Re: Kernel stack read with PTRACE_EVENT_EXIT and io_uring threads

From: Al Viro
Date: Mon Jun 21 2021 - 15:24:30 EST

On Mon, Jun 21, 2021 at 06:59:01PM +0000, Al Viro wrote:
> 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)...

FWIW, on alpha it's die_if_kernel(), do_entUna() and do_page_fault(),
all in not-from-userland cases. On m68k - die_if_kernel(), do_page_fault()
(both for non-from-userland cases) and something really odd - fpsp040_die().
Exception handling for floating point stuff on 68040? Looks like it has
an open-coded copy_to_user()/copy_from_user(), with faults doing hard
do_exit(SIGSEGV) instead of raising a signal and trying to do something

I really don't want to try and figure out how painful would it be to
teach that code how to deal with faults - _testing_ anything in that
area sure as hell will be. IIRC, details of recovery from FPU exceptions
on 68040 in the manual left impression of a minefield...