Re: Kernel stack read with PTRACE_EVENT_EXIT and io_uring threads

Michael Schmitz
2021-06-14T22:27:57Z

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.
> 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.

Fair enough :-)

Cheers,

Michael

>
> Today CONFIG_HAVE_ARCH_TRACEHOOK guards access to /proc/self/syscall.
> Which out of necessity ensures that user context is always readable.
> Which seems to solve both the PTRACE_EVENT_EXIT and the io_uring
> problems.
>
> What I especially like about that is there are a lot of other reasons
> to encourage architectures in a CONFIG_HAVE_ARCH_TRACEHOOK direction.
> I think the biggies are getting architectures to store the extra
> saved state on context switch into some place in task_struct
> and to implement the regset view of registers.
>
> Hmm. This is odd. CONFIG_HAVE_ARCH_TRACEHOOK is supposed to imply
> CORE_DUMP_USE_REGSET. But alpha, csky, h8300, m68k, microblaze, nds32
> don't implement CORE_DUMP_USE_REGSET but nds32 implements
> CONFIG_ARCH_HAVE_TRACEHOOK.
>
> I will keep digging and see what clean code I can come up with.
>
> Eric