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From:   ebiederm@xmission.com (Eric W. Biederman)
To:     Linus Torvalds <torvalds@linux-foundation.org>
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Date:   Wed, 16 Jun 2021 15:57:19 -0500
In-Reply-To: <CAHk-=whvNaYffE8Eaa4-jjYbF_r1u1sh5LF5zXFhdEP8hxySMQ@mail.gmail.com>
        (Linus Torvalds's message of "Wed, 16 Jun 2021 13:37:29 -0700")
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Subject: Re: [PATCH 1/2] alpha/ptrace: Record and handle the absence of switch_stack
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Linus Torvalds <torvalds@linux-foundation.org> writes:

> On Wed, Jun 16, 2021 at 1:00 PM Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
>>
>> And even for debugging, I think it would be both easier and cheaper to
>> just add a magic word to the entry stack instead.
>
> IOW, just add a
>
>       unsigned long magic;
>
> to "struct switch_stack", and then make the stack switch code push that value.
>
> That would be cheap enough to be just unconditional, but you could
> make it depend on a debug config option too, of course.
>
> It helps if 'xyz' is some constant that is easyish to generate. It
> might not be a constant - maybe it could be the address of that
> 'magic' field itself, so you'd just generate it with
>
>     stq $r,($r)
>
> and it would be equally easy to just validate at lookup for that WARN_ON_ONCE():
>
>     WARN_ON_ONCE(switch_stack->magic != (unsigned long)&switch_stack->magic);
>
> or whatever.
>
> It's for debugging, not security. So it doesn't have to be some kind
> of super-great magic number, just something easy to generate and check
> (that isn't a common value like "0" that trivially exist on the stack
> anyway).

Fair enough.

I was thinking for a moment that do_sigreturn might have a problem with
that but restore_sigcontext makes it clear that struct switch_stack is
not exposed to userspace.

Do you know if struct switch_stack or pt_regs is ever exposeed to
usespace?  They are both defined in arch/alpha/include/uapi/asm/ptrace.h
which makes me think userspace must see those definitions somewhere.

Eric