Subject: Re: [RFC PATCH] lkdtm: Replace lkdtm_rodata_do_nothing() by do_nothing()

From: Kees Cook <[email-masked]>
Date: Thu, 7 Apr 2022 20:42:32 -0700

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