Re: [PATCH RFT v4 0/5] fork: Support shadow stacks in clone3()

[Szabolcs Nagy <Szabolcs.Nagy@arm.com>]

The 11/30/2023 21:51, Mark Brown wrote:
> The concern Rick raised was that allowing user to pick the exact shadow
> stack pointer would allow userspace to corrupt or reuse the stack of an
> existing thread by starting a new thread with the shadow stack pointing
> into the existing shadow stack of that thread.  While in isolation

note that this can be prevented by map_shadow_stack adding
a token that clone3 verifies.

> that's not too much more than what userspace could just do directly
> anyway it might compose with other issues to something more "interesting"
> (eg, I'd be a bit concerned about overlap with pkeys/POE though I've not
> thought through potential uses in detail).
>
> > I'm not against clone3() getting a shadow_stack_size argument but asking
> > some more questions. If we won't pass a pointer as well, is there any
> > advantage in expanding this syscall vs a specific prctl() option? Do we
> > need a different size per thread or do all threads have the same shadow
> > stack size? A new RLIMIT doesn't seem to map well though, it is more
> > like an upper limit rather than a fixed/default size (glibc I think uses
> > it for thread stacks but bionic or musl don't AFAIK).
>
> I don't know what the userspace patterns are likely to be here, it's
> possible a single value for each process might be fine but I couldn't
> say that confidently.  I agree that a RLIMIT does seem like a poor fit.

user code can control the thread stack size per thread
and different size per thread happens in practice (even
in the libc e.g. timer_create with SIGEV_THREAD uses
different stack size than the dns resolver helper thread).
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