linux-kernel - Re: [RFC PATCH 04/13] powerpc/dexcr: Support userspace ROP protection

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Message-Id: <CRBSHXMZX79O.ESN0S8V6M644@bobo>
Date:   Tue, 21 Mar 2023 14:51:20 +1000
From:   "Nicholas Piggin" <npiggin@...il.com>
To:     "Benjamin Gray" <bgray@...ux.ibm.com>,
        <linuxppc-dev@...ts.ozlabs.org>
Cc:     <ajd@...ux.ibm.com>, <linux-kernel@...r.kernel.org>,
        <linux-hardening@...r.kernel.org>, <cmr@...escreens.de>
Subject: Re: [RFC PATCH 04/13] powerpc/dexcr: Support userspace ROP
 protection

On Tue Mar 7, 2023 at 3:37 PM AEST, Benjamin Gray wrote:
> On Tue, 2023-03-07 at 15:05 +1000, Nicholas Piggin wrote:
> > I think it is not quite per-process? I don't actually know how the
> > user
> > toolchain side is put together, but I'm thinking we can not give it a
> > new
> > salt on fork(), but we could on exec(). I think we could actually
> > give
> > each thread their own salt within a process too, right?
>
> Yeah, the error case is we return further than we called in a given
> execution context. A forked child may return after the fork, meaning it
> needs the same key as the parent for the hashchk to work. Exec can get
> a new key because we can't return with any existing hashes. I haven't
> seen enough of kernel thread support to know if/how we can give threads
> their own key. I believe they go through the fork() call that copies
> the parent key currently.

Could look at possibly doing per-thread keys afterward but what you're
doing makes sense so no problem.

Thanks,
Nick