Re: Kernel stack read with PTRACE_EVENT_EXIT and io_uring threads

Al Viro
2021-06-21T18:59:15Z

On Mon, Jun 21, 2021 at 01:54:56PM +0000, Al Viro wrote:
> On Tue, Jun 15, 2021 at 02:58:12PM -0700, Linus Torvalds wrote:
>
> > And I think our horrible "kernel threads return to user space when
> > done" is absolutely horrifically nasty. Maybe of the clever sort, but
> > mostly of the historical horror sort.
>
> How would you prefer to handle that, then? Separate magical path from
> kernel_execve() to switch to userland? We used to have something of
> that sort, and that had been a real horror...
>
> As it is, it's "kernel thread is spawned at the point similar to
> ret_from_fork(), runs the payload (which almost never returns) and
> then proceeds out to userland, same way fork(2) would've done."
> That way kernel_execve() doesn't have to do anything magical.
>
> Al, digging through the old notes and current call graph...

There's a large mess around do_exit() - we have a bunch of
callers all over arch/*; if nothing else, I very much doubt that really
want to let tracer play with a thread in the middle of die_if_kernel()
or similar.

We sure as hell do not want to arrange for anything on the kernel
stack in such situations, no matter what's done in exit(2)...