the deallocated memory cause an immediate memory exception rather than a blind access into memory that might now hold other data.” Environment Variables Injection For example, it places separate memory allocations on different virtual memory pages and then deletes the entire page when the memory is freed. Guard Malloc uses several techniques to try and crash your application at the specific point where a memory error occurs. “Guard Malloc is a special version of the malloc library that replaces the standard library during debugging. The idea is to leverage Guard Malloc or Valgrind, making the process crash right at the memory corruption occurs. We need to find a way to trigger the crash right at the point the memory corruption happens. However, having debug capability is not enough since the process crashes only when the corrupted memory is being used, which is AFTER the actual memory corruption. It’s great news since Simulator on MacOS provides better debug tools than iOS. We confirmed the assumption with the iPhone Simulator, since the crash happened on the iPhone Simulator. Meaning that it’s the iOS library that might have an issue. The important question is: how do we find out the source of the memory corruption? Open the PDF previewUI flashes and shows nothing: The PDF sample crashes previewUI with segmentation fault, meaning that a memory corruption was triggered by the PDF. The BackgroundĮarlier last year, we obtained a PDF file that cannot be previewed on iOS. It is possible that NSO noticed this incremental bug fix, and dived deeper into CoreGraphics. Are you able to reproduce this issue using any version of iOS 14? If so, we would appreciate any additional information you can provide us, such as an updated proof-of-concept.”). Apple contacted us on Octo– claiming that the bug was already fixed – (“ We were unable to reproduce this issue using any current version of iOS 14.
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