Re: Kernel stack read with PTRACE_EVENT_EXIT and io_uring threads

Eric W. Biederman
2021-06-15T19:31:59Z

Michael Schmitz <schmitzmic@gmail.com> writes:

> Hi Eric,
>
> On 15/06/21 4:26 am, Eric W. Biederman wrote:
>> Michael Schmitz <schmitzmic@gmail.com> writes:
>>
>>> On second thought, I'm not certain what adding another empty stack frame would
>>> achieve here.
>>>
>>> On m68k, 'frame' already is a new stack frame, for running the new thread
>>> in. This new frame does not have any user context at all, and it's explicitly
>>> wiped anyway.
>>>
>>> Unless we save all user context on the stack, then push that context to a new
>>> save frame, and somehow point get_signal to look there for IO threads
>>> (essentially what Eric suggested), I don't see how this could work?
>>>
>>> I must be missing something.
>> It is only designed to work well enough so that ptrace will access
>> something well defined when ptrace accesses io_uring tasks.
>>
>> The io_uring tasks are special in that they are user process
>> threads that never run in userspace. So as long as everything
>> ptrace can read is accessible on that process all is well.
> OK, I'm testing a patch that would save extra context in sys_io_uring_setup,
> which ought to ensure that for m68k.

I had to update ret_from_kernel_thread to pop that state to get Linus's
change to boot. Apparently kernel_threads exiting needs to be handled.

>> Having stared a bit longer at the code I think the short term
>> fix for both of PTRACE_EVENT_EXIT and io_uring is to guard
>> them both with CONFIG_HAVE_ARCH_TRACEHOOK.

Which does not work because nios2 which looks susceptible
sets CONFIG_HAVE_ARCH_TRACEHOOK.

A further look shows that there is also PTRACE_EVENT_EXEC that
needs to be handled so execve and execveat need to be wrapped
as well.

Do you happen to know if there is userspace that will run
in qemu-system-m68k that can be used for testing?

Eric