Re: Kernel stack read with PTRACE_EVENT_EXIT and io_uring threads

Linus Torvalds
2021-06-21T23:14:57Z

On Mon, Jun 21, 2021 at 12:45 PM Al Viro <viro@zeniv.linux.org.uk> 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)

(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