PatchBug1576767

PatchBug1576767 is a macOS kernel extension that is a temporary workaround for most of the crashes reported at Mozilla bug 1576767. It's also proof, for users who experience these crashes, that they're caused by an Apple bug. PatchBug1576767 requires macOS Catalina, minor version 10.15.4 or higher. It only has any effect if you're running AMD Radeon graphics hardware (since the bug is in Apple's kernel extension drivers for this hardware).

Building PatchBug1576767 requires XCode and its command line tools. If you have an Apple developer account, you can get them at https://developer.apple.com/download/more/. Installing PatchBug1576767 requires an admin account. You also need to turn off at least part of Apple's System Integrity Protection (SIP), and add "keepsyms=1" to your kernel boot args.

Here are brief instructions for building and installing PatchBug1576767:

The easiest way to build PatchBug1576767 is to run xcodebuild from the source distribution's PatchBug1576767/PatchBug1576767 directory. Doing this drops a release build into the project's build/Release directory. You should then copy PatchBug1576767.kext to a directory that only root has write permissions on, such as /usr/local/sbin:

    sudo cp -R PatchBug1576767.kext /usr/local/sbin

Next you need to completely disable SIP (possibly only temporarily), by doing the following:

Boot into your Recovery partition by restarting your computer and pressing Command-R immediately after you hear the Mac startup sound. Release these keys when you see the Apple logo.
Choose Utilties : Terminal, then do the following at the command line:
```
  csrutil disable
```
Quit Terminal and reboot your computer.

Now you can change your kernel boot args (an operation that isn't permitted while SIP is on). Do the following command in a Terminal window to see what kernel boot args you already have, if any:

    nvram boot-args

Then run something like the following command:

    sudo nvram boot-args="[oldbootargs] keepsyms=1"

Now you may wish to partially restore System Integrity Protection. You mustn't turn it back on completely, because that will prevent you from being able to load PatchBug1576767.kext.

Boot into your Recovery partition as above, run Terminal, then run the following command. You will see a warning from Apple that this isn't a supported configuration. Nonetheless, it has worked for many years.

    csrutil enable --without kext

Now quit Terminal and reboot your computer.

Now you should be able to load PatchBug1576767.kext. But first run the Console app and filter on messages containing "PatchBug1576767" or just the word "patch". This is the only way to see the messages that PatchBug1576767.kext displays while running. And thanks to an Apple design flaw, you'll need to load it twice to see any of its messages:

    sudo kextutil /usr/local/sbin/PatchBug1576767.kext
    sudo kextunload -b org.smichaud.PatchBug1576767
    sudo kextutil /usr/local/sbin/PatchBug1576767.kext

Among other messages, you should see the following two, or ones very much like them:

    PatchBug1576767: patch_bad_instructions(): AMDRadeonX4000_AMDHWVMContext::mapVA() patched
    PatchBug1576767: patch_bad_instructions(): AMDRadeonX4000_AMDHWVMContext::unmapVA() patched

To unload PatchBug1576767.kext and restore Apple's code to its original (unpatched) condition, do the following:

    sudo kextunload -b org.smichaud.PatchBug1576767

The bug that PatchBug1576767 fixes is as follows. Here is reconstructed C++ code for two methods in the AMDRadeonX4000 kernel extension (or AMDRadeonX5000 or AMDRadeonX6000):

    bool AMDRadeonX4000_AMDHWVMContext::mapVA(vm_address_t startAddress, IOAccelMemory* arg2,
                                              vm_size_t arg3, vm_size_t length,
                                              AMDRadeonX4000_IAMDHWVMM::VmMapFlags arg5)
    {
      if (startAddress < vmRangeStart) {
        return false;
      }
      if (startAddress + length >= vmRangeEnd) {
        return false;
      }
      ...
    }

    bool AMDRadeonX4000_AMDHWVMContext::unmapVA(vm_address_t startAddress, vm_size_t length)
    {
      if (startAddress < vmRangeStart) {
        return false;
      }
      if (startAddress + length >= vmRangeEnd) {
        return false;
      }
      ...
    }

The bug is in the fourth line of each method:

    if (startAddress + length >= vmRangeEnd) {

should be

    if (startAddress + length > vmRangeEnd) {

The crashes reported at Mozilla bug 1576767 happen when mapVA() tries to map in an object which has been allocated flush up to the end of "vmRange". There's enough space to map in the object, but the ">=" causes an unexpected failure, which causes a cascade of other errors, eventually leading to a crash.

steven-michaud / PatchBug1576767

PatchBug1576767

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