Re: Kernel stack read with PTRACE_EVENT_EXIT and io_uring threads

[Michael Schmitz]

Hi Linus,

On 22/06/21 11:14 am, Linus Torvalds wrote:
> On Mon, Jun 21, 2021 at 12:45 PM Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
> > > Looks like sys_exit() and do_group_exit() would be the two places to
> > > do it (do_group_exit() would handle the signal case and
> > > sys_group_exit()).
> > Maybe...  I'm digging through that pile right now, will follow up when
> > I get a reasonably complete picture
> We might have another possible way to solve this:
>
>   (a) make it the rule that everybody always saves the full (integer)
> register set in pt_regs
>
>   (b) make m68k just always create that switch-stack for all system
> calls (it's really not that big, I think it's like six words or
> something)

Correct - six words for registers, one for the return address. Probably 
still a win compared to setting and clearing flag bits all over the 
place in an attempt to catch any as yet undetected unsafe cases of 
ptrace_stop.

I'll have to see how much of a performance impact I can see (not that I 
can even remotely measure that accurately - it's more of a 'does it now 
feel real sluggish' thing).

Cheers,

    Michael

>   (c) admit that alpha is broken, but nobody really cares
>
> > In the meanwhile, do kernel/kthread.c uses look even remotely sane?
> > Intentional - sure, but it really looks wrong to use thread exit code
> > as communication channel there...
> I really doubt that it is even "intentional".
>
> I think it's "use some errno as a random exit code" and nobody ever
> really thought about it, or thought about how that doesn't really
> work. People are used to the error numbers, not thinking about how
> do_exit() doesn't take an error number, but a signal number (and an
> 8-bit positive error code in bits 8-15).
>
> Because no, it's not even remotely sane.
>
> I think the do_exit(-EINTR) could be do_exit(SIGINT) and it would make
> more sense. And the -ENOMEM might be SIGBUS, perhaps.
>
> It does look like the usermode-helper code does save the exit code
> with things like
>
>                  kernel_wait(pid, &sub_info->retval);
>
> and I see call_usermodehelper_exec() doing
>
>          retval = sub_info->retval;
>
> and treating it as an error code. But I think those have never been
> tested with that (bogus) exit code thing from kernel_wait(), because
> it wouldn't have worked.  It has only ever been tested with the (real)
> exit code things like
>
>                  if (pid < 0) {
>                          sub_info->retval = pid;
>
> which does actually assign a negative error code to it.
>
> So I think that
>
>                  kernel_wait(pid, &sub_info->retval);
>
> line is buggy, and should be something like
>
>                  int wstatus;
>                  kernel_wait(pid, &wstatus);
>                  sub_info->retval = WEXITSTATUS(wstatus) ? -EINVAL : 0;
>
> or something.
>
>              Linus