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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 15:42:05 -0500
In-Reply-To: <CAHk-=wiTEZN_3ipf51sh-csdusW4uGzAXq9m1JcMHu_c8OJ+pQ@mail.gmail.com>
        (Linus Torvalds's message of "Wed, 16 Jun 2021 13:00:52 -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 11:32 AM Eric W. Biederman
> <ebiederm@xmission.com> wrote:
>>
>> Prevent security holes by recording when all of the registers are
>> available so generic code changes do not result in security holes
>> on alpha.
>
> Please no, not this way. ldl/stc is extremely expensive on some alpha cpus.
>
> I really think thatTIF_ALLREGS_SAVED bit isn't worth it, except
> perhaps for debugging.
>
> And even for debugging, I think it would be both easier and cheaper to
> just add a magic word to the entry stack instead.

I think I can do something like that.

Looking at arch/alpha/asm/cache.h it looks like alpha had either 32byte
or 64bit cachelines.  Which makes struct switch_stack a full 10 or 5
cachelines in size.  So pushing something extra might hit an extra
cacheline.

However it looks like struct pt_regs is 16 bytes short of a full cache
line so struct switch_stack isn't going to be cacheline aligned. Adding
an extra 8 bytes of magic number will hopefully be in the noise.

If I can I would like to find something that is cheap enough that I can
always leave on.  Mostly because there is little enough testing that a
bug that allows anyone to stomp the kernel stack has existed for 17 years
without being noticed.

If you want it to be a debug option only I can certainly make that
happen.  I am still going "Eek! Arbitrary stack smash!" in my head.

Eric