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

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Date:   Tue, 07 Mar 2023 16:37:47 +1100
From:   Benjamin Gray <bgray@...ux.ibm.com>
To:     Nicholas Piggin <npiggin@...il.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, 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.