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From:   Michael Ellerman <mpe@ellerman.id.au>
To:     Russell Currey <ruscur@russell.cc>,
        Christophe Leroy <christophe.leroy@c-s.fr>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>
Cc:     linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org
Subject: Re: [RFC PATCH v1 3/6] powerpc: Add skeleton for Kernel Userspace Execution Prevention
In-Reply-To: <63177c72b94a653707d6b984ca789ecf8ebf0a95.camel@russell.cc>
References: <ab8fcb458e1a6ad86382767b3ce7f7cc0d5b7121.1541568127.git.christophe.leroy@c-s.fr> <1b27e980fa8dda09955b35be09c99bb1e22fef70.1541568127.git.christophe.leroy@c-s.fr> <63177c72b94a653707d6b984ca789ecf8ebf0a95.camel@russell.cc>
Date:   Wed, 21 Nov 2018 22:50:56 +1100
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Russell Currey <ruscur@russell.cc> writes:

> On Wed, 2018-11-07 at 16:56 +0000, Christophe Leroy wrote:
>> This patch adds a skeleton for Kernel Userspace Execution Prevention.
>> 
>> Then subarches implementing it have to define CONFIG_PPC_HAVE_KUEP
>> and provide setup_kuep() function.
>> 
>> Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
>
> An open question (with nothing to do specifically with this patch):
>
> For what reason would you ever disable execution prevention?  Clearly
> there must be something since "nosmep" is a thing, but I don't know why
> we'd ever do it.

Because depending on the implementation there might be a performance
overhead, and you may want to avoid that.

cheers