linux-kernel - Re: [RFC PATCH] lkdtm: Replace lkdtm_rodata_do_nothing() by do

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Message-ID: <202204072037.FE91C45E@keescook>
Date:   Thu, 7 Apr 2022 20:42:32 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Christophe Leroy <christophe.leroy@...roup.eu>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "James E.J. Bottomley" <James.Bottomley@...senpartnership.com>,
        Helge Deller <deller@....de>, Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-ia64@...r.kernel.org, linux-parisc@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [RFC PATCH] lkdtm: Replace lkdtm_rodata_do_nothing() by
 do_nothing()

On Sun, Oct 17, 2021 at 07:19:47PM +0200, Christophe Leroy wrote:
> But for EXEC_RODATA test, execute_location() uses
> lkdtm_rodata_do_nothing() which is already in rodata section
> at build time instead of using a copy of do_nothing(). However
> it still uses the function descriptor of do_nothing(). There
> is a risk that running lkdtm_rodata_do_nothing() with the
> function descriptor of do_thing() is wrong.

Wrong how? (Could there be two descriptors?)

> To remove the above risk, change the approach and do the same
> as for other EXEC tests: use a copy of do_nothing(). The copy
> cannot be done during the test because RODATA area is write
> protected. Do the copy during init, before RODATA becomes
> write protected.

Hmm, hmm. This is a nice way to handle it, but I'm not sure which
"weird" way is better. I kind of prefer the code going through all the
"regular" linking goo to end up in .rodata, but is it really any
different from doing this via the ro_after_init section? It makes me
nervous because they can technically be handled differently. For
example, .rodata is mapped differently on some architectures compared to
ro_after_init. Honestly, I actually this this patch should be modified
to _add_ a new test for EXEC_RO_AFTER_INIT, and leave the existing
.rodata one alone...

-Kees

-- 
Kees Cook